Python for Unix and Linux System Administration

Table of Contents

A Note Regarding Supplemental Files

Foreword

Preface

Conventions Used in This Book

Using Code Examples

Safari® Books Online

How to Contact Us

Acknowledgments

Noah’s Acknowledgments

Jeremy’s Acknowledgments

1. Introduction

Why Python?

Motivation

The Basics

Executing Statements in Python

Summary

Using Functions in Python

Reusing Code with the Import Statement

2. IPython

Installing IPython

Basic Concepts

Interacting with IPython

Tab Completion

Magic Edit

Configuring IPython

Help with Magic Functions

Unix Shell

alias

Shell Execute

rehash

rehashx

cd

bookmark

dhist

pwd

Variable Expansion

String Processing

sh Profile

Information Gathering

page

pdef

pdoc

pfile

pinfo

psource

psearch

who

who_ls

whos

History

Automation and Shortcuts

alias

macro

store

reset

run

save

rep

Summary

3. Text

Python Built-ins and Modules

str

re

Apache Config File Hacking

Working with Files

Standard Input and Output

StringIO

urllib

Log Parsing

ElementTree

Summary

4. Documentation and Reporting

Automated Information Gathering

Receiving Email

Manual Information Gathering

Information Formatting

Graphical Images

PDFs

Information Distribution

Sending Email

Trac

Summary

5. Networking

Network Clients

socket

httplib

ftplib

urllib

urllib2

Remote Procedure Call Facilities

XML-RPC

Pyro

SSH

Twisted

Scapy

Creating Scripts with Scapy

6. Data

Introduction

Using the OS Module to Interact with Data

Copying, Moving, Renaming, and Deleting Data

Working with Paths, Directories, and Files

Comparing Data

Using the filecmp Module

Merging Data

MD5 Checksum Comparisons

Pattern Matching Files and Directories

Wrapping Up rsync

Metadata: Data About Data

Archiving, Compressing, Imaging, and Restoring

Using tarfile Module to Create TAR Archives

Using a tarfile Module to Examine the Contents of TAR Files

7. SNMP

Introduction

Brief Introduction to SNMP

SNMP Overview

SNMP Installation and Configuration

IPython and Net-SNMP

Discovering a Data Center

Retrieving Multiple-Values with Net-SNMP

Finding Memory

Creating Hybrid SNMP Tools

Extending Net-SNMP

SNMP Device Control

Enterprise SNMP Integration with Zenoss

Zenoss API

8. OS Soup

Introduction

Cross-Platform Unix Programming in Python

Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

Creating a Cross-Platform, Systems Management Tool

Creating a Cross-Platform Build Network

PyInotify

OS X

Scripting DSCL or Directory Services Utility

OS X Scripting APIs

Automatically Re-Imaging Machines

Managing Plist Files from Python

Red Hat Linux Systems Administration

Ubuntu Administration

Solaris Systems Administration

Virtualization

VMware

Cloud Computing

Amazon Web Services with Boto

Google App Engine

Using Zenoss to Manage Windows Servers from Linux

9. Package Management

Introduction

Setuptools and Python Eggs

Using easy_install

easy_install Advanced Features

Search for Packages on a Web Page

Install Source Distribution from URL

Install Egg Located on Local or Network Filesystem

Upgrading Packages

Install an Unpacked Source Distribution in Current Working Directory

Extract Source Distribution to Specified Directory

Change Active Version of Package

Changing Standalone .py File into egg

Authenticating to a Password Protected Site

Using Configuration Files

Easy Install Advanced Features Summary

Creating Eggs

Entry Points and Console Scripts

Registering a Package with the Python Package Index

Where Can I Learn More About …

Distutils

Buildout

Using Buildout

Developing with Buildout

virtualenv

Creating a Custom Bootstrapped Virtual Environment

EPM Package Manager

EPM Package Manager Requirements and Installation

Creating a Hello World Command-Line Tool to Distribute

Creating Platform-Specific Packages with EPM

Making the Package

EPM Summary: It Really Is That Easy

10. Processes and Concurrency

Introduction

Subprocess

Using Return Codes with Subprocess

Using Supervisor to Manage Processes

Using Screen to Manage Processes

Threads in Python

Timed Delay of Threads with threading.Timer

Threaded Event Handler

Processes

Processing Module

Scheduling Python Processes

daemonizer

Summary

11. Building GUIs

GUI Building Theory

Building a Simple PyGTK App

Building an Apache Log Viewer Using PyGTK

Building an Apache Log Viewer Using Curses

Web Applications

Django

Apache Log Viewer Application

Simple Database Application

Conclusion

12. Data Persistence

Simple Serialization

Pickle

cPickle

shelve

YAML

ZODB

Relational Serialization

SQLite

Storm ORM

SQLAlchemy ORM

Summary

13. Command Line

Introduction

Basic Standard Input Usage

Introduction to Optparse

Simple Optparse Usage Patterns

No Options Usage Pattern

True/False Usage Pattern

Counting Options Usage Pattern

Choices Usage Pattern

Option with Multiple Arguments Usage Pattern

Unix Mashups: Integrating Shell Commands into Python Command-Line Tools

Kudzu Usage Pattern: Wrapping a Tool in Python

Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior

Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

Integrating Configuration Files

Summary

14. Pragmatic Examples

Managing DNS with Python

Using LDAP with OpenLDAP, Active Directory, and More with Python

Importing an LDIF File

Apache Log Reporting

FTP Mirror

A. Callbacks

Index

Python for Unix and Linux System Administration

Noah Gift

Jeremy M. Jones

Editor

Julie Steele

Copyright © 2009 Jeremy Jones and Noah Gift

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Dedication

I dedicate this book to Dr. Joseph E. Bogen, my mom, and my wife, Leah—three people who have loved me and believed in me when it counted the most.

—Noah

I dedicate this book to my wife, Debra, and my children, Zane and Justus. You encouraged me, bore with me with great patience, and gave me many smiles along the journey of writing this book. This book is as much yours as it is mine.

—Jeremy

A Note Regarding Supplemental Files

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All code files or examples referenced in the book will be available online. For physical books that ship with an accompanying disc, whenever possible, we’ve posted all CD/DVD content. Note that while we provide as much of the media content as we are able via free download, we are sometimes limited by licensing restrictions. Please direct any questions or concerns to booktech@oreilly.com.

Foreword

Æleen Frisch, July 2008

I was excited to preview this book on using Python for system administration. I remembered how I felt when I discovered Python after many years of programming in other languages: it was like a breath of spring air and the warmth of the sun after a long winter indoors. Code was suddenly easy and fun to write again, and I finished programs much more quickly than before.

As a system administrator, most of my own Python use is for system and network management tasks. I already knew how useful a good book focusing on system administration with Python would be. I am happy to say that this is that book. Overall, Noah and Jeremy have created an interesting, intelligent work on Python that is planted firmly in the system administration sphere. I found the book both very useful and enjoyable to read.

The two opening chapters are a great introduction to Python for system administrators (and others) who are new to Python. I consider myself an intermediate Python programmer, and I learned a lot from the book. I suspect even Python wizards will come across a few new tricks in here. I can especially recommend the chapters on networking and managing network services, SNMP, and management of heterogeneous systems as particularly useful and well focused on nontrivial, real-world tasks that system administrators face every day.

Preface

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates new terms, URLs, email addresses, filenames, and file extensions.

Constant width

Used for program listings, in text to refer to program elements, such as variable or function names, databases, data types, environment variables, statements, utilities, keywords, utilities, and modules.

Constant width bold

Shows commands or other text that should be typed literally by the user.

Constant width italic

Shows text that should be replaced with user-supplied values or by values determined by context.

Tip

This icon signifies a tip, suggestion, or general note.

Caution

This icon indicates a warning or caution.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code that is included in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission; selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission; incorporating a significant amount of example code from this book into your product’s documentation does require permission.

We appreciate, but do not require, attribution. An attribution usually includes the title, author, publisher, and ISBN, for example: “Python for Unix and Linux System Administration by Noah Gift and Jeremy M. Jones. Copyright 2008 Noah Gift and Jeremy M. Jones, 978-0-596-51582-9.”

If you feel your use of code examples falls outside fair use or the permission given above, feel free to contact us at permissions@oreilly.com.

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Acknowledgments

Noah’s Acknowledgments

As I sit writing an acknowledgment for this book, I have to first mention Dr. Joseph E. Bogen, because he made the single largest impact on me, at a time that it mattered the most. I met Dr. Bogen while I was working at Caltech, and he opened my eyes to another world giving me advice on life, psychology, neuroscience, math, the scientific study of consciousness, and much more. He was the smartest person I ever met, and was someone I loved. I am going to write a book about this experience someday, and I am saddened that he won’t be there to read it, his death was a big loss.

I want to thank my wife, Leah, who has been one of the best things to happen to me, ever. Without your love and support, I never could have written this book. You have the patience of a saint. I am looking forward to going where this journey takes us, and I love you. I also want to thank my son, Liam, who is one and a half, for being patient with me while I wrote this book. I had to cut many of our guitar, piano, and pushup lessons short, so I owe you payback times two, little goat.

To my mom, I love you, and thank you for encouraging me throughout life.

Of course, I want to thank Jeremy M. Jones, my coauthor, for agreeing to write this book with me. I think we were a great team with different, but complementary styles, and we wrote a great book. You have taught me a lot about Python, and have been a good partner and friend. Thanks!

Titus Brown, whom I suppose I have to call Dr. Brown now, was the person that got me interested in Python to begin with, when I met him at Caltech. He is another example of how one person can make a difference, and I am glad to consider him an “old” friend, the kind money can’t buy. He kept asking me, “Why don’t you use Python?” And then one day I did. If it wasn’t for Titus, I would certainly have continued down the Java and Perl path. You can read his blog here: http://ivory.idyll.org/blog.

Shannon Behrens has a heart of solid gold, a mind as sharp as a razor, and a knowledge of Python that is truly scary. I first met Shannon through Titus, ironic again, but he and I became quick friends. Shannon is the real deal in every sense of the word, and has taught me a tremendous amount about Python, in fact, staggering would be a better word. His help with Python, and editing this book has been incredible, and I owe him tremendously. I shudder to think of what it would have looked like without him. I can’t ever imagine a company being foolish enough to let him get away, and I look forward to helping him with his first book. Finally, he is just an incredible technical reviewer. You can read his blog here: http://jjinux.blogspot.com/.

Doug Hellmann was our other star technical reviewer and was exceptionally productive and helpful. Jeremy and I are extremely fortunate to get someone of his caliber to review the book. He went above and beyond his call of duty, and is truly a force of efficiency to reckon with. He was also a great source of motivation while we worked together at Racemi. You can read his blog here: http://blog.doughellmann.com/.

Thanks to Scott Leerseen for reviewing our book and giving us good advice along the way. I also especially enjoyed our code review battles. Just remember, I am always right.

Thanks to Alfredo Deza for the work on making an Ubuntu virtual machine for the book, your expertise was greatly appreciated.

A very large thanks to Liza Daly, for providing good feedback on some really early, and rough, parts of our book. This was tremendously helpful.

Special thanks to Jeff Rush for his advice and reference material on Buildout, Eggs, and Virtualenv.

Thanks to Aaron Hillegass who has given me some great advice and help along the way, and who has a great training company, Big Nerd Ranch. He is a special person, who I am lucky to have met. Thanks to Mark Lutz, who I had the pleasure of taking a Python training course from, and who has written some great books on Python.

Thanks to the people in the Python community in Atlanta, and the members of PyAtl: http://pyatl.org; you have all taught me a great deal. Rick Copeland, Rick Thomas, Brandon Rhodes, Derek Richardson, Jonathan La Cour, a.k.a Mr. Metaclass, Drew Smathers, Cary Hull, Bernard Matthews, Michael Langford, and many more I have forgotten to mention. Brandon and Rick Copeland in particular have been very helpful and are awesome Python programmers. You can read Brandon’s blog at http://rhodesmill.org/brandon/.

Thanks to Grig Gheorghiu for giving us expert sysadmin and testing advice and for giving us a kick in the butt when we needed one.

Thanks to my former employer Racemi, and the CTO/Founder, Charles Watt. I learned a lot from you and was glad you knew which competitive buttons to push. Just remember I will kick your butt at writing code, a 26-mile run, or a 200-mile bike ride any day, just tell me where and when.

Thanks to Dr. Nanda Ganesan, who was a great mentor in graduate school at CSULA. You taught me a lot about information technology and life and encouraged me to think big.

Thanks to Dr. Cindy Heiss, who was my professor for my undergraduate degree in nutritional science. You got me started on web development, encouraged me to believe in myself, and ultimately made an impact on my life, thanks!

Thanks to Sheldon Blockburger, who let me try out for Division I decathlon as a walk- on at Cal Poly SLO. Even though I didn’t make the team, you showed me how to be a fierce competitor and warrior, and taught me the self-discipline to run 200-meter intervals by myself. I believe weekly 200-meter interval workouts make me a better software engineer.

There were many other people who helped tremendously along the way, including Jennifer Davis, yet another friend from Caltech, who gave us some great feedback; some of my friends and coworkers at Turner; Doug Wake, Wayne Blanchard, Sam Allgood, Don Voravong; some of my friends and coworkers from Disney Feature animation, including Sean Someroff, Greg Neagle, and Bobby Lea. Greg Neagle in particular taught me a lot about OS X. Also, thanks to J.F. Panisset, who I met at Sony Imageworks, for teaching me quite a bit about engineering in general. Although he is now a CTO, he is another rare catch for any company.

I would like to thank a few others who made some important contributions: Mike Wagner, Chris McDowell, and Shaun Smoot.

Thanks to Bruce J. Bell, who I worked with at Caltech. He taught me quite a bit about Unix and programming over the years, and I owe him greatly for it. You can read his material here: http://www.ugcs.caltech.edu/~bruce/.

Also thanks to Alberto Valez, my boss at Sony Imageworks, for being possibly the best boss I ever had and giving me the chance to completely automate my job. Thanks to film editor Ed Fuller, who helped with advice on the book, and was a good friend during this process.

Thanks to many people in the Python community. First, thanks to Guido van Rossum for writing a great language, for being a great leader, and for being patient with me when I asked for advice on the book. There are so many rock stars in the Python community who crank out useful tools that I use everyday. They include Ian Bicking, Fernando Perez and Villi Vainio, Mike Bayer, Gustavo Niemeyer, etc. Thanks! Thanks to the great book by David Beazely, and his fantastic tutorial at PyCon 2008 on Generators. Thanks to other writers about Python and systems administration as well. You can find links to their work here: http://wiki.python.org/moin/systems_administration. Thanks also to the Repoze crew: Tres Seaver and Chris McDonough (http://repoze.org/index.html).

Special thanks to the great tools, advice, and tolerance from Phillip J. Eby on the setuptools section. Also, thanks to Jim Fulton who tolerated my questions about ZODB and buildout, with a crazy schedule. Additional thanks to Martijn Fassen, who taught me about ZODB and Grok. If you want to see the future of Python web development, check out Grok: http://grok.zope.org/.

Thanks to Red Hat Magazine staff, Julie Bryce, Jessica Gerber, Bascha Harris, and Ruth Suehle, for letting me try out ideas we used in this book in the form of articles. Also, thanks to Mike McCrary at IBM Developerworks, for letting me write articles to try out ideas we used in this book.

I want to thank the multitudes of people who told me at one point in my life that I couldn’t do something. At almost every step, I have met discouraging people who told me everything from I would never get into the college I wanted to to I would never learn to program. Thank you for giving me the extra motivation to succeed at my dreams. Humans can create their own reality if they truly believe in themselves, and I would encourage everyone to give themselves a chance to do what they truly want to do.

Finally, thanks to O’Reilly and Tatiana Apandi, for believing in my original pitch for a book on Python and Systems Administration. You took a chance and believed in me and Jeremy, and I thank you for that. Although Tatiana left O’Reilly near the end of our book to pursue her dreams, her impact was still felt. I also want to thank our new editor Julie Steele, who has been supportive and helpful every step of the way. You have really provided a sea of calm that I personally appreciated greatly. I look forward to hearing great things in the future from Julie, and I’m excited to work with her again.

Jeremy’s Acknowledgments

After reading Noah’s list of thanks, it makes me feel both ungrateful, because I know my list won’t be that long, and at a loss, because I think he covered nearly everyone that I wanted to thank.

First, I must thank my God, through Whom I can do all things and without Whom, I can do nothing.

First in an earthly sense, I thank my wife, Debra. You kept the children engaged with other activities while I worked on the book. You enforced the so-often reapeated rule “Don’t bother Daddy while he’s working on his book.” You encouraged me when I needed it, and you also gave me a lot of space, which is what I needed most. Thank you. I love you. I could not have written this book without you.

I also must thank my sweet children, Zane and Justus, for their patience through the process of my writing this book. I missed out on a lot of trips to Stone Mountain with you both. I still put one of you to bed most nights, but I missed out on staying in there long enough to fall asleep with you, like I used to. I missed out on the last several weeks of Kid’s Rock on Wednesday nights. I missed out on so much, but you bore it patiently. So, thank you for your patience. And thank you for your excitement as you hear that I’m almost done with the book. I love you both.

I want to thank my parents, Charles and Lynda Jones, for their support through the course of my writing this book. But more than that, I want to thank them for being a living example of a strong work ethic, of earning everything you have, of working hard to better yourself, and of spending money wisely. Those are lessons I hope to pass on to Zane and Justus.

Thank you to Noah Gift, my coauthor, for getting me into this mess. It has been hard, harder than I thought and definitely one of the hardest things I’ve ever done in my life. I think it says a lot about a person when you work on something like this with him and at the end, you can still think of him as your friend. Thanks, Noah. This book would not have begun if not for you.

I want to thank our team of reviewers. I think that Noah has already thanked all of you, but I want to thank everyone that I can: Doug Hellman, Jennifer Davis, Shannon JJ Behrens, Chris McDowell, Titus Brown, and Scott Leerseen. You guys were awesome. There were times when I thought that I had something spot-on and you readjusted my thinking. Or you just brought a completely different perspective to the book and helped me see my work through a different set of eyes. (That was mostly you, Jennifer. If the text processing chapter is useful for sysadmins, it’s mostly because of you.) Thank you all.

I also want to thank our editors, Tatiana Apandi and Julie Steele. You guys handled the hard stuff, freeing us up to work on the book. You both eased our burden along the way. Thank you.

I’d also like to thank Fernando Perez and Ville Vainio for your amazing feedback. I hope I’ve done IPython justice. And thank you for IPython. I feel like I couldn’t live without it.

Thank you Duncan McGreggor, for helping me get the Twisted code in better shape. Your comments were extemely helpful. And thank you for working on Twisted. It is an amazing framework. I hope to use it more, real soon now.

I thank Bram Moolenaar and everyone who has ever worked on the Vim editor. Almost every word and XML tag that I wrote flowed through capabilities Vim. I picked up a few tricks along the way that I’ll incorporate into my daily editing habits. Vim made me more productive. Thank you.

I also want to thank Linus Torvalds, the Debian folks, the Ubuntu folks, and anyone else who has ever worked on Linux. Almost every word that I typed was done on Linux. You made it incredibly simple to set up new environments and test different things. Thank you.

Finally, but by no means least, I want to thank Guido van Rossum and everyone who has ever done any work on Python. I have been benefitting from your work for a number of years now. I was hired for my last two jobs because of Python. Python, the language, and Python, the community, have been both a great joy for me since I started working with it sometime around 2001–2002. Thank you. Python has been very good to me.

Chapter 1. Introduction

Why Python?

If you are a system administrator, it is likely that you have encountered Perl, Bash, ksh, or some other scripting language. You may have even used one or more yourself. Scripting languages are often used to do repetitive, tedious work at a rate and with an accuracy that far surpass what you could accomplish without them. All languages are tools. They are simply a means to get work done. They have value only insofar as they help you get your job done better. We believe that Python is a valuable tool, specifically because it enables you to get your work done efficiently.

So is Python better than Perl, Bash, Ruby, or any other language? It’s really difficult to put that sort of qualitative label on a programming language, since the tool is so closely tied to the thought process of the programmer who is using it. Programming is a subjective, deeply personal activity. For the language to be excellent, it must fit the person using it. So we’re not going to argue that Python is better, but we will explain the reasons that we believe Python can be an excellent choice. We’ll also explain why it is a great fit for performing sysadmin tasks.

The first reason that we think that Python is excellent is that it is easy to learn. If a language can’t help you become productive pretty quickly, the lure of that language is severely diminished. Why would you want to spend weeks or months studying a language before you are able to write a program that does something useful? This is especially the case for sysadmins. If you are a sysadmin, your work can pile up faster than you can unpile it. With Python, you can start writing useful scripts literally in hours rather than in days or weeks. If you can’t learn a language quickly enough to start writing scripts with it almost immediately, you should strongly question whether you should be learning it.

However, a language that is easy to learn but doesn’t allow you to do fairly complex tasks isn’t worth much either. So the second reason that we consider Python to be an excellent programming language is that, while it lets you start simply, it also allows you to perform tasks that are as complex as you can imagine. Do you need to read through a logfile line by line and pull out some pretty basic information? Python can handle that. Or do you need to parse through a logfile, extract every piece of information that it provides, compare usage from each IP address in this logfile to usage in each logfile (which are stored in a relational database, by the way) from the past three months, and then store the results to a relational database? Sure, Python can do that as well. Python is being used on some pretty complex problems, such as analysis of genomic sequences, multithreaded web servers, and heavy duty statistical analysis. You may never have to work on anything like that, but it’s nice to know that when you need to do complex things, the language is able to work with you.

Additionally, if you are able to perform complex operations, but the maintainability of your code suffers along the way, that isn’t a good thing. Python doesn’t prevent code maintenance problems, but it does allow you to express complex ideas with simple language constructs. Simplicity is a huge factor in writing code that is easy to maintain later. Python has made it pretty simple for us to go back over our own code and work on it after we haven’t touched it in months. It has also been pretty simple for us to work on code that we haven’t seen before. So the language, that is the language’s syntax and common idioms, are clear and concise and easy to work with over long periods of time.

The next reason we consider Python to be an excellent language is its readability. Python relies on whitespace to determine where code blocks begin and end. The indentation helps your eyes quickly follow the flow of a program. Python also tends to be “word-based.” By that we mean that while Python uses its share of special characters, features are often implemented as keywords or with libraries. The emphasis on words rather than special characters helps the reading and comprehension of code.

Now that we’ve outlined a few of Python’s benefits, we’ll show some comparisons of code examples in Python, Perl, and Bash. Along the way, we’ll also look at a few more of Python’s benefits. Here is a simple example, in Bash, of showing all the combinations of 1, 2 and a, b:

#!/bin/bash

for a in 1 2; do
for b in a b; do
echo "$a $b"
done
done

And here is a comparable piece of Perl:

#!/usr/bin/perl

foreach $a ('1', '2') {
foreach $b ('a', 'b') {
print "$a $b\n";
}
}

This is a pretty simple nested loop. Let’s compare these looping mechanisms with a for loop in Python:

#!/usr/bin/env python

for a in [1, 2]:
for b in ['a', 'b']:
print a, b

Next, we’ll demonstrate using conditionals in Bash, Perl, and Python. We have a simple if/else condition check here. We’re just checking to see whether a certain file path is a directory:

#!/bin/bash

if [ -d "/tmp" ] ; then
echo "/tmp is a directory"
else
echo "/tmp is not a directory"
fi

Here is the Perl equivalent of the same script:

#!/usr/bin/perl

if (-d "/tmp") {
print "/tmp is a directory\n";
}
else {
print "/tmp is not a directory\n";
}

And here is the Python equivalent of the script:

#!/usr/bin/env python

import os

if os.path.isdir("/tmp"):
print "/tmp is a directory"
else:
print "/tmp is not a directory"

Another point in favor of Python’s excellence is its simple support for object-oriented programming (OOP). And, actually, the converse of that is that you don’t have to do OOP if you don’t want to. But if you do, it’s dead simple in Python. OOP allows you to easily and cleanly break problems apart and bundle pieces of functionality together into single “things” or “objects.” Bash doesn’t support OOP, but both Perl and Python do. Here is a module in Perl that defines a class:

package Server;
use strict;

sub new {
my $class = shift;
my $self  = {};
$self->{IP} = shift;
$self->{HOSTNAME} = shift;
bless($self);
return $self;
}

sub set_ip {
my $self = shift;
$self->{IP} = shift;
return $self->{IP};
}

sub set_hostname {
my $self = shift;
$self->{HOSTNAME} = shift;
return $self->{HOSTNAME};
}

sub ping {
my $self = shift;
my $external_ip = shift;
my $self_ip = $self->{IP};
my $self_host = $self->{HOSTNAME};
print "Pinging $external_ip from $self_ip ($self_host)\n";
return 0;
}

1;

And here is a piece of code that uses it:

#!/usr/bin/perl

use Server;

$server = Server->new('192.168.1.15', 'grumbly');
$server->ping('192.168.1.20');

The code that makes use of the OO module is straightforward and simple. The OO module may take a bit more mental parsing if you’re not familiar with OOP or with the way that Perl tackles OOP.

A comparable Python class and use of the class looks something like this:

#!/usr/bin/env python

class Server(object):
def __init__(self, ip, hostname):
self.ip = ip
self.hostname = hostname
def set_ip(self, ip):
self.ip = ip
def set_hostname(self, hostname):
self.hostname = hostname
def ping(self, ip_addr):
print "Pinging %s from %s (%s)" % (ip_addr, self.ip, self.hostname)

if __name__ == '__main__':
server = Server('192.168.1.20', 'bumbly')
server.ping('192.168.1.15')

Both the Perl and Python examples demonstrate some of the fundamental pieces of OOP. The two examples together display the different flavors that each respective language provides while reaching toward its respective goals. They both do the same thing, but are different from one another. So, if you want to use OOP, Python supports it. And it’s quite simple and clear to incorporate it into your programming.

Another element of Python’s excellence comes not from the language itself, but from the community. In the Python community, there is much consensus about the way to accomplish certain tasks and the idioms that you should (and should not) use. While the language itself may support certain phrasings for accomplishing something, the consensus of the community may steer you away from that phrasing. For example, from module import * at the top of a module is valid Python. However, the community frowns upon this and recommends that you use either: import module or: from module import resource. Importing all the contents of a module into another module’s namespace can cause serious annoyance when you try to figure out how a module works, what functions it is calling, and where those functions come from. This particular convention will help you write code that is clearer and will allow people who work on your code after you to have a more pleasant maintenance experience. Following common conventions for writing your code will put you on the path of best practices. We consider this a good thing.

The Python Standard Library is another excellent attribute of Python. If you ever hear the phrase “batteries included” in reference to Python, it simply means that the standard library allows you to perform all sorts of tasks without having to go elsewhere for modules to help you get it done. For example, though it isn’t built-in to the language directly, Python includes regular expression functionality; sockets; threads; date/time functionality; XML parsers; config file parser; file and directory functionality; data persistence; unit test capabilities; and http, ftp, imap, smpt, and nntp client libraries; and much more. So once Python is installed, modules to support all of these functions will be imported by your scripts as they are needed. You have all the functionality we just listed here. It is impressive that all of this comes with Python without requiring anything else. All of this functionality will help you out immensely as you write Python programs to do work for you.

Easy access to numerous third-party packages is another real advantage of Python. In addition to the many libraries in the Python Standard Library, there are a number of libraries and utilities that are easily accessible on the internet that you can install with a single shell command. The Python Package Index, PyPI (http://pypi.python.org), is a place where anyone who has written a Python package can upload it for others to use. At the time we are writing this book, there are over 3,800 packages available for download and use. Packages include IPython, which we cover in the following chapter; Storm (an object-relational mapper, which we cover in Chapter 12); and Twisted, a network framework, which we cover in Chapter 5—just to name 3 of the over 3,800 packages. Once you start using PyPI, you’ll find it nearly indispensible for finding and installing useful packages.

Many of the benefits that we see in Python stem from the central philosophy of Python. When you type import this at a Python prompt, you will see The Zen of Python by Tim Peters. Here it is:

In [1]: import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

This statement isn’t a dogmatic imperative that is strictly enforced at all levels of development of the language, but the spirit of it seems to permeate much of what happens in and with the language. And we have found this spirit to be a beautiful thing. This is perhaps the essence of why we choose to use Python day after day. This philosophy resonates within us as what we want and expect from a language. And if this resonates with you, then Python is probably a good choice for you as well.

Motivation

If you justpicked up this book in a bookstore or are reading an introduction online somewhere, you may be asking yourself, how hard it is going to be to learn Python and if it is even worth it. Although Python is catching on like wildfire, there are many sysadmins who have been exposed to Bash and Perl only. If you find yourself in this category, you should take comfort in knowing that Python is very easy to learn. In fact, although it is a matter of opinion, Python is considered by many to be the easiest language to learn and teach, period!

If you already know Python, or are a programming guru in another language, you will probably be able to jump right into any of the following chapters without reading this intro and immediately start being productive using our examples. We made a concerted effort to create examples that will actually help you get your job done. There are examples of ways to discover and monitor subnets automatically with SNMP, to convert to an interactive Python shell called IPython, to build data processing pipelines, to write custom metadata management tools with object-relational mappers, to perform network programming, to write command-line tools, and much more.

If you are coming from a shell programming/scripting background, though, don’t worry at all. You, too, can learn Python quite easily. You need only motivation, curiosity, and determination, the same factors that led you to pick up this book and look at the introduction in the first place.

We sense there are still a few skeptics out there. Maybe some of the things you have heard about programming have scared you. One common, and horribly false, misconception is that only some people can learn to program, and they are a mysterious and elite few. The frank truth is that anyone can learn how to program. A second, equally false, misconception is that earning a computer science degree is the only way a person can truly become a software engineer. But some of the most prolific software developers do not have engineering degrees. There are people with philosophy, journalism, nutritional science, and English degrees who are competent Python programmers. Having a degree in computer science is not a requirement to learn Python, although it certainly doesn’t hurt.

Another funny, and false, misconception is that you must have started to program in your teenage years, or you will never learn to program. While this makes people who were lucky enough to have someone in their life that encouraged them to program at a young age feel good, it is another myth. It is very helpful to have started learning programming at a young age, but age is not a requirement to learn Python. Learning Python is most certainly not a “young person’s game,” as we have heard some people say. There are countless cases of developers who learned to program in their late 20s, 30s, 40s, and onward.

If you have gotten this far, we should point out that you, the reader, have an advantage many people do not. If you decided to pick up a book on Python for Unix and Linux system administration, then you most likely know something about how to execute commands from a shell. This is a tremendous advantage to learning to become a Python programmer. Having an understanding of the way to execute commands from a terminal is all that is required for this introduction to Python. If you truly believe you will learn how to program with Python, then read the next section immediately. If you don’t believe it yet, then reread this section again, and convince yourself it really is just a matter of getting your mind to understand you do have the power to learn how to program in Python. It is really that simple; if you make this decision, it will change your life.

The Basics

This introduction to Python is going to be very different from any other one we’ve seen, as it will use an interactive shell called IPython and a regular Bash shell. You will need to open two terminal windows, one with IPython and one with Bash. In every example, we will compare what we do in Python with a Bash example. The first steps are to download the correct version of IPython for your platform and install it. You can get a copy at http://ipython.scipy.org/moin/Download. If for some reason, you can’t get IPython to install you can also just use a regular Python shell. You can also download a copy of the virtual machine that includes all of the software for the book, as we have a copy of IPython preconfigured and ready to run. You just need to type in ipython, and you will get a prompt.

Once you have installed IPython and have an IPython shell prompt, it should look something like this:

[ngift@Macintosh-7][H:10679][J:0]# ipython
Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17)
Type "copyright", "credits" or "license" for more information.

IPython 0.8.2 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object'. ?object also works, ?? prints more.

In [1]:

An IPython shell is quite a bit like a regular Bash shell and can execute commands such as ls, cd, and pwd, but you can read the next chapter for more of a scoop on IPython. This chapter is about learning Python, so on to the tutorial.

In your Python terminal, type in the following:

In [1]: print "I can program in Python"
I can program in Python

In your Bash terminal, type in the following:

[ngift@Macintosh-7][H:10688][J:0]# echo "I can program in Bash"
I can program in Bash

In these two examples, there isn’t much of a difference in Python and Bash; we hope it takes some of the mystery out of Python.

Executing Statements in Python

If you spend a lot of your day typing commands into a terminal, then you are used to executing statements and, perhaps, redirecting the output to a file or to another Unix command. Let’s look at the way we would execute a command in Bash and then compare that to the way it works in Python. In the Bash terminal, type the following:

[ngift@Macintosh-7][H:10701][J:0]# ls -l /tmp/
total 0
-rw-r--r--  1 ngift  wheel  0 Apr  7 00:26 file.txt

In the Python terminal, type the following:

In [2]: import subprocess

In [3]: subprocess.call(["ls","-l ","/tmp/"])
total 0
-rw-r--r--  1 ngift  wheel  0 Apr  7 00:26 file.txt
Out[3]: 0

The Bash example shouldn’t need any explanation as it is a simple ls command, but if you have never seen Python code before, the Python example probably looks a bit strange. You might be thinking, “What the heck is this import subprocess thing?” One of the powerful features of Python is its ability to import modules or other files that contain code and reuse them in a new program. If you are familiar with “sourcing” a file in Bash, then you will recognize some similarities. In this particular situation, all that is important to know is that you import the subprocess and use it in the syntax that is shown. We will get into the particulars of how subprocess and import work later, but for now, ignore why it works and copy the code:

subprocess.call(["some_command", "some_argument", "another_argument_or_path"])

You can run any shell command in Python just as it would be run with Bash. Given this bit of information, you can now create a Python version of ls. Just open up your favorite text editor in another terminal tab or window and place this in a file named pyls.py, and make it executable by using chmod +x pyls.py. See Example 1-1.

Example 1-1. Python wrapper for ls command

#!/usr/bin/env python
#Python wrapper for the ls command

import subprocess

subprocess.call(["ls","-l"])

Now if you run this script, you will get the exact same output that you would get if you ran ls -ls from the command line:

[ngift@Macintosh-7][H:10746][J:0]# ./pyls.py
total 8
-rwxr-xr-x  1 ngift  staff  115 Apr  7 12:57 pyls.py

While this may seem silly, (and it is silly actually), it gives you a good idea of a common use of Python in systems programming. Often, you use Python to “wrap” other scripts or Unix commands. Given this new bit of information, you could happily start writing some basic scripts if you just put one command after another in a file and ran it. Let’s take a look at something pretty simple that does just that. To follow along from home, either cut and paste the code in Example 1-2, or run the scripts pysysinfo.py and bashsysinfo.sh located in the source code that is included with this chapter. See Examples 1-2 and 1-3.

Example 1-2. System information script—Python

#!/usr/bin/env python
#A System Information Gathering Script
import subprocess

#Command 1
uname = “uname”
uname_arg = “-a”
print "Gathering system information with %s command:\n" % uname
subprocess.call([uname, uname_arg])

#Command 2
diskspace = "df"
diskspace_arg = "-h"
print "Gathering diskspace information %s command:\n" % diskspace
subprocess.call([diskspace, diskspace_arg])

Example 1-3. System information script—Bash

#!/usr/bin/env bash
#A System Information Gathering Script

#Command 1
UNAME="uname -a"
printf “Gathering system information with the $UNAME command: \n\n"
$UNAME

#Command 2
DISKSPACE="df -h"
printf "Gathering diskspace information with the $DISKSPACE command: \n\n"
$DISKSPACE

If we look at both of the scripts, we see that they look a lot a like. And if we run them, we see that the output of each is identical. One quick note though: splitting the command from the argument is completely optional using subprocess.call. You can also use this syntax:

subprocess.call("df -h", shell=True)

So far so good, but we still haven’t explained import and subprocess completely. In the Python version of the script, we imported the subprocess module because it already contained the code to make system calls in Python.

As we mentioned earlier, importing a module like subprocess is just importing a file that contains code you can use. You can create your own module or file and reuse code you have written in the same way you import subprocess. Importing is not magic at all, it is just a file with some code in it. One of the nice things about the IPython shell that you have open is its ability to inspect inside modules and files, and see the attributes that are available inside them. In Unix terms, this is a lot like running the ls command inside of /usr/bin. If you happen to be on a new system such as Ubuntu or Solaris, and you are used to Red Hat, you might do an ls of /usr/bin to see if tools such as wget, curl, or lynx are available. If you want to use a tool you find inside /usr/bin, you would simply type /usr/bin/wget, for example.

Modules such as subprocess are very similar. With IPython you can use tab complete to look at the tools that are available inside a module. Let’s walk through subprocess using tab complete to look at the attributes available inside of it. Remember, a module is just a file with some code in it. Here is what a tab complete looks like with the subprocess module in IPython:

In [12]: subprocess.
subprocess.CalledProcessError  subprocess.__hash__            subprocess.call
subprocess.MAXFD               subprocess.__init__            subprocess.check_call
subprocess.PIPE                subprocess.__name__            subprocess.errno
subprocess.Popen               subprocess.__new__             subprocess.fcntl
subprocess.STDOUT              subprocess.__reduce__          subprocess.list2cmdline
subprocess.__all__             subprocess.__reduce_ex__       subprocess.mswindows
subprocess.__builtins__        subprocess.__repr__            subprocess.os
subprocess.__class__           subprocess.__setattr__         subprocess.pickle
subprocess.__delattr__         subprocess.__str__             subprocess.select
subprocess.__dict__            subprocess._active             subprocess.sys
subprocess.__doc__             subprocess._cleanup            subprocess.traceback
subprocess.__file__            subprocess._demo_posix         subprocess.types
subprocess.__getattribute__    subprocess._demo_windows

To replicate this same behavior, you simply need to type:

import subprocess

and then type:

subprocess.

and press Tab to get a tab completion of the attributes available. In the third column of our example, notice that you see subprocess.call. Now, to see more information about how to use subprocess.call, type:

In [13]: subprocess.call?

Type:        function
Base Class:    <type 'function'>
String Form:    <function call at 0x561370>
Namespace:    Interactive
File:        /System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
subprocess.py
Definition:    subprocess.call(*popenargs, **kwargs)
Docstring:
Run command with arguments. Wait for command to complete, then
return the returncode attribute.

The arguments are the same as for the Popen constructor. Example:

retcode = call(["ls", "-l"])

Think of the special question mark syntax as a manpage query. If you want to know how a tool works in Unix, simply type:

man name_of_tool

It is the same with an attribute located inside a module such as subprocess.call. In IPython, when you type a question mark after the attribute you want to find information about, the documentation that has been included with the attribute will print out. If you do this on most attributes in the standard library, you should find some helpful information to properly use them. Keep in mind that you can also refer to the Python Standard Library documentation as well.

When we look at this documentation, “Docstring” is the official term, we see an example of the way to use subprocess.call and a description of what it does.

Summary

You now have enough information to call yourself a Python programmer. You know how to write a simple Python script, how to translate simple scripts from Bash and call them with Python, and, finally, how to find documentation about new modules and attributes. In the next section, you’ll see how to better organize these flat sequences of commands into functions.

Using Functions in Python

In the previous section we went through executing statements one after another, which is pretty useful, because it means we were able to automate something that we would normally have to do manually. The next step to automating our code execution is to create functions. If you are not already familiar with writing functions in Bash or another language, then one way to think about functions is as miniscripts. A function allows you to create blocks of statements that get called in groups that live inside of the function. This is quite a bit like the Bash script we wrote in which there were two commands enclosed in a script. One of the differences between a Bash script and a function is that you can include many function scripts. Ultimately, you can have multiple functions that group statements together in a script, and then that group of statements can be called to run a miniprogram at the proper time in your script.

At this point, we need to talk about the topic of whitespace. In Python, a uniform level of indentation must be maintained in nesting code. In another language, like Bash, when you define a function you put brackets around the code inside of a function. With Python, you must indent the code inside of the bracket. This can trip up newcomers to the language, at first, but after a while it will grow on you, and you will realize that this encourages readable code. If you have trouble getting any of these examples to work interactively, make sure you refer to the actual source code to see the proper indentation level. The most common practice is to set a tab to indent exactly four spaces.

Let’s take a look at how this works in Python and Bash. If you still have the IPython shell open, you don’t need to create a Python script file, although you can if you like. Just type the following into the interactive IPython prompt:

In [1]: def pyfunc():
...:     print "Hello function"
...:
...:

In [2]: pyfunc
Out[2]: <function pyfunc at 0x2d5070>

In [3]: pyfunc()
Hello function

In [4]: for i in range(5):
...:     pyfunc()
...:
...:
Hello function
Hello function
Hello function
Hello function
Hello function

In this example, you can see that putting a print statement in a function allows you not only to call the function later but also to call it as many times as we want. In line [4], we use a programming idiom, or technique, that executes the function five times. If you haven’t seen that technique before, understand that it calls the function five times.

We can do the same thing in a live Bash shell as well. Here is one way:

bash-3.2$ function shfunc()
> {
>     printf "Hello function\n"
> }
bash-3.2$ for (( i=0 ; i < 5 ; i++))
> do
>     shfunc
> done
Hello function
Hello function
Hello function
Hello function
Hello function

In the Bash example, we created a simple function shfunc, and then called it five times, just like we did with the Python function earlier. One thing to notice is that the Bash example requires more “baggage” to do the same thing that Python does. Notice the difference between the Bash for loop and the Python for loop. If this is your first exposure to a function in Bash or Python, you should make some other functions in your IPython window before you continue.

Functions are not magic, and writing multiple functions interactively is a great way to take away the mystery if this is your first experience with them. Here are a couple of examples of simple functions:

In [1]: def print_many():
...:     print "Hello function"
...:     print "Hi again function"
...:     print "Sick of me yet"
...:
...:

In [2]: print_many()
Hello function
Hi again function
Sick of me yet

In [3]: def addition():
...:     sum = 1+1
...:     print "1 + 1 = %s" % sum
...:
...:

In [4]: addition()
1 + 1 = 2

Now we have a few silly examples under our belt, in addition to the silly examples that you tried out on your own as well, right? So we can go back to the script we wrote that prints system information and convert those statements into functions. See Example 1-4.

Example 1-4. Converted Python system info script: pysysinfo_func.py

#!/usr/bin/env python
#A System Information Gathering Script
import subprocess

#Command 1
def uname_func():

uname = "uname"
uname_arg = "-a"
print "Gathering system information with %s command:\n" % uname
subprocess.call([uname, uname_arg])

#Command 2
def disk_func():

diskspace = "df"
diskspace_arg = "-h"
print "Gathering diskspace information %s command:\n" % diskspace
subprocess.call([diskspace, diskspace_arg])

#Main function that call other functions
def main():
uname_func()
disk_func()

main()

Given our experiments with functions, this converted example of our previous script that we simply placed these statements inside functions and then used the main function to call them all at once. If you are not familiar with this style, you might not have known that it is common to create several functions inside a script and then call them all with one main function. One of many reasons for this is that if you decide to reuse this script for another program, you can either call the functions independently or together with the main method. The key is that you decide after the module is imported.

When there is no control flow, or main function, then all of the code gets executed immediately when it is imported. This may be OK for a one-off script, but if you plan to create reusable tools, and you should, then it is a good practice to create functions that encapsulate specific actions, and then have a main function that executes the whole program.

For comparison’s sake, let’s convert our previous Bash system information script to use functions as well. See Example 1-5.

Example 1-5. Converted Bash system info script: bashsysinfo_func.sh

#!/usr/bin/env bash
#A System Information Gathering Script

#Command 1
function uname_func ()
{
UNAME="uname -a"
printf "Gathering system information with the $UNAME command: \n\n"
$UNAME
}
#Command 2
function disk_func ()
{
DISKSPACE="df -h"
printf "Gathering diskspace information with the $DISKSPACE command: \n\n"
$DISKSPACE
}

function main ()
{
uname_func
disk_func
}

main

Looking at our Bash example, you can see it has quite a bit in common with its Python cousin. We created two functions and then called those two functions by calling the main function. If this is your first experience with functions, then we would highly recommend that you comment out the main method by placing a pound sign in front of both the Bash and the Python scripts and running them again. You should get absolutely nothing when you run both scripts, because the program should execute, but won’t call the two functions inside.

At this point, you are now a programmer capable of writing simple functions in both Bash and Python. Programmers learn by doing, though, so at this point we highly recommend that you change the system calls in these two Bash and Python programs and make them your own. Give yourself some bonus points if you add several new functions to the script and call them from a main function.

Reusing Code with the Import Statement

One problem with learning something new is that, if it is abstract, like calculus, for example, it is hard to justify caring about it. When was the last time you used the math you learned in high school at the grocery store? In our previous examples, we showed you how to create functions as an alternative to executing shell commands one after another in a script. We also told you that a module is really just a script, or some lines of code in a file. It isn’t anything tricky, but it does need to be arranged in a particular way so that it can be reused in another future program. Here is the point where we show you why you should care. Let’s import the previous system information scripts in both Bash and Python and execute.

Open the IPython and Bash windows if you closed them so that we can demonstrate very quickly why functions are important for code reuse. One of the first scripts we created in Python was a sequence of commands in a file named pysysinfo.py. In Python because a file is a module and vice versa, we can import this script file into IPython. Keep in mind that you never need to specify the .py portion of the file you are importing. In fact if you do this, the import will not work. Here is what it looks like when we do that on Noah’s Macbook Pro laptop:

In [1]: import pysysinfo
Gathering system information with uname command:

Darwin Macintosh-8.local 9.2.2 Darwin Kernel Version 9.2.2: /
Tue Mar  4 21:17:34 PST 2008; root:xnu-1228.4.31~1/RELEASE_I386 i386
Gathering diskspace information df command:

Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   88Gi  4.2Gi    96%    /
devfs          110Ki  110Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
/dev/disk1s2   298Gi  105Gi  193Gi    36%    /Volumes/Backup
/dev/disk2s3   466Gi  240Gi  225Gi    52%    /Volumes/EditingDrive

Wow, that is pretty cool, right? If you import a file full of Python code it seems to runs great. But, actually, there are a few problems with this. If you plan to run Python code, it should always be executed from the command line as a part of a script or program you write. Using import is to help with this “reusing code” idea we keep throwing around. Here is the punch line: what if you only wanted to print the output of the diskspace portion of the script? The answer is you can’t. That is why you use functions. They allow you to control when and how parts of your program run so that they don’t all run at once, as they do in this example. Don’t just take our word for it, though. If you import the example of a script that puts these commands into functions, you’ll see what we mean.

Here is the output from the IPython terminal:

In [3]: import pysysinfo_func
Gathering system information with uname command:

Darwin Macintosh-8.local 9.2.2 Darwin Kernel Version 9.2.2:
Tue Mar  4 21:17:34 PST 2008; root:xnu-1228.4.31~1/RELEASE_I386 i386
Gathering diskspace information df command:

Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   88Gi  4.1Gi    96%    /
devfs          110Ki  110Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
/dev/disk1s2   298Gi  105Gi  193Gi    36%    /Volumes/Backup
/dev/disk2s3   466Gi  240Gi  225Gi    52%    /Volumes/EditingDrive

Now we get the exact same output that we get from script that doesn’t contain functions. If you are puzzled, this is a good sign. To see the reason we get the same exact output, we just need to look at the source code. If you are following along at home, open up another terminal tab or window and look at the script pysysinfo_func:

#Main function that call other functions
def main():
uname_func()
disk_func()

main()

The problem is that main function we created at the end of the last chapter is coming back to bite us. On one hand we want to be able to run our script on the command line to get the output, but on the other hand when we import it we don’t want all of the output all at once. Fortunately, the need to use a module as both a script that gets executed from the command line and as a reusable module is very common in Python. The solution is to change the way the main method gets called by replacing the last part of the script to look like this:

#Main function that call other functions
def main():
uname_func()
disk_func()

if __name__ == "__main__":
main()

This is an “idiom,” a technique that is commonly used to solve a problem. Any code that you indent underneath this statement gets run only when it is executed from the command line. To see this, either replace this in your copy of the script or import the fixed version of the script pysysinfo_func_2.py.

Now, if we go back to our IPython interpreter and import this new script, we should see this:

In [1]: import pysysinfo_func_2

This time, the main method is not called, because of the fix we made. So, to return to our original point about reusable code, we have three functions that we can use in other programs or use to interact with the IPython shell. Remember earlier we said how it would be nice to call only the function that prints the disk usage without having to call the function that calls the other commands, too. First, though, we need to review an IPython trick that we showed you before. Remember that you can use Tab to complete a module, and it will show you all of the attributes that are available to use. Here’s what that looks like:

In [2]: pysysinfo_func_2.
pysysinfo_func_2.__builtins__      pysysinfo_func_2.disk_func
pysysinfo_func_2.__class__         pysysinfo_func_2.main
pysysinfo_func_2.__delattr__       pysysinfo_func_2.py
pysysinfo_func_2.__dict__          pysysinfo_func_2.pyc
pysysinfo_func_2.__doc__           pysysinfo_func_2.subprocess
pysysinfo_func_2.__file__          pysysinfo_func_2.uname_func
pysysinfo_func_2.__getattribute__
pysysinfo_func_2.__hash__

In this example, we can ignore anything with double underscores, because these are special methods that are beyond the scope of this introduction. Because IPython is also a regular shell, it picks up the filename and the byte-compiled Python file with the .pyc extension. Once we filter past all of those names, we can see that there is a pysysinfo_func_2.disk_func. Let’s go ahead and call that function:

In [2]: pysysinfo_func_2.disk_func()
Gathering diskspace information df command:

Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   89Gi  4.1Gi    96%    /
devfs          111Ki  111Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
/dev/disk1s2   298Gi  105Gi  193Gi    36%    /Volumes/Backup
/dev/disk2s3   466Gi  240Gi  225Gi    52%    /Volumes/EditingDrive

You might have realized by now that functions are always “called” or run by attaching the “()” after the name. In this case, we ran just that one function inside of a file that contained three functions: the function we just called disk_func, the uname_func, and finally the main function. Aha! We finally have our code reuse. We were able to import something we wrote earlier and interactively run just the part of it we needed. Of course, we can also run the other two functions we wrote separately. Let’s take a look at that:

In [3]: pysysinfo_func_2.uname_func()
Gathering system information with uname command:

Darwin Macintosh-8.local 9.2.2 Darwin Kernel Version 9.2.2:
Tue Mar  4 21:17:34 PST 2008; root:xnu-1228.4.31~1/RELEASE_I386 i386

In [4]: pysysinfo_func_2.main()
Gathering system information with uname command:

Darwin Macintosh-8.local 9.2.2 Darwin Kernel Version 9.2.2:
Tue Mar  4 21:17:34 PST 2008; root:xnu-1228.4.31~1/RELEASE_I386 i386
Gathering diskspace information df command:

Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   89Gi  4.1Gi    96%    /
devfs          111Ki  111Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
/dev/disk1s2   298Gi  105Gi  193Gi    36%    /Volumes/Backup
/dev/disk2s3   466Gi  240Gi  225Gi    52%    /Volumes/EditingDrive

If you look carefully, you’ll see that we ran both of the other functions. Remember, the main function runs everything at once.

Often, the point of writing a reusable module is so that you can take some of the code and use it over and over again in a new script. So practice that by writing another script that uses one of the functions. See Example 1-6.

Example 1-6. Reusing code with import: new_pysysinfo

#Very short script that reuses pysysinfo_func_2 code
from pysysinfo_func_2 import disk_func
import subprocess

def tmp_space():
tmp_usage = "du"
tmp_arg = "-h"
path = "/tmp"
print "Space used in /tmp directory"
subprocess.call([tmp_usage, tmp_arg, path])

def main():
disk_func()
tmp_space()

if __name__ == "__main__":
main()

In this example, not only do we reuse the code we wrote earlier, but we use a special Python syntax that allows us to import the exact function we need. What’s fun about reusing code is that it is possible to make a completely different program just by importing the function from our previous program. Notice that in the main method we mix the function from the other module we created, disk_func(), and the new one we just created in this file.

In this section, we learned the power of code reuse and how simple it really is. In a nutshell, you put a function or two in a file and then, if you also want it to run as script, place that special if__name__ == "__main__": syntax. Later you can either import those functions into IPython or simply reuse them in another script. With the information you have just learned, you are truly dangerous. You could write some pretty sophisticated Python modules and reuse them over and over again to create new tools.

Chapter 2. IPython

One of Python’s strengths is its interactive interpreter, or shell. The shell provides a way to quickly test ideas, test features and interfaces of modules you are working with, and perform some one-off tasks for which you would otherwise have written a three line script. The way that we tend to code is by simultaneously running a text editor and a Python prompt (actually, an IPython prompt, but we’ll get to that in a moment), frequently interacting with them both, switching back and forth between shell and editor, and often pasting code from one to the other. This approach allows us to see immediate results from Python about the way it handles code and to quickly get the code in the text editor working the way we want it to.

At its heart, IPython is a bundle of interactive Python goodness. It is an amazing Python shell, far superior to the standard Python shell. It also provides the ability to create highly customized console-based command environments; it allows for easy inclusion of an interactive Python shell into any Python application; and it can even be used as a system shell, with some level of success. This chapter will focus on using IPython to improve your productivity on *nix-shell and Python-related tasks.

IPython also has an active, incredibly helpful community behind it. You can sign up for the mailing list at http://lists.ipython.scipy.org/mailman/listinfo/ipython-user. There is an excellent wiki at http://ipython.scipy.org/moin. And, as part of the wiki, there is a cookbook at http://ipython.scipy.org/moin/Cookbook. So, you can read or contribute to any of these resources as you wish. Another area that you can contribute to is the development of IPython. IPython development recently switched to a distributed source control system, so you can just branch their code and start hacking. And if you do something that could benefit others, you can submit your changes to them.

Celebrity Profile: IPython: Fernando Perez

Fernando Perez received his Ph.D. in physics and then worked on numerical algorithm development at the Applied Mathematics Department of the University of Colorado. He is currently a research scientist at the Helen Wills Neuroscience Institute at the University of California, Berkeley, focusing on the development of new analysis methods for brain imaging problems and high-level scientific computing tools. Toward the end of his graduate studies, he became involved with the development of Python tools for scientific computing. He started the open source IPython project in 2001, when he was looking for a more efficient interactive workflow for everyday scientific tasks. Thanks to the participation of a growing community of collaborators, this project has grown over the years to be useful to other programmers outside of scientific fields.

Celebrity Profile: IPython: Ville Vainio

Ville Vainio received his B.Sc. degree in software engineering, in 2003, from Satakunta University of Applied Sciences, Pori Faculty of Technology in Finland. As of this book’s writing, he is employed as a software specialist in the Smartphone department of Digia Plc, doing C++ development for Symbian OS platforms from Nokia and UIQ. During his studies, he functioned as software engineer at Cimcorp Oy, developing communications software, in Python, for industrial robots. Ville is a long-time IPython enthusiast, and the maintainer of the stable branch (0.x series), since January 2006. His IPython work started with a series of patches to make IPython behave better as a system shell on Windows, and the system shell use case is still a strong personal focus. He lives with his fiancée in Pori, Finland, and is doing his MSc thesis project at Tampere University of Technology’s Pori Unit, on ILeo, the IPython-Leo bridge that makes Leo a full-fledged IPython notebook.

Installing IPython

There are a few options for installing IPython. The first and most rudimentary is source distribution. The download page for IPython is located at http://ipython.scipy.org/dist/. At the time of this writing, the latest IPython release was 0.8.2 and 0.8.3 was nearing completion. To install from source, go to http://ipython.scipy.org/dist/ipython-0.8.2.tar.gz and download the tar.gz file. You can unzip the downloaded file using tar zxvf ipython-0.8.2.tar.gz. The unzipped directory will contain a setup.py file. Call Python on the setup.py with an install paramater (e.g., python setup.py install). This will install the libraries for IPython in your site-packages directory and create an ipython script in your scripts directory. On UNIXes, this is typically the same directory that the python binary is located in. If you are using a python that was installed with your system’s package manager, it (and consequently ipython) will likely be located in /usr/bin. We’ve installed the source distribution of the latest development code for IPython, so you will see 0.8.3 in some examples.

A second option for installing IPython is to use a package from your system’s package management system. On Debian and Ubuntu, .deb packages are available to install. Installation is simply apt-get install ipython. Ubuntu installs the IPython libraries to the appropriate location (/usr/share/python-support/ipython with a tangle of .pth files and symbolic links to make things work right). It also installs the ipython binary to /usr/bin/ipython.

The third option for installing IPython is to use a Python package. You may be unaware that such a thing as a Python package exists, but they do. Python packages are files bundled together in a ZIP format, and the resulting file has a .egg extension. Eggs (as they are often simply called) can be installed with the easy_install utility. One of the cool features of easy_install is that it checks a central repository of eggs and then figures out what to install. There is a bit more to it behind the scenes, but as far as the user is concerned, it is relatively easy.The repository is called the Python Package Index or PyPI for short (though some affectionately call it the Python CheeseShop). To easy_install IPython, simply log on as the user that has permissions to write to your Python’s site-packages directory and easy_install ipython.

The fourth option is to use IPython without installing it at all. “What?” You might well ask. Well, if you download the source distribution and just run ipython.py from the root of that set of files, you will have a running instance of the IPython version that you downloaded. That can be a pretty useful way to go if you don’t want to clutter up your site-packages directory, but you’ll want to be aware of some limitations. If you run IPython from the directory to which you untarred it and you don’t modify your PYTHONPATH environment variable, you won’t be able to use it as a library.

Basic Concepts

After you’ve installed IPython, when you run ipython for the first time, you will see something like this:

jmjones@dink:~$ ipython
**********************************************************************
Welcome to IPython. I will try to create a personal configuration directory
where you can customize many aspects of IPython's functionality in:

/home/jmjones/.ipython

Successful installation!

Please read the sections 'Initial Configuration' and 'Quick Tips' in the
IPython manual (there are both HTML and PDF versions supplied with the
distribution) to make sure that your system environment is properly configured
to take advantage of IPython's features.

Important note: the configuration system has changed! The old system is
still in place, but its setting may be partly overridden by the settings in
"~/.ipython/ipy_user_conf.py" config file. Please take a look at the file
if some of the new settings bother you.

Please press <RETURN> to start IPython.

After you hit the Return key, your cursor will be waiting for you at a prompt, and IPython will display the following text:

jmjones@dinkgutsy:stable-dev$ python ipython.py
Python 2.5.1 (r251:54863, Mar  7 2008, 03:39:23)
Type "copyright", "credits" or "license" for more information.

IPython 0.8.3.bzr.r96 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object'. ?object also works, ?? prints more.

In [1]:

Interacting with IPython

It’s common to feel some helplessness when presented with a new shell prompt for the very first time. It’s not at all obvious exactly what to do. Remember the first time you logged into a Unix box and were presented with a (ba|k|c|z)sh prompt? Since you are reading this book, we assume you have attained some level of familiarity over the Unix shell. If that is the case, then gaining some level of mastery over IPython will be easy.

One of the reasons that it is unclear what you should do when you first see the IPython prompt, is that what you can do is virtually unlimited. So it is more appropriate to think in terms of what you want to do. All of the features of the Python language are available to you from within the IPython prompt. Plus, there are a number of IPython “magic” functions available to you. And you can easily execute any Unix shell command from within IPython and then store the output into a Python variable. The next few examples will demonstrate what you can expect from IPython with its default configuration.

Here are the input and output of a few simple assignment operations:

In [1]: a = 1

In [2]: b = 2

In [3]: c = 3

This doesn’t look much different from what you would see if you entered the same thing at a standard Python prompt. We simply assigned 1, 2, 3 to a, b, c, respectively. The biggest difference you’ll see between the IPython and standard Python shells is that IPython gives you a numbered prompt.

Now that we have values (1, 2, and 3) stored in a few variables (a, b, and c, respectively), we can see the values that those variables contain:

In [4]: print a
1

In [5]: print b
2

In [6]: print c
3

This is a contrived example. We just typed in the values, so we can, at worst, just scroll up to see what the values were. Each variable that we displayed took six characters more than was necessary to display its value. Here’s a slightly less verbose way of showing the value:

In [7]: a
Out[7]: 1

In [8]: b
Out[8]: 2

In [9]: c
Out[9]: 3

While the outlook values look pretty much the same, there is a difference. The print statements use the “unofficial” string representation, while the bare variable names use the “official” string representation. This distinction is typically more important when you are dealing with custom classes than when you are dealing with built-in classes. Here is an example of the different string representations in use:

In [10]: class DoubleRep(object):
....:     def __str__(self):
....:         return "Hi, I'm a __str__"
....:     def __repr__(self):
....:         return "Hi, I'm a __repr__"
....:
....:

In [11]: dr = DoubleRep()

In [12]: print dr
Hi, I'm a __str__

In [13]: dr
Out[13]: Hi, I'm a __repr__

We created the class DoubleRep with two methods, __str__ and __repr__, to demonstrate the difference between printing an object and showing the “official” string representation of it. The special method __str__ on an object will be called when the “unofficial” string representation is needed. The special method __repr__ on an object will be called when the “official” representation is needed. After instantiating our DoubleRep object and assigning the variable dr as its value, we printed out the value of dr. The __str__ method was called. Next, we simply typed in the variable name and the __repr__ method was called. The point of all this is that when we simply type in the name of a variable, the result that IPython displays is the “official” string representation. When we tell IPython to print the variable, we see the “unofficial” string representation. In Python in general, __str__ is what gets called when you call str(obj) on an object or when it is used in a formatted string like this: "%s" % obj. When repr(obj) gets called, or when it is used in a formatting string like this: "%r" % obj, __repr__ is what gets called.

This behavior isn’t particular to IPython, however. This is exactly how the standard Python shell works. Here is the same DoubleRep example using the standard Python shell:

>>> class DoubleRep(object):
...    def __str__(self):
...        return "Hi, I'm a __str__"
...    def __repr__(self):
...        return "Hi, I'm a __repr__"
...
>>>
>>> dr = DoubleRep()
>>> print dr
Hi, I'm a __str__
>>> dr
Hi, I'm a __repr__

You may have noticed that the standard Python prompt and the IPython prompt look different. The standard Python prompt consists of three greater-than signs (>>>), whereas the IPython prompt consists of the word “In,” followed by a number in brackets, followed by a colon (In [1]:). A likely reason for this is that IPython keeps track of the commands that you input and stores them in a list named In. After you’ve assigned 1, 2, 3 to a, b, c, in the previous example, here is what the In list would look like:

In [4]: print In
['\n', u'a = 1\n', u'b = 2\n', u'c = 3\n', u'print In\n']

The output from the IPython prompt is different from the output from the standard Python prompt as well. The IPython prompt seems to distinguish between two types of output: written output and evaluated output. In reality, IPython really doesn’t distinguish between the two types. What happens is that print calls are a side effect of computation; so IPython doesn’t see them, and so it can’t trap them. These print side effects just wind up on stdout, which is where the calling process sent them. However, as IPython executes user’s code, it checks return values. If the return value is not None, it prints it at an Out [number]: prompt.

The standard Python prompt does not even appear to distinguish between these two types of output. If a statement that you typed into an IPython prompt evaluates to some value other than None, IPython will write it to a line that starts with Out, followed by a bracketed number, followed by a colon, and, finally, followed by the value that the statement evaluated to (i.e., Out[1]: 1). Here is an example of what happens when IPython assigns an integer to a variable, evaluates the variable, displays its value, and then prints out that value. Note the differences among the tasks of assigning to the variable, showing what the variable value evaluates to, and printing the value of the variable. First, the IPython prompt:

In [1]: a = 1

In [2]: a
Out[2]: 1

In [3]: print a
1

In [4]:

Next, the standard Python prompt:

>>> a = 1
>>> a
1
>>> print a
1
>>>

There is really no difference between the way IPython and Python assign the integer, IPython prompt, and the standard Python prompt. Both immediately returned an input prompt to the user. But in showing the “official” string representation of a variable, IPython and standard Python are different. IPython shows only an Out prompt, while Python showed the output. For printing, there was no difference; both showed the output with no prompt.

This In [some number]: and Out [some number]: business may have you wondering if there is a deeper difference between IPython and standard Python, or if the difference is purely cosmetic. The difference is definitely deeper. In fact, the difference represents an area of functionality that places IPython into what seems to be a different category of interactive shell from the standard Python shell.

There are two built-in variables that you will want to be aware of. They are In and Out. The former is an IPython input list object and the latter is a dict object. Here is what type says about the ins and outs of In and Out:

In [1]: type(In)
Out[1]: <class 'IPython.iplib.InputList'>

In [2]: type(Out)
Out[2]: <type 'dict'>

After you start using In and Out, this will make sense.

So, what do these datatypes hold?

In [3]: print In
['\n', u'type(In)\n', u'type(Out)\n', u'print In\n']

In [4]: print Out
{1: <class 'IPython.iplib.InputList'>, 2: <type 'dict'>}

As you may expect, In and Out, respectively, hold the input that you typed in and the output that non-None statements and expressions evaluated to. Since each line must necessarily have input, it would seem to make sense to keep track of input in some sort of list-like structure. But keeping track of the output in a list-like structure would result in a number of empty fields or fields containing only None. So, since not every line will have evaluatable non-None output, it makes sense to keep track of output in a dictionary-like data structure or even a pure dict object.

Tab Completion

Another of the incredibly useful data-entry features of IPython is tab completion by default. The standard Python shell has tab-completion capability if it is compiled with readline support, but you have to do something like the following:

>>> import rlcompleter, readline
>>> readline.parse_and_bind('tab: complete')

This will give us functionality as follows:

>>> import os
>>> os.lis<TAB>
>>> os.listdir
>>> os.li<TAB><TAB>
os.linesep  os.link     os.listdir

After importing rlcompleter and readline and setting the readline tab complete option, we were able to import os, type in os.lis, hit the Tab key once, and have it complete to os.listdir. We were also able to enter os.li, hit the Tab key twice, and get a list of possible completions.

We get this same behavior with IPython for without any extra configuration necessary. Well, it’s free with the standard Python shell as well, but with IPython, it’s the default behavior. Here is the previous example run with IPython:

In [1]: import os

In [2]: os.lis<TAB>
In [2]: os.listdir
In [2]: os.li<TAB>
os.linesep  os.link     os.listdir

Notice that we had to hit the Tab key only once on the last part of the example.

The os.TAB example really only shows off the attribute lookup and completion functionality of IPython, but another cool thing that IPython will complete on is module imports. Open a new IPython shell so that you can see IPython help us find a module to import:

In [1]: import o
opcode       operator     optparse     os           os2emxpath   ossaudiodev

In [1]: import xm
xml        xmllib     xmlrpclib

Notice that all of the items that import completed on were modules, so this wasn’t accidental behavior. This is a feature.

IPython exposes two types of completion: “complete” and “menu-complete.” The difference between the two is that “complete” expands the current “word” as far as it can and then provides a list of alternatives, while “menu-complete” expands the word fully to match one of the alternatives, and then each subsequent press of the Tab key morphs the word into the next alternative. IPython’s default completion option is “complete.” We’ll get into configuring your IPython in just a bit.

Magic Edit

The last basic input and output topic we will cover is the “magic” edit function. (We will go over magic functions in the next section.) Strictly line-oriented user interaction with a shell has tremendous, but limited, usefulness. Since that statement sounds like a contradiction, we’ll unpack it. Typing commands into a shell one line at a time is very useful. You type in a command; the shell goes off and does its thing; you sometimes sit and wait for it to return; you type in your next command. This is not a bad cycle. In fact, it’s quite effective. But sometimes it would be nice to work with a block of lines all at the same time. And it would be nice to work with them in your text editor of choice, although readline support in IPython does improve its usefulness, in this respect. We are aware of using a text editor to create Python modules, but that isn’t what we’re talking about here. We’re talking about more of a compromise between line-oriented input and text editor input to feed commands to the shell. If we can say that adding support for working with blocks of lines of commands would be better, then we can say that a strictly line-oriented interface is limited. So, we can say that a strictly line-oriented interface is exceptionally useful but limited at the same time.

The magic edit function acts as the compromise we just mentioned between pure command-line interaction with the Python shell and interaction using a text editor. The benefit of the compromise is that you have the full power of both environments at your fingertips. You have the benefit of the full-featured power of your text editor of choice. You can easily edit blocks of code and change lines of code around within a loop or a method or function. Plus, you have the nimbleness and agility that comes from directly interacting with the shell. When you combine these two approaches to working with code, a synergistic benefit emerges. You are able to maintain the environment you were working in directly from within your shell, and you can pause, edit, and execute code from within an editor. When you resume working within your shell, you will see the changes you just made in your editor.

Configuring IPython

The final “basic” information you need to know in order to begin is how to configure IPython. If you didn’t assign a different location when you ran IPython for the first time, it created an .ipython directory in your home directory. Inside the .ipython directory is a file called ipy_user_conf.py. This user file is simply a configuration file that uses Python syntax. In order to help you give IPython the look and feel that you want it to have, the config file contains a wide variety of elements that you can customize. For example, you can choose the colors used in the shell, the components of the shell prompt, and the text editor that will automatically be used use when you %edit text. We won’t go into any more detail than that here. Just know that the config file exists, and it is worth looking through to see if there are some elements you need to or want to configure.

Help with Magic Functions

As we’ve already said, IPython is incredibly powerful. One reason for this power is that there is an almost overwhelming number of built-in magic functions. Just what is a magic function? The IPython documentation says:

IPython will treat any line whose first character is a % as a special call to a ‘magic’ function. These allow you to control the behavior of IPython itself, plus a lot of system-type features. They are all prefixed with a % character, but parameters are given without parentheses or quotes.

Example: typing ‘%cd mydir’ (without the quotes) changes your working directory to ‘mydir’, if it exists.

Two of the “magic” functions can help you wade through all of this functionality and sort out what might be useful for you. The first magic help function that we’ll look at is lsmagic. lsmagic gives a listing of all the “magic” functions. Here is the output of running lsmagic:

In [1]: lsmagic
Available magic functions:
%Exit  %Pprint  %Quit  %alias  %autocall  %autoindent  %automagic  %bg
%bookmark  %cd  %clear  %color_info  %colors  %cpaste  %debug  %dhist  %dirs
%doctest_mode  %ed  %edit  %env  %exit  %hist  %history  %logoff  %logon
%logstart  %logstate  %logstop  %lsmagic  %macro  %magic  %p  %page  %pdb
%pdef  %pdoc  %pfile  %pinfo  %popd  %profile  %prun  %psearch  %psource
%pushd  %pwd  %pycat  %quickref  %quit  %r  %rehash  %rehashx  %rep  %reset
%run  %runlog  %save  %sc  %store  %sx  %system_verbose  %time  %timeit
%unalias  %upgrade  %who  %who_ls  %whos  %xmode

Automagic is ON, % prefix NOT needed for magic functions.

As you can see, there is an almost unwieldy number of functions for you to work with. In fact, as of this writing, there are 69 magic functions for you to use. You may find it helpful to list the magic functions like this:

In [2]: %<TAB>
%Exit            %debug           %logstop         %psearch         %save
%Pprint          %dhist           %lsmagic         %psource         %sc
%Quit            %dirs            %macro           %pushd           %store
%alias           %doctest_mode    %magic           %pwd             %sx
%autocall        %ed              %p               %pycat           %system_verbose
%autoindent      %edit            %page            %quickref        %time
%automagic       %env             %pdb             %quit            %timeit
%bg              %exit            %pdef            %r               %unalias
%bookmark        %hist            %pdoc            %rehash          %upgrade
%cd              %history         %pfile           %rehashx         %who
%clear           %logoff          %pinfo           %rep             %who_ls
%color_info      %logon           %popd            %reset           %whos
%colors          %logstart        %profile         %run             %xmode
%cpaste          %logstate        %prun            %runlog

Typing %-TAB will give you a nicer view of all 69 magic functions. The point of using the lsmagic function and %-TAB is to see a quick rundown of all the available functions when you’re looking for something specific. Or, you can use them to quickly browse through all the functions to see what is available. But unless you see a description, the list isn’t going to help you understand what each function does.

That is where magic, the next help function comes in. The name of this magic function is itself “magic.” Running magic brings up a pageable help document that the program uses for all of the built-in magic functions in IPython. The help format includes the function name, the use of the function (where applicable), and a description of the way the function works. Here is the help on the magic page function:

%page:
Pretty print the object and display it through a pager.

%page [options] OBJECT

If no object is given, use _ (last output).

Options:

-r: page str(object), don't pretty-print it.

Depending on your pager, you can search and scroll after executing the magic function. This can come in handy if you know what function you need to look up and want to jump right to it rather than scrolling around hunting for it. The functions are arranged alphabetically, so that will help you find what you’re looking for whether you search or scroll.

You can also use another help method that we will get to later in this chapter. When you type in the name of the magic function for which you want help, followed by a question mark (?), it will give you almost the same information that %magic will give you. Here is the output of %page ?:

In [1]: %page ?
Type:           Magic function
Base Class:     <type 'instancemethod'>
String Form:    <bound method InteractiveShell.magic_page of
<IPython.iplib.InteractiveShell object at 0x2ac5429b8a10>>
Namespace:      IPython internal
File:           /home/jmjones/local/python/psa/lib/python2.5/site-packages/IPython/Magic.py
Definition:     %page(self, parameter_s='')
Docstring:
Pretty print the object and display it through a pager.

%page [options] OBJECT

If no object is given, use _ (last output).

Options:

-r: page str(object), don't pretty-print it.

And here is one final piece of IPython help that is great for generating a summary of the way things work, as well as a summary of the magic functions themselves. When you type in %quickref at an IPython prompt, you’ll see a paged reference that begins this way:

IPython -- An enhanced Interactive Python - Quick Reference Card
================================================================

obj?, obj??      : Get help, or more help for object (also works as
?obj, ??obj).
?foo.*abc*       : List names in 'foo' containing 'abc' in them.
%magic           : Information about IPython's 'magic' % functions.

Magic functions are prefixed by %, and typically take their arguments without
parentheses, quotes or even commas for convenience.

Example magic function calls:

%alias d ls -F   : 'd' is now an alias for 'ls -F'
alias d ls -F    : Works if 'alias' not a python name
alist = %alias   : Get list of aliases to 'alist'
cd /usr/share    : Obvious. cd -<tab> to choose from visited dirs.
%cd??            : See help AND source for magic %cd

System commands:

!cp a.txt b/     : System command escape, calls os.system()
cp a.txt b/      : after %rehashx, most system commands work without !
cp ${f}.txt $bar : Variable expansion in magics and system commands
files = !ls /usr : Capture sytem command output
files.s, files.l, files.n: "a b c", ['a','b','c'], 'a\nb\nc'

and ends with this:

%time:
Time execution of a Python statement or expression.
%timeit:
Time execution of a Python statement or expression
%unalias:
Remove an alias
%upgrade:
Upgrade your IPython installation
%who:
Print all interactive variables, with some minimal formatting.
%who_ls:
Return a sorted list of all interactive variables.
%whos:
Like %who, but gives some extra information about each variable.
%xmode:
Switch modes for the exception handlers.

The starting portion of %quickref is a reference to various usage scenarios for IPython. The rest of %quickref is a minisummary of each of the %magic functions. The mini-summaries in %quickref each contain the first line of the full help on each of the %magic functions found elsewhere. For example, here is the full help description of %who:

In [1]: %who ?
Type:           Magic function
Base Class:     <type 'instancemethod'>
String Form:    <bound method InteractiveShell.magic_who of
<IPython.iplib.InteractiveShell object at 0x2ac9f449da10>>
Namespace:      IPython internal
File:           /home/jmjones/local/python/psa/lib/python2.5/site-packages/IPython/
Magic.py
Definition:     who(self, parameter_s='')
Docstring:
Print all interactive variables, with some minimal formatting.

If any arguments are given, only variables whose type matches one of
these are printed. For example:

%who function str

will only list functions and strings, excluding all other types of
variables. To find the proper type names, simply use type(var) at a
command line to see how python prints type names. For example:

In [1]: type('hello')
Out[1]: <type 'str'>

indicates that the type name for strings is 'str'.

%who always excludes executed names loaded through your configuration
file and things which are internal to IPython.

This is deliberate, as typically you may load many modules and the
purpose of %who is to show you only what you've manually defined.

The help line for %who in the %quickref is identical to the first line of the Docstring that is returned by %who ?.

Unix Shell

Working in a Unix shell certainly has its benefits (a unified approach to working through problems, a rich set of tools, a fairly terse yet simple syntax, standard I/O streams, pipes, and redirection to name a few), but it’s nice for us to be able to add a touch of Python to this old friend. IPython has some features that make bridging the two very valuable.

alias

The first feature of a Python/Unix shell bridge that we will look at is the alias magic function. With alias you can create an IPython shortcut to execute system commands. To define an alias, simply type alias followed by the system command (and any arguments for that command). For example:

In [1]: alias nss netstat -lptn

In [2]: nss
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN

There are a few ways to get different input into an alias. One option is the do-nothing approach. If all the extras you wanted to pass into your command can be lumped together, the do-nothing approach may be for you. For example, if you wanted to grep the results of the netstat command above for 80, you could do this:

In [3]: nss | grep 80
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN     -

This isn’t passing in extra options, but for the sake of how things are happening, it winds up being the same thing.

Next, there is the do-everything approach. It’s pretty similar to the do-nothing approach except that, by implicitly handling all arguments, you’re explicitly handling all subsequent arguments. Here is an example that shows how to treat the subsequent arguments as a single group:

In [1]: alias achoo echo "|%l|"

In [2]: achoo
||

In [3]: achoo these are args
|these are args|

This demonstrates the %l (percent sign followed by the letter “l”) syntax that is used to insert the rest of the line into an alias. In real life, you would be most likely to use this to insert everything after the alias somewhere in the middle of the implemented command that the alias is standing in for.

And here is the do-nothing example retooled to handle all arguments explicitly:

In [1]: alias nss netstat -lptn %l

In [2]: nss
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN

In [3]: nss | grep 80
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN

In this example, we really didn’t need to put the %l in there at all. If we had just left it out, we would have gotten up with the same result.

To insert different parameters throughout a command string, we would use the %s substitution string. This example shows how to run the parameters:

In [1]: alias achoo echo first: "|%s|", second: "|%s|"

In [2]: achoo foo bar
first: |foo|, second: |bar|

This can be a bit problematic, however. If you supply only one parameter and two were expected, you can expect an error:

In [3]: achoo foo
ERROR: Alias <achoo> requires 2 arguments, 1 given.
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)

On the other hand, providing more parameters than expected is safe:

In [4]: achoo foo bar bam
first: |foo|, second: |bar| bam

foo and bar are properly inserted into their respective positions, while bam is appended to the end, which is where you would expect it to be placed.

You can also persist your aliases with the %store magic function, and we will cover how to do that later in this chapter. Continuing with the previous example, we can persist the achoo alias so that the next time we open IPython, we’ll be able to use it:

In [5]: store achoo
Alias stored: achoo (2, 'echo first: "|%s|", second: "|%s|"')

In [6]:
Do you really want to exit ([y]/n)?
(psa)jmjones@dinkgutsy:code$ ipython -nobanner

In [1]: achoo one two
first: |one|, second: |two|

Shell Execute

Another, and possibly easier, way of executing a shell command is to place an exclamation point (!) in front of it:

In [1]: !netstat -lptn
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN

You can pass in variables to your shell commands by prefixing them with a dollar sign ($). For example:

In [1]: user = 'jmjones'

In [2]: process = 'bash'

In [3]: !ps aux | grep $user | grep $process
jmjones   5967  0.0  0.4  21368  4344 pts/0    Ss+  Apr11   0:01 bash
jmjones   6008  0.0  0.4  21340  4304 pts/1    Ss   Apr11   0:02 bash
jmjones   8298  0.0  0.4  21296  4280 pts/2    Ss+  Apr11   0:04 bash
jmjones  10184  0.0  0.5  22644  5608 pts/3    Ss+  Apr11   0:01 bash
jmjones  12035  0.0  0.4  21260  4168 pts/15   Ss   Apr15   0:00 bash
jmjones  12943  0.0  0.4  21288  4268 pts/5    Ss   Apr11   0:01 bash
jmjones  15720  0.0  0.4  21360  4268 pts/17   Ss   02:37   0:00 bash
jmjones  18589  0.1  0.4  21356  4260 pts/4    Ss+  07:04   0:00 bash
jmjones  18661  0.0  0.0    320    16 pts/15   R+   07:06   0:00 grep bash
jmjones  27705  0.0  0.4  21384  4312 pts/7    Ss+  Apr12   0:01 bash
jmjones  32010  0.0  0.4  21252  4172 pts/6    Ss+  Apr12   0:00 bash

This listed all bash sessions belonging to jmjones.

Here’s an example of the way to store the result of a ! command:

In [4]: l = !ps aux | grep $user | grep $process

In [5]: l

Out[5]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: jmjones   5967  0.0  0.4  21368  4344 pts/0    Ss+  Apr11   0:01 bash
1: jmjones   6008  0.0  0.4  21340  4304 pts/1    Ss   Apr11   0:02 bash
2: jmjones   8298  0.0  0.4  21296  4280 pts/2    Ss+  Apr11   0:04 bash
3: jmjones  10184  0.0  0.5  22644  5608 pts/3    Ss+  Apr11   0:01 bash
4: jmjones  12035  0.0  0.4  21260  4168 pts/15   Ss   Apr15   0:00 bash
5: jmjones  12943  0.0  0.4  21288  4268 pts/5    Ss   Apr11   0:01 bash
6: jmjones  15720  0.0  0.4  21360  4268 pts/17   Ss   02:37   0:00 bash
7: jmjones  18589  0.0  0.4  21356  4260 pts/4    Ss+  07:04   0:00 bash
8: jmjones  27705  0.0  0.4  21384  4312 pts/7    Ss+  Apr12   0:01 bash
9: jmjones  32010  0.0  0.4  21252  4172 pts/6    Ss+  Apr12   0:00 bash

You may notice that the output stored in the variable l is different from the output in the previous example. That’s because the variable l contains a list-like object, while the previous example showed the raw output from the command. We’ll discuss that list-like object later in String Processing.”

An alternative to ! is !!, except that you can’t store the result in a variable as you are running it, !! does the same thing that ! does. But you can access it with the _ or _[0-9]* notation that we’ll discuss later in History results.”

Programming a quick ! or !! before a shell command is definitely less work than creating an alias, but you may be better off creating aliases in some cases and using the ! or !! in others. For example, if you are typing in a command you expect to execute all the time, create an alias or macro. If this is a one time or infrequent occurrence, then just use ! or !!.

rehash

There is another option for aliasing and/or executing shell commands from IPython: rehashing. Technically, this is creating an alias for shell commands, but it doesn’t really feel like that is what you’re doing. The rehash “magic” function updates the “alias table” with everything that is on your PATH. You may be asking, “What is the alias table?” When you create an alias, IPython has to map the alias name to the shell command with which you wanted it to be associated. The alias table is where that mapping occurs.

Note

The preferred way of rehashing the alias table is to use the rehashx magic function rather than rehash. We will present both to demonstrate the ways they work, and then we will describe their differences.

IPython exposes a number of variables that you have access to when running IPython, such as In and Out, which we saw earlier. One of the variables that IPython exposes is __IP, which is actually the interactive shell object. An attribute named alias_table hangs on that object. This is where the mapping of alias names to shell commands takes place. We can look at this mapping in the same way we would look at any other variable:

In [1]: __IP.alias_table

Out[1]:
{'cat': (0, 'cat'),
'clear': (0, 'clear'),
'cp': (0, 'cp -i'),
'lc': (0, 'ls -F -o --color'),
'ldir': (0, 'ls -F -o --color %l | grep /$'),
'less': (0, 'less'),
'lf': (0, 'ls -F -o --color %l | grep ^-'),
'lk': (0, 'ls -F -o --color %l | grep ^l'),
'll': (0, 'ls -lF'),
'lrt': (0, 'ls -lart'),
'ls': (0, 'ls -F'),
'lx': (0, 'ls -F -o --color %l | grep ^-..x'),
'mkdir': (0, 'mkdir'),
'mv': (0, 'mv -i'),
'rm': (0, 'rm -i'),
'rmdir': (0, 'rmdir')}

It looks like a dictionary:

In [2]: type(__IP.alias_table)

Out[2]: <type 'dict'>

Looks can be deceiving, but they’re not this time.

Right now, this dictionary has 16 entries:

In [3]: len(__IP.alias_table)

Out[3]: 16

After we rehash, this mapping gets much larger:

In [4]: rehash

In [5]: len(__IP.alias_table)

Out[5]: 2314

Let’s look for something that wasn’t there before, but should be there now—the transcode utility should be in the alias table now:

In [6]: __IP.alias_table['transcode']

Out[6]: (0, 'transcode')

Note

When you see a variable or attribute name that begins with a double underscore (__), it usually means that the author of that code doesn’t want you to change. We’re accessing __IP here, but it’s only to show you the internals structure. If we wanted to access the official API for IPython, we would use the _ip object that is accessible at the IPython prompt.

rehashx

Excepting that rehashx looks for things on your PATH that it thinks are executable to add to the alias table, rehashx is similar to rehash. So, when we start a new IPython shell and rehashx, we would expect the alias table to be the same size as or smaller than the result of rehash:

In [1]: rehashx

In [2]: len(__IP.alias_table)

Out[2]: 2307

Interesting; rehashx produces an alias table with seven fewer items than rehash. Here are the seven differences:

In [3]: from sets import Set

In [4]: rehashx_set = Set(__IP.alias_table.keys())

In [5]: rehash

In [6]: rehash_set = Set(__IP.alias_table.keys())

In [7]: rehash_set - rehashx_set

Out[7]: Set(['fusermount', 'rmmod.modutils', 'modprobe.modutils', 'kallsyms', 'ksyms', /
'lsmod.modutils', 'X11'])

And if we look to see why rmmod.modutils didn’t show up when we ran rehashx but did show up when we ran for rehash, here is what we find:

jmjones@dinkgutsy:Music$ slocate rmmod.modutils
/sbin/rmmod.modutils
jmjones@dinkgutsy:Music$ ls -l /sbin/rmmod.modutils
lrwxrwxrwx 1 root root 15 2007-12-07 10:34 /sbin/rmmod.modutils -> insmod.modutils
jmjones@dinkgutsy:Music$ ls -l /sbin/insmod.modutils
ls: /sbin/insmod.modutils: No such file or directory

So, you can see that rmmod.modutils is a link to insmod.modutils, and insmod.modutils doesn’t exist.

cd

If you have the standard Python shell, you may have noticed that it can be hard to determine which directory you’re in. You can use os.chdir() to change the directory, but that isn’t very convenient. You could also get the current directory via os.getcwd(), but that’s not terribly convenient either. Since you are executing Python commands rather than shell commands with the standard Python shell, maybe it isn’t that big of a problem, but when you are using IPython and have easy access to the system shell, having comparably easy access to directory navigation is critical.

Enter cd magic. It seems like we’re making a bigger deal out of this than it warrants: this isn’t a revolutionary concept; it’s not all that difficult. But just imagine if it were missing. That would be painful.

In IPython, cd works mostly as it does in Bash. The primary usage is cd directory_name. That works as you would expect it to from your Bash experience. With no arguments, cd takes you to your home directory. With a space and hyphen as an argument, cd - takes you to your previous directory. There are three additional options that Bash cd doesn’t give you.

The first is the -q, or quiet, option. Without this option, IPython will output the directory into which you just changed. The following example shows the ways to change a directory both with and without the -q option:

In [1]: cd /tmp
/tmp

In [2]: pwd

Out[2]: '/tmp'

In [3]: cd -
/home/jmjones

In [4]: cd -q /tmp

In [5]: pwd

Out[5]: '/tmp'

Using the -q prevented IPython from outputting the /tmp directory we had gone into.

Another feature that IPython’s cd includes is the ability to go to defined bookmarks. (We’ll explain how to create bookmarks soon.) Here is an example of how to change a directory for which you have created a bookmark:

In [1]: cd -b t
(bookmark:t) -> /tmp
/tmp

This example assumes that we have bookmarked /tmp the name t. The formal syntax to change to a bookmarked directory is cd -b bookmark_name, but, if a bookmark of bookmark_name is defined and there is not a directory called bookmark_name in the current directory, the -b flag is optional; IPython can figure out that you are intending to go into a bookmarked directory.

The final extra feature that cd offers in IPython is the ability to change into a specific directory given a history of directories that have been visited. The following is an example that makes use of this directory history:

0: /home/jmjones
1: /home/jmjones/local/Videos
2: /home/jmjones/local/Music
3: /home/jmjones/local/downloads
4: /home/jmjones/local/Pictures
5: /home/jmjones/local/Projects
6: /home/jmjones/local/tmp
7: /tmp
8: /home/jmjones

In [2]: cd -6
/home/jmjones/local/tmp

First, you see there is a list of all the directories in our directory history. We’ll get to where it came from in a moment. Next, we pass the numerical argument –6. This tells IPython that we want to go to the item in our history marked “6”, or /home/jmjones/local/tmp. Finally, you can see that these are now in /home/jmjones/local/tmp.

bookmark

We just showed you how to use a cd option to move into a bookmarked directory. Now we’ll show you how to create and manage your bookmarks. It deserves mentioning that bookmarks persist across IPython sessions. If you exit IPython and start it back up, your bookmarks will still be there. There are two ways to create bookmarks. Here is the first way:

In [1]: cd /tmp
/tmp

In [2]: bookmark t

By typing in bookmark t while we’re in /tmp, a bookmark named t is created and pointing at /tmp. The next way to create a bookmark requires typing one more word:

In [3]: bookmark muzak /home/jmjones/local/Music

Here, we created a bookmark named muzak that points to a local music directory. The first argument is the bookmark’s name, while the second is the directory the bookmark points to.

The -l option tells IPython to get the list of bookmarks, of which we have only two. Now, let’s see a list of all our bookmarks:

In [4]: bookmark -l
Current bookmarks:
muzak -> /home/jmjones/local/Music
t     -> /tmp

There are two options for removing bookmarks: remove them all, or remove one at a time. In this example, we’ll create a new bookmark, remove it, and then remove all in the following example:

In [5]: bookmark ulb /usr/local/bin

In [6]: bookmark -l
Current bookmarks:
muzak -> /home/jmjones/local/Music
t     -> /tmp
ulb   -> /usr/local/bin

In [7]: bookmark -d ulb

In [8]: bookmark -l
Current bookmarks:
muzak -> /home/jmjones/local/Music
t     -> /tmp

An alternative to using bookmark -l is to use cd -b:

In [9]: cd -b<TAB>
muzak  t      txt

And after a few backspaces, we’ll continue where we left off:

In [9]: bookmark -r

In [10]: bookmark -l
Current bookmarks:

We created a bookmark named ulb pointing to /usr/local/bin. Then, we deleted it with the -d bookmark_name option for bookmark. Finally, we deleted all bookmarks with the -r option.

dhist

In the cd example above, we show a list of the directories we had visited. Now we’ll show you how to view that list. The magic command is dhist, which not only saves the session list, but also saves the list of directories across IPython sessions. Here is what happens when you run dhist with no arguments:

In [1]: dhist
Directory history (kept in _dh)
0: /home/jmjones
1: /home/jmjones/local/Videos
2: /home/jmjones/local/Music
3: /home/jmjones/local/downloads
4: /home/jmjones/local/Pictures
5: /home/jmjones/local/Projects
6: /home/jmjones/local/tmp
7: /tmp
8: /home/jmjones
9: /home/jmjones/local/tmp
10: /tmp

A quick way to access directory history is to use cd -<TAB> like this:

In [1]: cd -
-00 [/home/jmjones]                  -06 [/home/jmjones/local/tmp]
-01 [/home/jmjones/local/Videos]     -07 [/tmp]
-02 [/home/jmjones/local/Music]      -08 [/home/jmjones]
-03 [/home/jmjones/local/downloads]  -09 [/home/jmjones/local/tmp]
-04 [/home/jmjones/local/Pictures]   -10 [/tmp]
-05 [/home/jmjones/local/Projects]

There are two options that make dhist more flexible than cd -<TAB>. The first is that you can provide a number to specify how many directories should be displayed. To specify that we want to see only the last five directories that were visited, we would input the following:

In [2]: dhist 5
Directory history (kept in _dh)
6: /home/jmjones/local/tmp
7: /tmp
8: /home/jmjones
9: /home/jmjones/local/tmp
10: /tmp

The second option is that you can specify a range of directories that were visited. For example, to view from the third through the sixth directories visited, we would enter the following:

In [3]: dhist 3 7
Directory history (kept in _dh)
3: /home/jmjones/local/downloads
4: /home/jmjones/local/Pictures
5: /home/jmjones/local/Projects
6: /home/jmjones/local/tmp

Notice that the ending range entry is noninclusive, so you have to indicate the directory immediately following the final directory you want to see.

pwd

A simple but nearly necessary function for directory navigation, pwd simply tells you what your current directory is. Here is an example:

In [1]: cd /tmp
/tmp

In [2]: pwd

Out[2]: '/tmp'

Variable Expansion

The previous eight or so IPython features are definitely helpful and necessary, but the next three features will give great joy to power users. The first of these is variable expansion. Up to this point, we’ve mostly kept shell stuff with shell stuff and Python stuff with Python stuff. But now, we’re going to cross the line and mingle the two of them. That is, we’re going to take a value that we get from Python and hand it to the shell:

In [1]: for i in range(10):
...:     !date > ${i}.txt
...:
...:

In [2]: ls
0.txt  1.txt  2.txt  3.txt  4.txt  5.txt  6.txt  7.txt  8.txt  9.txt

In [3]: !cat 0.txt
Sat Mar  8 07:40:05 EST 2008

This example isn’t all that realistic. It is unlikely that you will want to create 10 text files that all contain the date. But the example does show how to mingle Python code and shell code. We iterated over a list created by the range() function and stored the current item in the list in variable i. For each time through the iteration, we use the shell execution ! notation to call out to the date command-line system utility. Notice that the syntax we use for calling date is identical to the way we would call it if we had defined a shell variable i. So, the date utility is called, and the output is redirected to the file {current list item}.txt. We list the files after creating them and even cat one out to see that it contains something that looks like a date.

You can pass any kind of value you can come up with in Python into your system shell. Whether it is in a database or in a pickle file, generated by computation, an XMLRPC service, or data you extract from a text file, you can pull it in with Python and then pass it to the system shell with the ! execution trick.

String Processing

Another incredibly powerful feature that IPython offers is the ability to string process the system shell command output. Suppose we want to see the PIDs of all the processes belonging to the user jmjones. We could do that by inputting the following:

ps aux | awk '{if ($1 == "jmjones") print $2}'

This is pretty tight, succinct, and legible. But let’s tackle the same task using IPython. First, let’s grab the output from an unfiltered ps aux:

In [1]: ps = !ps aux

In [2]:

The result of calling ps aux, which is stored in the variable ps, is a list-like structure whose elements are the lines that were returned from the shell system call. It is list-like, in this case, because we mean that it inherits from the built-in list type, so it supports all the methods of that type. So, if you have a function or method that expects a list, you can pass one of these result objects to it as well. In addition to supporting the standard list methods, it also supports a couple of very interesting methods and one attribute that will come in handy. Just to show what the “interesting methods” do, we’ll divert from our end goal of finding all processes owned by jmjones for just a moment. The first “interesting method” we’ll look at is the grep() method. This is basically a simple filter that determines which lines of the output to keep and which to leave off. To see if any of the lines in the output match lighttpd, we would input the following:

In [2]: ps.grep('lighttpd')

Out[2]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: www-data  4905  0.0  0.1........0:00 /usr/sbin/lighttpd -f /etc/lighttpd/l

We called the grep() method and passed it the regular expression 'lighttpd'. Remember, regular expressions passed to grep() are case-insensitive. The result of this grep() call was a line of output that showed that there was a positive match for the 'lighttpd' regular expression. To see all records except those that match a certain regular expression, we would do something more like this:

In [3]: ps.grep('Mar07', prune=True)

Out[3]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
1: jmjones  19301  0.0  0.4  21364  4272 pts/2    Ss+  03:58   0:00 bash
2: jmjones  21340  0.0  0.9 202484 10184 pts/3    Sl+  07:00   0:06 vim ipytho
3: jmjones  23024  0.0  1.1  81480 11600 pts/4    S+   08:58   0:00 /home/jmjo
4: jmjones  23025  0.0  0.0      0     0 pts/4    Z+   08:59   0:00 [sh] <defu
5: jmjones  23373  5.4  1.0  81160 11196 pts/0    R+   09:20   0:00 /home/jmjo
6: jmjones  23374  0.0  0.0   3908   532 pts/0    R+   09:20   0:00 /bin/sh -c
7: jmjones  23375  0.0  0.1  15024  1056 pts/0    R+   09:20   0:00 ps aux

We passed in the regular expression 'Mar07' to the grep() method and found that most of the processes on this system were started on March 7, so we decided that we wanted to see all processes not started on March 7. In order to exclude all 'Mar07' entries, we had to pass in another argument to grep(), this time a keyword argument: prune=True. This keyword argument tells IPython, “Any records you find that match the stated regular expression—throw them out.” And as you can see, there are no records that match the 'Mar07' regex.

Callbacks can also be used with grep(). This just means that grep() will take a function as an argument and call that function. It will pass the function to the item in the list that it is working on. If the function returns True on that item, the item is included in the filter set. For example, we could do a directory listing and filter out only files or only directories:

In [1]: import os

In [2]: file_list = !ls

In [3]: file_list

Out[3]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: ch01.xml
1: code
2: ipython.pdf
3: ipython.xml

This directory listing shows four “files.” We can’t tell from this list which are files and which are directories, but if we filter using the os.path.isfile() test, we can see which ones are files:

In [4]: file_list.grep(os.path.isfile)

Out[4]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: ch01.xml
1: ipython.pdf
2: ipython.xml

This left out the “file” named code, so code must not be a file at all. Let’s filter for directories:

In [5]: file_list.grep(os.path.isdir)

Out[5]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: code

Now that we see that code is, in fact, a directory, another interesting method is fields(). After (or, we guess, even before) you filter your result set down to the desired level of specificity, you can display exactly the fields that you want to display. Let’s take the non-Mar07 example that we just walked through and output the user, pid, and start columns:

In [4]: ps.grep('Mar07', prune=True).fields(0, 1, 8)

Out[4]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: USER PID START
1: jmjones 19301 03:58
2: jmjones 21340 07:00
3: jmjones 23024 08:58
4: jmjones 23025 08:59
5: jmjones 23373 09:20
6: jmjones 23374 09:20
7: jmjones 23375 09:20

First, notice that whatever it is that fields() does, we’re doing it to the result of the grep() method call. We are able to do this because grep() returns an object of the same type as the ps object that we started with. And fields() itself returns the same object type as grep(). Since that is the case, you can chain grep() and fields() calls together. Now, on to what is going on here. The fields() method takes an indefinite number of arguments, and these arguments are expected to be the “columns” from the output, if the output lines were split on whitespace. You can think of this very much like the default splitting that awk does to lines of text. In this case, we called fields() to view columns 0, 1, and 8. These are, respectively, USERNAME, PID, and STARTTIME.

Now, back to showing the PIDs of all processes belonging to jmjones:

In [5]: ps.fields(0, 1).grep('jmjones').fields(1)

Out[5]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: 5385
1: 5388
2: 5423
3: 5425
4: 5429
5: 5431
6: 5437
7: 5440
8: 5444
<continues on...>

This example first trims the result set back to only two columns, 0 and 1, which are the username and PID, respectively. Then, we take that narrower result set and grep() for 'jmjones'. Finally, we take that filtered result set and request the second field by calling fields(1). (Remember, lists start at zero.)

The final piece of string processing that we want to showcase is the s attribute of the object trying to directly access your process list. This object is probably not going to give you the results you were looking for. In order to get the system shell to work with your output, use the s attribute on your process list object:

In [6]: ps.fields(0, 1).grep('jmjones').fields(1).s

Out[6]: '5385 5388 5423 5425 5429 5431 5437 5440 5444 5452 5454 5457 5458 5468
5470 5478 5480 5483 5489 5562 5568 5593 5595 5597 5598 5618 5621 5623 5628 5632
5640 5740 5742 5808 5838 12707 12913 14391 14785 19301 21340 23024 23025 23373
23374 23375'

The s attribute gives us a nice space-separated string of PIDs that we can work with in a system shell. We wanted to, we could store that stringified list in a variable called pids and do something like kill $pids from within IPython. But that would send a SIGTERM to every process owned by jmjones, and it would kill his text editor and his IPython sessions.

Earlier, we demonstrated that we could accomplish the stated goals for our IPython script with the following awk one-liner:

ps aux | awk '{if ($1 == "jmjones") print $2}'

We will be ready to accomplish this goal after we’ve introduced one more concept. The grep() method takes a final optional parameter called field. If we specify a field parameter, the search criteria has to match that field in order for that item to be included in the result set:

In [1]: ps = !ps aux

In [2]: ps.grep('jmjones', field=0)

Out[2]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: jmjones   5361  0.0  0.1  46412  1828 ?        SL   Apr11
0:00 /usr/bin/gnome-keyring-daemon -d
1: jmjones   5364  0.0  1.4 214948 14552 ?        Ssl  Apr11
0:03 x-session-manager
....
53: jmjones  32425  0.0  0.0   3908   584 ?        S    Apr15
0:00 /bin/sh /usr/lib/firefox/run-mozilla.
54: jmjones  32429  0.1  8.6 603780 88656 ?        Sl   Apr15
2:38 /usr/lib/firefox/firefox-bin

This matched the exact rows that we wanted, but printed out the whole row. To get at just the PID, we’ll have to do something like this:

In [3]: ps.grep('jmjones', field=0).fields(1)

Out[3]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: 5361
1: 5364
....
53: 32425
54: 32429

And with that, we are able to meet the goal of performing that specific awk filter.

sh Profile

One IPython concept that we haven’t described yet is a profile. A profile is simply a set of configuration data that is loaded when you start IPython. You can customize a number of profiles to make IPython perform in different ways depending on a session’s needs. To invoke a specific profile, use the -p command-line option and specify the profile you’d like to use.

The sh, or shell, profile is one of the built-in profiles for IPython. The sh profile sets some configuration items so that IPython becomes a more friendly system shell. Two examples of configuration values that are different from the standard IPython profile are that sh displays the current directory and it rehashes your PATH so that you have instant access to all of the same executables that you would have in, say, Bash.

In addition to setting certain configuration values, the sh profile also enables a few shell-helpful extensions. For example, it enables the envpersist extension. The envpersist extension allows you to modify various environment variables easily and persistently for your IPython sh profile, and you don’t have to update a .bash_profile or .bashrc.

Here, is what our PATH looks like:

jmjones@dinkgutsy:tmp$ ipython -p sh
IPython 0.8.3.bzr.r96   [on Py 2.5.1]
[~/tmp]|2> import os
[~/tmp]|3> os.environ['PATH']
<3> '/home/jmjones/local/python/psa/bin:
/home/jmjones/apps/lb/bin:/home/jmjones/bin:
/usr/local/sbin:/usr/local/bin:/usr/sbin:
/usr/bin:/sbin:/bin:/usr/games'

Now we add :/appended to the end of our current PATH:

[~/tmp]|4> env PATH+=:/appended
PATH after append = /home/jmjones/local/python/psa/bin:
/home/jmjones/apps/lb/bin:/home/jmjones/bin:
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:
/sbin:/bin:/usr/games:/appended

and /prepended: to the beginning of our current PATH:

[~/tmp]|5> env PATH-=/prepended:
PATH after prepend = /prepended:/home/jmjones/local/python/psa/bin:
/home/jmjones/apps/lb/bin:/home/jmjones/bin:/usr/local/sbin:
/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/appended

This shows the PATH environment variable using os.environ:

[~/tmp]|6> os.environ['PATH']
<6> '/prepended:/home/jmjones/local/python/psa/bin:
/home/jmjones/apps/lb/bin:/home/jmjones/bin:
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:
/bin:/usr/games:/appended'

Now we’ll exit our IPython shell:

[~/tmp]|7>
Do you really want to exit ([y]/n)?
jmjones@dinkgutsy:tmp$

Finally, we’ll open a new IPython shell to see what the PATH environment variable shows:

jmjones@dinkgutsy:tmp$ ipython -p sh
IPython 0.8.3.bzr.r96   [on Py 2.5.1]
[~/tmp]|2> import os
[~/tmp]|3> os.environ['PATH']
<3> '/prepended:/home/jmjones/local/python/psa/bin:
/home/jmjones/apps/lb/bin:/home/jmjones/bin:/usr/local/sbin:
/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/appended'

Interestingly, it shows our prepended and appended values, even though we didn’t update any profile scripts. It persisted the change to PATH without any additional work on our part. Now let’s display all persistent changes to environment variables:

[~/tmp]|4> env -p
<4> {'add': [('PATH', ':/appended')], 'pre': [('PATH', '/prepended:')], 'set': {}}

We can delete any persistent changes to PATH:

[~/tmp]|5> env -d PATH
Forgot 'PATH' (for next session)

and we can check to see the value of PATH:

[~/tmp]|6> os.environ['PATH']
<6> '/prepended:/home/jmjones/local/python/psa/bin:/home/jmjones/apps/lb/bin:
/home/jmjones/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:
/sbin:/bin:/usr/games:/appended'

Even after we’ve told IPython to remove the persistent entries for PATH, they are still there. But that makes sense. That just means that IPython should remove the directive to persist those entries. Note that the process started with certain values in an environment variable will retain those values unless something changes them. The next time the IPython shell starts, things should be different:

[~/tmp]|7>
Do you really want to exit ([y]/n)?
jmjones@dinkgutsy:tmp$ ipython -p sh
IPython 0.8.3.bzr.r96   [on Py 2.5.1]
[~/tmp]|2> import os
[~/tmp]|3> os.environ['PATH']
<3> '/home/jmjones/local/python/psa/bin:/home/jmjones/apps/lb/bin:
/home/jmjones/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:
/sbin:/bin:/usr/games'

And, just as we would expect, this is back to what it was before we started making changes to our PATH.

One other useful feature in the sh profile is mglob. mglob has a simpler syntax for a lot of common uses. For example, to find all of the .py files in the Django project, we could just do this:

[django/trunk]|3> mglob rec:*py
<3> SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: ./setup.py
1: ./examples/urls.py
2: ./examples/manage.py
3: ./examples/settings.py
4: ./examples/views.py
...
1103: ./django/conf/project_template/urls.py
1104: ./django/conf/project_template/manage.py
1105: ./django/conf/project_template/settings.py
1106: ./django/conf/project_template/__init__.py
1107: ./docs/conf.py
[django/trunk]|4>

The rec directive simply says to look recursively for the following pattern. In this case, the pattern is *py. To show all directories in the Django root directory, we would issue a command like this:

[django/trunk]|3> mglob dir:*
<3> SList (.p, .n, .l, .s, .grep(), .fields() available).
Value:
0: examples
1: tests
2: extras
3: build
4: django
5: docs
6: scripts
</3>

The mglob command returns a Python list-like object, so anything we can do in Python, we can do to this list of returned files or folders.

This was just a taste of how the sh behaves. There are some sh profile features and feature options that we didn’t cover.

Information Gathering

IPython is much more than just a shell in which you can actively get work done. It also works as a tool to gather up all sorts of information about the code and objects you are working with. It can be such an asset in digging up information that it can feel like a forensic or investigatory tool. This section will outline a number of the features that can help you gather information.

page

If an object you are dealing with has too much of a representation to fit on one screen, you may want to try the magic page function. You can use page to pretty print your object and run it through a pager. The default pager on many systems is less, but yours might use something different. Standard usage is as follows:

In [1]: p = !ps aux
==
['USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND',
'root         1  0.0  0.1   5116  1964 ?        Ss   Mar07   0:00 /sbin/init',
< ... trimmed result ... >
In [2]: page p
['USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND',
'root         1  0.0  0.1   5116  1964 ?        Ss   Mar07   0:00 /sbin/init',
< ... trimmed result ... >

Here, we stored the result of the system shell command ps aux in the variable p. Then, we called page and passed the process result object to it. The page function then opened less.

There is one option for page: -r. This option tells page not to pretty print the object, but to run its string representation (result of str()) through the pager instead. For our process output object, that would look like this:

In [3]: page -r p
ilus-cd-burner/mapping-d', 'jmjones   5568  0.0  1.0 232004 10608 ?        S
Mar07   0:00 /usr/lib/gnome-applets/trashapplet --', 'jmjones   5593  0.0  0.9
188996 10076 ?        S    Mar07   0:00 /usr/lib/gnome-applets/battstat-apple',
'jmjones   5595  0.0  2.8 402148 29412 ?        S    Mar07   0:01 p
< ... trimmed result ... >

This non-pretty-printed result is not pretty, indeed. We recommend starting out with the pretty printer and then working from there.

pdef

The magic pdef function prints the definition headers or the function signature of any callable object. In this example, we create our own function with a docstring and return statement:

In [1]: def myfunc(a, b, c, d):
...:     '''return something by using a, b, c, d to do something'''
...:     return a, b, c, d
...:

In [2]: pdef myfunc
myfunc(a, b, c, d)

The pdef function ignored our docstring and return statement, but printed out the function signature portion of the function. You can use this on any callable function. This function even works if the source code is not available as long as it has access to either the .pyc file or the egg.

pdoc

The pdoc function prints the docstring of the function you pass to it. Here, we run the same myfunc() function that we used in the pdef example through pdoc:

In [3]: pdoc myfunc
Class Docstring:
return something by using a, b, c, d to do something
Calling Docstring:
x.__call__(...) <==> x(...)

This one is pretty self-explanatory.

pfile

The pfile function will run the file that contains an object through the pager if it can find the containing file:

In [1]: import os

In [2]: pfile os

r"""OS routines for Mac, NT, or Posix depending on what system we're on.

This exports:
- all functions from posix, nt, os2, mac, or ce, e.g. unlink, stat, etc.

< ... trimmed result ... >

This opened the os module and ran it through less. This can definitely be handy if you are trying to understand the reason a piece of code is behaving in a particular way. It will not work if the only access to the file is an egg or a .pyc file.

Note

You can see the same information from the ?? operator that you can from the magic functions %pdef, %pdoc, and %pfile. The preferred method is ??.

pinfo

The pinfo function and related utilities have become such a convenience for us that it’s hard to imagine not having them. The pinfo function provides information such as type, base class, namespace, and docstring. If we have a module that contains:

#!/usr/bin/env python

class Foo:
"""my Foo class"""
def __init__(self):
pass

class Bar:
"""my Bar class"""
def __init__(self):
pass

class Bam:
"""my Bam class"""
def __init__(self):
pass

then we can request information from the module itself:

In [1]: import some_module

In [2]: pinfo some_module
Type:           module
Base Class:     <type 'module'>
String Form:    <module 'some_module' from 'some_module.py'>
Namespace:      Interactive
File:           /home/jmjones/code/some_module.py
Docstring:
<no docstring>

We can request information from a class in the module:

In [3]: pinfo some_module.Foo
Type:           classobj
String Form:    some_module.Foo
Namespace:      Interactive
File:           /home/jmjones/code/some_module.py
Docstring:
my Foo class

Constructor information:
Definition:     some_module.Foo(self)

We can request information from an instance of one of those classes:

In [4]: f = some_module.Foo()

In [5]: pinfo f
Type:           instance
Base Class:     some_module.Foo
String Form:    <some_module.Foo instance at 0x86e9e0>
Namespace:      Interactive
Docstring:
my Foo class

A question mark (?) preceeding or following an object name provides the same functionality as pinfo:

In [6]: ? f
Type:           instance
Base Class:     some_module.Foo
String Form:    <some_module.Foo instance at 0x86e9e0>
Namespace:      Interactive
Docstring:
my Foo class

In [7]: f ?
Type:           instance
Base Class:     some_module.Foo
String Form:    <some_module.Foo instance at 0x86e9e0>
Namespace:      Interactive
Docstring:
my Foo class

But two question marks (??) preceeding or following an object name provides us with even more information:

In [8]: some_module.Foo ??
Type:           classobj
String Form:    some_module.Foo
Namespace:      Interactive
File:           /home/jmjones/code/some_module.py
Source:
class Foo:
"""my Foo class"""
def __init__(self):
pass
Constructor information:
Definition:     some_module.Foo(self)

The ?? notation provides us with all the information that pinfo provided us plus the source code for the requested object. Because we only asked for the class, ?? gave us the source code for this class rather than for the whole file. This is one of the features of IPython that we find ourselves using more than nearly any other.

psource

The psource function shows the source code for the element you define, whether that’s a module or something in a module, like a class or function. It runs the source code through a pager in order to display it. Here is an example of psource for a module:

In [1]: import some_other_module

In [2]: psource some_other_module
#!/usr/bin/env python

class Foo:
"""my Foo class"""
def __init__(self):
pass

class Bar:
"""my Bar class"""
def __init__(self):
pass

class Bam:
"""my Bam class"""
def __init__(self):
pass

def baz():
"""my baz function"""
return None

Here is an example of psource for a class in a module:

In [3]: psource some_other_module.Foo
class Foo:
"""my Foo class"""
def __init__(self):
pass

and here is an example of psource for a function in a module:

In [4]: psource some_other_module.baz
def baz():
"""my baz function"""
return None

psearch

The psearch magic function will look for Python objects by name, with the aid of wildcards. We’ll just briefly describe the psearch function here and if you want to know more, you can find documentation on the magic functions by typing magic at an IPython prompt, and then searching within the alphabetical list for psearch.

Let’s start by declaring the following objects:

In [1]: a = 1

In [2]: aa = "one"

In [3]: b = 2

In [4]: bb = "two"

In [5]: c = 3

In [6]: cc = "three"

We can search for all of the objects starting with a, b, or c as follows:

In [7]: psearch a*
a
aa
abs
all
any
apply

In [8]: psearch b*
b
basestring
bb
bool
buffer

In [9]: psearch c*
c
callable
cc
chr
classmethod
cmp
coerce
compile
complex
copyright
credits

Notice all the objects that were found in addition to a, aa, b, bb, c, cc; those are built-ins.

There is a quick and dirty alternative to using psearch: the ? operator. Here’s an example:

In [2]: import os

In [3]: psearch os.li*
os.linesep
os.link
os.listdir

In [4]: os.li*?
os.linesep
os.link
os.listdir

Instead of psearch, we were able to use *?.

There is an option to search -s or exclude searching -e a given namespace built-in to psearch. Namespaces include builtin, user, user_global, internal, and alias. By default, psearch searches builtin and user. To explicitly search user only, we would pass a -e builtin psearch option to exclude searching the builtin namespace. This is a little counterintuitive, but it makes an odd sort of sense. The default search path for psearch is builtin and user, so if we specify a -s user, searching builtin and user would still be what we’re asking it to do. In this example, the search is run again; notice that these results exclude the built-ins:

In [10]: psearch -e builtin a*
a
aa

In [11]: psearch -e builtin b*
b
bb

In [12]: psearch -e builtin c*
c
cc

The psearch function also allows searching for specific types of objects. Here, we search the user namespace for integers:

In [13]: psearch -e builtin * int
a
b
c

and here we search for strings:

In [14]: psearch -e builtin * string
__
___
__name__
aa
bb
cc

The __ and ___ objects that were found are IPython shorthand for previous return results. The __name__ object is a special variable that denotes the name of the module. If __name__ is '__main__', it means that the module is being run from the interpreter rather than being imported from another module.

who

IPython provides a number of facilities for listing all interactive objects. The first of these is the who function. Here is the previous example, including the a, aa, b, bb, c, cc variables, with the addition of the magic who function:

In [15]: who
a       aa      b       bb      c       cc

That’s pretty straightforward; it returns a simple listing of all interactively defined objects. You can also use who to filter on types. For example:

In [16]: who int
a       b       c

In [17]: who str
aa      bb      cc

who_ls

Except that it returns a list rather than printing the names of the matching variables, who_ls is similar to who. Here is an example of the who_ls function with no arguments:

In [18]: who_ls

Out[18]: ['a', 'aa', 'b', 'bb', 'c', 'cc']

and here is an example of filtering based on the types of objects:

In [19]: who_ls int

Out[19]: ['a', 'b', 'c']

In [20]: who_ls str

Out[20]: ['aa', 'bb', 'cc']

Since who_ls returns a list of the names, you can access the list of names using the _ variable, which just means “the last output.” Here is the way to iterate the last returned list of matching variable names:

In [21]: for n in _:
....:     print n
....:
....:
aa
bb
cc

whos

The whos function is similar to the who function except that whos prints out information that who doesn’t. Here is an example of the whos function used with no command-line arguments:

In [22]: whos
Variable   Type    Data/Info
----------------------------
a          int     1
aa         str     one
b          int     2
bb         str     two
c          int     3
cc         str     three
n          str     cc

And as we can with who, we can filter on type:

In [23]: whos int
Variable   Type    Data/Info
----------------------------
a          int     1
b          int     2
c          int     3

In [24]: whos str
Variable   Type    Data/Info
----------------------------
aa         str     one
bb         str     two
cc         str     three
n          str     cc

History

There are two ways to gain access to your history of typed-in commands in IPython. The first is readline-based; the second is the hist magic function.

Readline support

In IPython, you have access to all the cool features that you would expect to be in a readline-enabled application. If you are used to searching your Bash history using Ctrl-s, you won’t have a problem transitioning to the same functionality in IPython. Here, we’ve defined a few variables, then searched back through the history:

In [1]: foo = 1

In [2]: bar = 2

In [3]: bam = 3

In [4]: d = dict(foo=foo, bar=bar, bam=bam)

In [5]: dict2 = dict(d=d, foo=foo)

In [6]: <CTRL-s>

(reverse-i-search)`fo': dict2 = dict(d=d, foo=foo)

<CTRL-r>

(reverse-i-search)`fo': d = dict(foo=foo, bar=bar, bam=bam)

We typed Ctrl-r to start the search, then typed in fo as the search criteria. It brought up the line we entered that is denoted by IPython as In [5]. Using readline’s search functionality, we hit Ctrl-r and it matched the line we entered that is denoted by IPython as In [4].

There are many more things you can do with readline, but we’ll touch only briefly on them. Ctrl-a will take you to the beginning of a line and Ctrl-e will take you to the end of a line. Ctrl-f will move forward one character and Ctrl-b will move backward one character. Ctrl-d deletes one character and Ctrl-h deletes one character backward (backspace). Ctrl-p moves one line backward in the history and Ctrl-n moves one line forward in your history. For more readline functionality, enter man readline on your *nix system of choice.

hist command

In addition to providing access to the history functionality of the readline library, IPython also provides its own history function named history or hist for short. With no parameters, hist prints a sequential list of the input commands received from the user. By default, this list will be numbered. In this example, we set a few variables, change the directory, and then run the hist command:

In [1]: foo = 1

In [2]: bar = 2

In [3]: bam = 3

In [4]: cd /tmp
/tmp

In [5]: hist
1: foo = 1
2: bar = 2
3: bam = 3
4: _ip.magic("cd /tmp")
5: _ip.magic("hist ")

Items 4 and 5 in the history above are magic functions. Note that they have been modified by IPython and you can see what is going on under the covers through the IPython magic() function call.

To suppress the line numbers, use the -n option. Here is an example using the -n option for hist:

kIn [6]: hist -n
foo = 1
bar = 2
bam = 3
_ip.magic("cd /tmp")
_ip.magic("hist ")
_ip.magic("hist -n")

It is very helpful if you’ve been working in IPython and want to paste a section of your IPython code into a text editor.

The -t option returns a “translated” view of the history that shows the way IPython sees the commands that have been entered. This is the default. Here is the history we’ve built up so far run through with the -t flag:

In [7]: hist -t
1: foo = 1
2: bar = 2
3: bam = 3
4: _ip.magic("cd /tmp")
5: _ip.magic("hist ")
6: _ip.magic("hist -n")
7: _ip.magic("hist -t")

The “raw history,” or -r, flag will show you exactly what you typed. Here is the result of the earlier example, adding the “raw history” flag:

In [8]: hist -r
1: foo = 1
2: bar = 2
3: bam = 3
4: cd /tmp
5: hist
6: hist -n
7: hist -t
8: hist -r

IPython’s -g flag function also provides a facility to search through your history for a specific pattern. Here is the earlier example with the -g flag used to search for hist:

In [9]: hist -g hist
0187: hist
0188: hist -n
0189: hist -g import
0190: hist -h
0191: hist -t
0192: hist -r
0193: hist -d
0213: hist -g foo
0219: hist -g hist
===
^shadow history ends, fetch by %rep <number> (must start with 0)
=== start of normal history ===
5 : _ip.magic("hist ")
6 : _ip.magic("hist -n")
7 : _ip.magic("hist -t")
8 : _ip.magic("hist -r")
9 : _ip.magic("hist -g hist")

Notice that the term “shadow history” is returned in the previous example. “Shadow history” is a history of every command you have ever entered. Those items are displayed at the beginning of the result set and begin with a zero. History results from this session are stored at the end of the result set and do not start with a zero.

History results

In both Python and IPython, you can access not only your history of the commands you entered, but also access the history of your results. The first way to do this is using the _ flag, which means “the last output.” Here is an example of the way the _ function works in IPython:

In [1]: foo = "foo_string"

In [2]: _

Out[2]: ''

In [3]: foo

Out[3]: 'foo_string'

In [4]: _

Out[4]: 'foo_string'

In [5]: a = _

In [6]: a

Out[6]: 'foo_string'

When we defined foo in In [1], the _ in In [2] returned an empty string. When we output foo in In [3], we were able to use _ to get the result back in In [4]. And in In [5], we were able to save it off to a variable named a.

Here is the same example using the standard Python shell:

>>> foo = "foo_string"
>>> _
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name '_' is not defined
>>> foo
'foo_string'
>>> _
'foo_string'
>>> a = _
>>> a
'foo_string'

We see pretty much the same thing in the standard Python shell that we see in IPython, except that trying to access _ before anything has been output results in a NameError exception.

IPython takes this “last output” concept a step further. In the description of the Shell Execute” function, we described the ! and !! operators and explained that you can’t store the results of !! in a variable but can use it later. In a nutshell, you can access any result that was output using the syntax underscore (_) followed by a number_[0-9]* syntax. The number must correspond to the Out [0-9]* result that you want to see.

To demonstrate this, we’ll first list files but not do anything with the output:

In [1]: !!ls apa*py

Out[1]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: apache_conf_docroot_replace.py
1: apache_log_parser_regex.py
2: apache_log_parser_split.py

In [2]: !!ls e*py

Out[2]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: elementtree_system_profile.py
1: elementtree_tomcat_users.py

In [3]: !!ls t*py

Out[3]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: test_apache_log_parser_regex.py
1: test_apache_log_parser_split.py

We should have access to Out [1-3] by using _1, _2, and _3. So, we’ll attach a more meaningful name to them:

In [4]: apache_list = _1

In [5]: element_tree_list = _2

In [6]: tests = _3

Now, apache_list, element_tree_list, and tests contain the same elements that were output in Out [1], Out [2], and Out [3], respectively:

In [7]: apache_list

Out[7]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: apache_conf_docroot_replace.py
1: apache_log_parser_regex.py
2: apache_log_parser_split.py

In [8]: element_tree_list

Out[8]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: elementtree_system_profile.py
1: elementtree_tomcat_users.py

In [9]: tests

Out[9]: SList (.p, .n, .l, .s, .grep(), .fields() available). Value:
0: test_apache_log_parser_regex.py
1: test_apache_log_parser_split.py

But the whole point of all this is that, in IPython, you can access previous output results with either the naked _ special variable, or with an explicit numbered output reference by using _ followed by a number.

Automation and Shortcuts

As if IPython hasn’t done enough to improve your productivity, it also provides a number of functions and features to help you automate your IPython tasks and usage.

alias

We’ll first mention the alias “magic” command. We already covered this earlier in this chapter, so we won’t rehash usage of it again. But we wanted to just point out here that alias cannot only help you use *nix shell commands directly from within IPython, it can help you automate tasks as well.

macro

The macro function lets you define a block of code that can be executed later inline with whatever code you are working on. This is different from creating functions or methods. The macro, in a sense, becomes aware of the current context of your code. If you have a common set of processing steps you frequently execute on all your files, you can create a macro to work on the files. To get a feel for the way a macro will work on a list of files, look at the following example:

In [1]: dirlist = []

In [2]: for f in dirlist:
...:     print "working on", f
...:     print "done with", f
...:     print "moving %s to %s.done" % (f, f)
...:     print "*" * 40
...:
...:

In [3]: macro procdir 2
Macro `procdir` created. To execute, type its name (without quotes).
Macro contents:
for f in dirlist:
print "working on", f
print "done with", f
print "moving %s to %s.done" % (f, f)
print "*" * 40

At the time that we created the loop in In [2], there were no items in dirlist for the loop to walk over, but because we anticipated that future iterations would include items in dirlist, we created a macro named procdir to walk over the list. The syntax for creating a macro is macro macro_name range_of_lines, where the range of lines is a list of the lines from your history that you want incorporated into the macro. The lines for your macro list should be designated by a space-separated list of either numbers or ranges of numbers (such as 1-4).

In this example, we create a list of filenames and store them in dirlist, then execute the macro procdir. The macro will walk over the list of files in dirlist:

In [4]: dirlist = ['a.txt', 'b.txt', 'c.txt']

In [5]: procdir
------> procdir()
working on a.txt
done with a.txt
moving a.txt to a.txt.done
****************************************
working on b.txt
done with b.txt
moving b.txt to b.txt.done
****************************************
working on c.txt
done with c.txt
moving c.txt to c.txt.done
****************************************

Once you have a macro defined, you can edit it. This will open in your defined text editor. This can be very helpful when you are tweaking a macro to make sure it is right before you persist it.

store

You can persist your macros and plain Python variables with the store magic function. The simple standard use of store is store variable. However, store also takes a number of parameters that you may find useful: the -d variable function deletes the specified variable from the persistence store; -z function deletes all stored variables; and the -r function reloads all variables from the persistence store.

reset

The reset function deletes all variables from the interactive namespace. In the following example, we define three variables, use whos to verify they are set, reset the namespace, and use whos again to verify that they are gone:

In [1]: a = 1

In [2]: b = 2

In [3]: c = 3

In [4]: whos
Variable   Type    Data/Info
----------------------------
a          int     1
b          int     2
c          int     3

In [5]: reset
Once deleted, variables cannot be recovered. Proceed (y/[n])?  y

In [6]: whos
Interactive namespace is empty.

run

The run function executes the specified file in IPython. Among other things, this allows you to work on a Python module in an external text editor and interactively test changes you are making in it from within IPython. After executing the specified program, you are returned back to the IPython shell. The syntax for using run is run options specified_file args.

The -n option causes the module’s __name__ variable to be set not to '__main__', but to its own name. This causes the module to be run much as it would be run if it were simply imported.

The -i option runs the module in IPython’s current namespace and, thereby, gives the running module access to all defined variables.

The -e option causes IPython to ignore calls to sys.exit() and SystemExit exceptions. If either of these occur, IPython will just continue.

The -t option causes IPython to print out information about the length of time it took the module to run.

The -d option causes the specified module to be run under the Python debugger (pdb).

The -p option runs the specified module under the Python profiler.

save

The save function will save the specified input lines to the specified output file. Syntax for using save is save options filename lines. The lines may be specified in the same range format as is used for macro. The only save option is -r, which designates that raw input rather than translated should be saved. Translated input, which is standard Python, is the default.

rep

The final automation-enabling function is rep. The rep function takes a number of parameters that you might find useful. Using rep without parameters takes the last result that was processed and places a string representation of it on the next input line. For example:

In [1]: def format_str(s):
...:     return "str(%s)" % s
...:

In [2]: format_str(1)

Out[2]: 'str(1)'

In [3]: rep

In [4]: str(1)

The rep call at In [3] causes the text you see to be placed on In [4]. This allows you to programatically generate input for IPython to process. This comes in handy, particularly when you are using a combination of generators and macros.

A fairly common use case for rep without arguments is lazy, mouseless editing. If you have a variable containing some value, you can edit that value directly. As an example, assume that we are using a function that returns to the bin directory for specific installed packages. We’ll store the bin directory in a variable called a:

In [2]: a = some_blackbox_function('squiggly')

In [3]: a

Out[3]: '/opt/local/squiggly/bin'

If we type rep right here, we’ll see /opt/local/squiggly/bin on a new input line with a blinking cursor expecting us to edit it:

In [4]: rep

In [5]: /opt/local/squiggly/bin<blinking cursor>

If we wanted to store the base directory of the package rather than the bin directory, we can just delete the bin from the end of the path, prefix the path with a new variable name, follow that with an equal sign and quotation marks, and suffix it with just a quotation mark:

In [5]: new_a = '/opt/local/squiggly'

Now we have a new variable containing a string that is the base directory for this package.

Sure, we could have just copied and pasted, but that would have been more work. Why should you leave the comfort of your cozy keyboard to reach for the mouse? You can now use new_a as a base directory for anything that you need to do regarding the squiggly package.

When one number is given as an argument to rep, IPython brings up the input from that particular line of history and places it on the next line, and then places the cursor at the end of that line. This is helpful for executing, editing, and re-executing single lines or even small blocks of code. For example:

In [1]: map = (('a', '1'), ('b', '2'), ('c', '3'))

In [2]: for alph, num in map:
...:     print alph, num
...:
...:
a 1
b 2
c 3

Here, we edit In [2] and print the number value times 2 rather than a noncomputed value. We could either type the for loop in again, or we can use rep:

In [3]: rep 2

In [4]: for alph, num in map:
print alph, int(num) * 2
...:
...:
a 2
b 4
c 6

The rep function also takes ranges of numbers for arguments. The numeric range syntax is identical to the macro numeric range syntax that we discussed elsewhere in this chapter. When you specify a range for rep, the lines are executed immediately. Here is an example of rep:

In [1]: i = 1

In [2]: i += 1

In [3]: print i
2

In [4]: rep 2-3
lines [u'i += 1\nprint i\n']
3

In [7]: rep 2-3
lines [u'i += 1\nprint i\n']
4

We defined a counter incrementer and code that prints out the current count in In [1] through In [3]. In In [4] and In [7], we told rep to repeat lines 2 and 3. Notice that 2 lines (5 and 6) are missing since they were executed after In [4].

The last option for rep that we’ll go over is passing in a string. This is more like “passing in a word to rep” or even “passing in a nonquoted search string to rep.” Here is an example:

In [1]: a = 1

In [2]: b = 2

In [3]: c = 3

In [4]: rep a

In [5]: a = 1

We defined a few variables and told rep to repeat the last line that has an “a” in it. It brought In [1] back to us to edit and re-execute.

Summary

IPython is one of the most well-worn tools in our toolbox. Having mastery of a shell is like having mastery of a text editor: the more proficient you are, the more quickly you can cut through the tedious parts of the task you are working on. When we started working with IPython a few years ago, it was an amazingly powerful tool. Since then, it has grown into even more. The grep function and the ability to do string processing are just two of the things that come to mind when we think about the really useful, powerful features that keep emerging from the IPython community. We highly recommend that you dig deeper into IPython. Mastering it is a time investment that you won’t regret.

Chapter 3. Text

Nearly every system administrator has to deal with text whether it is in the form of logfiles, application data, XML, HTML, configuration files, or the output of some command. Often, utilities like grep and awk are all you need, but sometimes a tool that is more expressive and elegant is needed to tackle complex problems. When you need to create files with data extracted from other files, redirecting text from the output of a process (again, grep and awk come to mind) to a file is often good enough. But there are also times when a tool that is more easily extensible is better-suited for the job.

As we explained in the Introduction,” our experience has shown that that Python qualifies as more elegant, expressive, and extensible than Perl, Bash, or other languages we have used for programming. For more discussion of why we value Python more highly than Perl or Bash (and you could make application to sed and awk), see Chapter 1. Python’s standard library, language features, and built-in types are powerful tools for reading text files, manipulating text, and extracting information from text files. Python and its standard library contain a wealth of flexibility and functionality for text processing using the string type, the file type, and the regular expression module. A recent addition to the standard library, ElementTree, is immensely helpful when you need to work with XML. In this chapter, we will show you how to effectively use the standard library and built-in components that help with processing text.

Python Built-ins and Modules

str

A string is simply a sequence of characters. If you ever need to deal with textual data, you’ll almost certainly need to work with it as a string object or a series of string objects. The string type, str, is a powerful, flexible means for manipulating string data. This section shows you how to create strings and what you can do with them once they’ve been created.

Creating strings

The most common way to create a string is to surround the text with quotation marks:

In [1]: string1 = 'This is a string'

In [2]: string2 = "This is another string"

In [3]: string3 = '''This is still another string'''

In [4]: string4 = """And one more string"""

In [5]: type(string1), type(string2), type(string3), type(string4)
Out[5]: (<type 'str'>, <type 'str'>, <type 'str'>, <type 'str'>)

Single, double, and triple quotation marks accomplish the same thing: they all create an object of type str. Single and double quotation marks are identical in the creation of strings; you can use them interchangeably. This is different from the way quotation marks work in Unix shells, in which the marks cannot be used interchangeably. For example:

jmjones@dink:~$ FOO=sometext
jmjones@dink:~$ echo "Here is $FOO"
Here is sometext
jmjones@dink:~$ echo 'Here is $FOO'
Here is $FOO

Perl also uses between single and double quotes in string creation. Here’s a comparable example in a Perl script:

#!/usr/bin/perl

$FOO = "some_text";
print "-- $FOO --\n";
print '-- $FOO --\n';

And here is the output from this simple Perl script:

jmjones@dinkgutsy:code$ ./quotes.pl
-- some_text --
-- $FOO --\njmjones@dinkgutsy:code$

This is a distinction that Python does not make. Python leaves the distinction to the programmer. For example, if you needed to embed double quotation marks within the string and did not want to have to escape them (with a backslash). Conversely, if you needed to embed single quotes within the string and did not want to have to escape them, you would use double quotes. See Example 3-1.

Example 3-1. Python single/double quote comparison

In [1]: s = "This is a string with 'quotes' in it"

In [2]: s
Out[2]: "This is a string with 'quotes' in it"

In [3]: s = 'This is a string with \'quotes\' in it'

In [4]: s
Out[4]: "This is a string with 'quotes' in it"

In [5]: s = 'This is a string with "quotes" in it'

In [6]: s
Out[6]: 'This is a string with "quotes" in it'

In [7]: s = "This is a string with \"quotes\" in it"

In [8]: s
Out[8]: 'This is a string with "quotes" in it'

Notice in lines 2 and 4 that embedding an escaped quote of the same type as the enclosing quote coerces the enclosing quotation mark to the opposite quotation mark type. (Actually, it’s just coercing the representation of the string to show the “right” quotation mark types.)

There are times when you might want a string to span multiple lines. Sometimes embedding \n in the string where you want line breaks solves the problem for you, but this can get unwieldy. Another, often cleaner alternative is to use triples quotes, which allow you to create multiline strings. Example 3-2 is an example of trying to use single quotes for multiline strings and succeeding with triple quotes.

Example 3-2. Triple quotes

In [6]: s = 'this is
------------------------------------------------------------
File "<ipython console>", line 1
s = 'this is
^
SyntaxError: EOL while scanning single-quoted string

In [7]: s = '''this is a
...: multiline string'''

In [8]: s
Out[8]: 'this is a\nmultiline string'

And just to complicate matters, there is another way to denote strings in Python called “raw” strings. You create a raw string by placing the letter r immediately before the quotation mark when you are creating a string. Basically, the effect of creating a raw string as opposed to a non-raw (would that be cooked?) string is that Python does not interpret escape sequences in raw strings, whereas it does interpret escape sequences in regular strings. Python follows a set of rules similar to those used by Standard C regarding escape sequences. For example, in regular strings, \t is interpreted as a tab character, \n as a newline, and \r as a line feed. Table 3-1 shows escape sequences in Python.

Table 3-1. Python escape sequences

Sequence

Interpreted as

\newline

Ignored

\\

Backslash

\'

Single quote

\”

Double quote

\a

ASCII Bell

\b

ASCII backspace

\f

ASCII form feed

\n

ASCII line feed

\N{name}

Named character in Unicode database (Unicode strings only)

\r

ASCII carriage return

\t

ASCII horizontal tab

\uxxxx

Character with 16-bit hex value xxxx (Unicode only)

\Uxxxxxxxx

Character with 32-bit hex value xxxx (Unicode only)

\v

ASCII vertical tab

\ooo

Character with octal value oo

\xhh

Character with hex value hh

Escape sequences and raw strings are useful to remember, particularly when you are dealing with regular expressions, which we will get to later in this chapter. Example 3-3 shows escape sequences used with raw strings.

Example 3-3. Escape sequences and raw strings

In [1]: s = '\t'

In [2]: s
Out[2]: '\t'

In [3]: print s

In [4]: s = r'\t'

In [5]: s
Out[5]: '\\t'

In [6]: print s
\t

In [7]: s = '''\t'''

In [8]: s
Out[8]: '\t'

In [9]: print s

In [10]: s = r'''\t'''

In [11]: s
Out[11]: '\\t'

In [12]: print s
\t

In [13]: s = r'\''

In [14]: s
Out[14]: "\\'"

In [15]: print s
\'

When escape sequences are interpreted, \t is a tab character. When escape sequences are not interpreted, \t is simply a string that contains the two characters \ and t. Strings created with any of the quote characters, whether double or single, laid out individually or three in a row, allow \t to be interpreted as a tab character. Any of those same strings prefixed with an r allow \t to be interpreted as the two characters \ and t.

Another bit of fun from this example is the distinction between __repr__ and __str__. When you type a variable name at an IPython prompt and hit enter, its __repr__ representation is displayed. When we type print followed by a variable name and then hit enter, its __str__ representation is printed out. The print function interprets the escape sequences in the string and displays them appropriately. For more discussion on __repr__ and __str__ , see “Basic Concepts” in Chapter 2.

Built-in methods for str data extraction

Because strings are objects, they provide methods that can be called to perform operations. But by “method,” we don’t mean only those methods that the str type provides for us; we mean all the ways that are available to extract data from an object of str type. This includes all the str methods, and it also includes the in and not in text operators you saw in our first example.

Technically, the in and not in test operators call a method on your str object, __contains__() in Example 3-1 (shown earlier). For more information on how this works, see the Appendix. You can use both in and not in to determine if a string is a part of another string. See Example 3-4.

Example 3-4. In and not in

In [1]: import subprocess

In [2]: res = subprocess.Popen(['uname', '-sv'], stdout=subprocess.PIPE)

In [3]: uname = res.stdout.read().strip()

In [4]: uname

Out[4]: 'Linux #1 SMP Tue Feb 12 02:46:46 UTC 2008'

In [5]: 'Linux' in uname

Out[5]: True

In [6]: 'Darwin' in uname

Out[6]: False

In [7]: 'Linux' not in uname

Out[7]: False

In [8]: 'Darwin' not in uname

Out[8]: True

If string2 contains string1, string1 in string2 returns True, otherwise, it returns False. So, checking to see if "Linux" was in our uname string returned True, but checking to see if "Darwin" was in our uname returned false. And we demonstrated not in just for fun.

Sometimes you only need to know if a string is a substring of another string. Other times, you need to know where in a string the substring occurs. find() and index() let you do that. See Example 3-5.

Example 3-5. find() and index()

In [9]: uname.index('Linux')

Out[9]: 0

In [10]: uname.find('Linux')

Out[10]: 0

In [11]: uname.index('Darwin')
---------------------------------------------------------------------------
<type 'exceptions.ValueError'>            Traceback (most recent call last)

/home/jmjones/code/<ipython console> in <module>()

<type 'exceptions.ValueError'>: substring not found

In [12]: uname.find('Darwin')

Out[12]: -1

If string1 is in string2 (as in our previous example), string2.find(string1) returns the index of the first character of string1, otherwise, it returns –1. (Don’t worry—we’ll get into indexes in a moment.) Likewise, if string1 is in string2, string2.index(string1) returns the index of the first character of string1, otherwise, it raises a ValueError exception. In the example, the find() method found "Linux" at the beginning of the string, so it returned 0 indicating that the index of the first character of "Linux" was 0. However, the find() method couldn’t find "Darwin" anywhere, so it returned –1. When Python was looking for Linux, the index() method behaved in the same way the find() method does when looking for "Linux". However, when looking for "Darwin", index() threw a ValueError exception, indicating that it could not find that string.

So, what can you do with these “index” numbers? What good are they? Strings are treated as lists of characters. The “index” that find() and index() return simply shows which character of the larger string is the beginning of the match. See Example 3-6.

Example 3-6. String slice

In [13]: smp_index = uname.index('SMP')

In [14]: smp_index

Out[14]: 9

In [15]: uname[smp_index:]

Out[15]: 'SMP Tue Feb 12 02:46:46 UTC 2008'

In [16]: uname[:smp_index]

Out[16]: 'Linux #1 '

In [17]: uname

Out[17]: 'Linux #1 SMP Tue Feb 12 02:46:46 UTC 2008'

We were able to see every character from the index of finding "SMP" to the end of the string with the slice syntax string[index:]. We were also able to see every character from the beginning of the uname string to the index of finding "SMP" with the slice syntax string[:index]. The slight variation between these two is which side of the index the colon (:) finds itself on.

The point of this string slicing example, and of the in/not in tests, is to show you that strings are sequences and so they behave in a way that is similar to the way that sequences such as lists work. For a more thorough discussion of the way sequences work, see “Sequence Operations” in Chapter 4 of Python in a Nutshell (O’Reilly) by Alex Martelli (also available online on Safari at http://safari.oreilly.com/0596100469/pythonian-CHP-4-SECT-6).

Two other strings that are occasionally methods are startswith() and endswith(). As their names imply, they can help you determine whether a string “starts with” or “ends with” a particular substring. See Example 3-7.

Example 3-7. startswith() and endswith()

In [1]: some_string = "Raymond Luxury-Yacht"

In [2]: some_string.startswith("Raymond")
Out[2]: True

In [3]: some_string.startswith("Throatwarbler")
Out[3]: False

In [4]: some_string.endswith("Luxury-Yacht")
Out[4]: True

In [5]: some_string.endswith("Mangrove")
Out[5]: False

So, you can see that Python returns the information that the string “Raymond Luxury-Yacht” begins with “Raymond” and ends with “Luxury-Yacht.” It does not begin with “Throatwarbler,” nor does it end with “Mangrove.” It is pretty simple to achieve the same result using slicing, but slicing is messy and can be tedious as well. See Example 3-8.

Example 3-8. Startswith() endswith() replacement hack

In [6]: some_string[:len("Raymond")] == "Raymond"
Out[6]: True

In [7]: some_string[:len("Throatwarbler")] == "Throatwarbler"
Out[7]: False

In [8]: some_string[-len("Luxury-Yacht"):] == "Luxury-Yacht"
Out[8]: True

In [9]: some_string[-len("Mangrove"):] == "Mangrove"
Out[9]: False

Note

A slice operation creates and returns a new string object rather than modifying the string in line. Depending on how frequently you slice a string in a script, there could be a noticable memory and performance impact. Even if there is no discernible performance impact, it’s probably a good habit to refrain from using the slice operation in cases in which startswith() and endswith() will do what you need to do.

We were able to see that the string “Raymond” appeared in some_string from its beginning through however many characters are in the string “Raymond.” In other words, we were able to see that some_string starts with the string “Raymond” without calling the startswith() method. And likewise for ending with “Luxury-Yacht.”

Without any arguments, lstrip(), rstrip(), and strip() are methods that remove leading, trailing, and both leading and trailing whitespace, respectively. Examples of whitespace include tabs, space characters, carriage returns, and line feeds. Using lstrip() without arguments removes any whitespace that appears at the beginning of a string and then returns a new string. Using rstrip() without arguments removes any whitespace that appears at the end of a string and then returns a new string. Using strip() without arguments removes all whitespace at the beginning or end of a string and then returns a new string. See Example 3-9.

Note

All of the strip() methods create and return new string objects rather than modifying the strings in line. This might never cause problems for you, but it’s something to be aware of.

Example 3-9. lstrip(), rstrip(), and strip()

In [1]: spacious_string = "\n\t Some Non-Spacious Text\n \t\r"

In [2]: spacious_string
Out[2]: '\n\t Some Non-Spacious Text\n \t\r'

In [3]: print spacious_string

Some Non-Spacious Text

In [4]: spacious_string.lstrip()
Out[4]: 'Some Non-Spacious Text\n \t\r'

In [5]: print spacious_string.lstrip()
Some Non-Spacious Text

In [6]: spacious_string.rstrip()
Out[6]: '\n\t Some Non-Spacious Text'

In [7]: print spacious_string.rstrip()

Some Non-Spacious Text

In [8]: spacious_string.strip()
Out[8]: 'Some Non-Spacious Text'

In [9]: print spacious_string.strip()
Some Non-Spacious Text

But strip(), rstrip(), and lstrip() all take one optional argument: a string whose characters are to be appropriately stripped off of the string. This means that the strip()s don’t just remove whitespace; they’ll remove anything you tell them to remove:

In [1]: xml_tag = "<some_tag>"

In [2]: xml_tag.lstrip("<")

Out[2]: 'some_tag>'

In [3]: xml_tag.lstrip(">")

Out[3]: '<some_tag>'

In [4]: xml_tag.rstrip(">")

Out[4]: '<some_tag'

In [5]: xml_tag.rstrip("<")

Out[5]: '<some_tag>'

Here, we stripped off the left and right angle brackets from an XML tag one at a time. But what if we wanted to strip off both of them at the same time? Then we could do this:

In [6]: xml_tag.strip("<").strip(">")

Out[6]: 'some_tag'

Since the strip()s return a string, we can call another string operation directly after a strip() call. Here, we chained strip() calls together. The first strip() call took off the starting character (the left angle bracket) and returned a string, and the second strip() call took off the ending character (the right angle bracket) and returned the string "some_tag". But there’s an easier way:

In [7]: xml_tag.strip("<>")

Out[7]: 'some_tag'

You might have assumed that the strip()s stripped off an exact occurrence of the string you fed it, but the strips actually remove any sequential occurrence of the specified characters from the appropriate side of the string. In that last example, we told strip() to remove "<>". That doesn’t mean to exactly match "<>" and remove any occurrences of those two characters that are adjacent to one another in that order; it means remove any occurrences of "<" or ">" that are adjacent to one another on either end of the string.

Here is perhaps a clearer example:

In [8]: gt_lt_str = "<><><>gt lt str<><><>"

In [9]: gt_lt_str.strip("<>")

Out[9]: 'gt lt str'

In [10]: gt_lt_str.strip("><")

Out[10]: 'gt lt str'

We stripped off any occurrences of "<" or ">" on either side of the string. So we wound up with something that was just letters and spaces.

This still might not work exactly as you’re expecting. For example:

In [11]: foo_str = "<fooooooo>blah<foo>"

In [12]: foo_str.strip("<foo>")

Out[12]: 'blah'

You may have expected strip() to match and strip the right side but not the left. But it matched and stripped the sequential occurrence of "<", "f", "o", and ">". And no, we didn’t leave out an "o". Here is one final clarifying example for the strip():

In [13]: foo_str.strip("><of")

Out[13]: 'blah'

This stripped "<", "f", "o", even though the characters were not in that order.

The methods upper() and lower() are useful, particularly when you need to compare two strings without regard to whether the characters are upper- or lowercase. The upper() method returns a string, which is the uppercase of the original. The lower() method returns a string, which is the lowercase of the original. See Example 3-10.

Example 3-10. upper() and lower()

In [1]: mixed_case_string = "VOrpal BUnny"

In [2]: mixed_case_string == "vorpal bunny"
Out[2]: False

In [3]: mixed_case_string.lower() == "vorpal bunny"
Out[3]: True

In [4]: mixed_case_string == "VORPAL BUNNY"
Out[4]: False

In [5]: mixed_case_string.upper() == "VORPAL BUNNY"
Out[5]: True

In [6]: mixed_case_string.upper()
Out[6]: 'VORPAL BUNNY'

In [7]: mixed_case_string.lower()
Out[7]: 'vorpal bunny'

If you need to extract a piece of a string based on some kind of delimiter, the split() method may provide exactly what you are looking for. See Example 3-11.

Example 3-11. split()

In [1]: comma_delim_string = "pos1,pos2,pos3"

In [2]: pipe_delim_string = "pipepos1|pipepos2|pipepos3"

In [3]: comma_delim_string.split(',')
Out[3]: ['pos1', 'pos2', 'pos3']

In [4]: pipe_delim_string.split('|')
Out[4]: ['pipepos1', 'pipepos2', 'pipepos3']

Typical use of the split() method is to pass in the string that you want to split. Often, this is a single character such as a comma or pipe, but it can also be a string of more than one character. We split comma_delim_string on a comma and pipe_delim_string on the pipe (|) character by passing the comma and the pipe characters to split(). The return value of split() is a list of strings, each of which is a contiguous group of characters that fell between the specified delimiters. When you need to split on a number of characters rather than just a single character, the split() method accommodates that, too. As we are writing this book, there is no character type in Python, so what we passed in to split(), although it was a single character in both cases, was actually a string. So when we pass several characters in to split(), it will work with them. See Example 3-12.

Example 3-12. split() multiple delimiter example

In [1]: multi_delim_string = "pos1XXXpos2XXXpos3"

In [2]: multi_delim_string.split("XXX")
Out[2]: ['pos1', 'pos2', 'pos3']

In [3]: multi_delim_string.split("XX")
Out[3]: ['pos1', 'Xpos2', 'Xpos3']

In [4]: multi_delim_string.split("X")
Out[4]: ['pos1', '', '', 'pos2', '', '', 'pos3']

Notice that we first specified "XXX" as the delimiting string for multi_delim_string. As we expected, this returned ['pos1', 'pos2', 'pos3']. Next, we specified "XX" as the delimiting string and split() returned ['pos1', 'Xpos2', 'Xpos3']. Split() looked for the characters that appeared between each instance of the "XX" delimiter. "Pos1" appeared from the beginning of the string to the first "XX" delimiter; "Xpos2" appeared from the first occurrence of "XX" to the second appearance of it; and "Xpos3" appeared from the second occurrence of "XX" to the end of the string. The last split() used a single "X" character as the delimiting string. Notice that, in the positions where there were adjacent "X" characters, there is an empty string ("") in the returned list. This simply means that there is nothing between the adjacent "X" characters.

But what if you only want to split the string on the first “n” occurrences of the specified delimiters? Split() takes a second parameter, called max_split. When an integer value for max_split is passed in, split() will only split the string the number of times the max_split argument dictates:

In [1]: two_field_string = "8675309,This is a freeform, plain text, string"

In [2]: two_field_string.split(',', 1)
Out[2]: ['8675309', 'This is a freeform, plain text, string']

We split on a comma and told split() to only split on the first occurrence of the delimiter. Although there are multiple commas in this example, the string is split only on the first one.

If you need to split on whitespace in order to retrieve, for example, words from a piece of prose-like text, split() is an easy tool for accomplishing that:

In [1]: prosaic_string = "Insert your clever little piece of text here."

In [2]: prosaic_string.split()
Out[2]: ['Insert', 'your', 'clever', 'little', 'piece', 'of', 'text', 'here.']

Because no parameters have been passed in, split() defaults to splitting on whitespace.

Most of the time, you will probably see the results you expected to see. However, if you have a multiline piece of text, you might see results that you were not expecting. Often, when you have a multiline piece of text, you intend to deal with one line at a time. But you might find that the program split on every word in the string:

In [1]: multiline_string = """This
...: is
...: a multiline
...: piece of
...: text"""

In [2]: multiline_string.split()
Out[2]: ['This', 'is', 'a', 'multiline', 'piece', 'of', 'text']

In this case, splitlines() will get you closer to what you wanted:

In [3]: lines = multiline_string.splitlines()

In [4]: lines

Out[4]: ['This', 'is', 'a multiline', 'piece of', 'text']

Splitlines() returned a list of each line within the string and preserved groups of “words.” From here, you can iterate over each line and split the line into words:

In [5]: for line in lines:
...:     print "START LINE::"
...:     print line.split()
...:     print "::END LINE"
...:
START LINE::
['This']
::END LINE
START LINE::
['is']
::END LINE
START LINE::
['a', 'multiline']
::END LINE
START LINE::
['piece', 'of']
::END LINE
START LINE::
['text']
::END LINE

Sometimes you don’t want to pull a string apart or extract information from it; sometimes you need to piece a string together from data you already have. In these cases, join() can help:

In [1]: some_list = ['one', 'two', 'three', 'four']

In [2]: ','.join(some_list)
Out[2]: 'one,two,three,four'

In [3]: ', '.join(some_list)
Out[3]: 'one, two, three, four'

In [4]: '\t'.join(some_list)
Out[4]: 'one\ttwo\tthree\tfour'

In [5]: ''.join(some_list)
Out[5]: 'onetwothreefour'

Given the list some_list, we were able to assemble the strings 'one', 'two', 'three', and 'four' into a number of variations. We joined the list some_list with a comma, a comma and a space, a tab, and an empty string. Join() is a string method, so calling join() on a string literal such as ',' is perfectly valid. Join() takes a sequence of strings as an argument. It packs the sequence of strings together into a single string so that each item of the sequence appears in order, but the string on which you called join() appears between each item in the sequence.

We have a word of warning regarding join() and the argument it expects. Note that join() expects a sequence of strings. What happens if you pass in a sequence of integers? Kaboom!

In [1]: some_list = range(10)

In [2]: some_list
Out[2]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

In [3]: ",".join(some_list)
---------------------------------------------------------------------------
exceptions.TypeError                                 Traceback (most recent call last)

/Users/jmjones/<ipython console>

TypeError: sequence item 0: expected string, int found

The traceback to the exception that join() raises is pretty self-explanatory, but since this is a common error, it is worth understanding. You can easily avoid this pitfall with a simple list comprehension. Here we enlist the help of a list comprehension to convert all the elements of some_list, all of which are integers, to strings:

In [4]: ",".join([str(i) for i in some_list])
Out[4]: '0,1,2,3,4,5,6,7,8,9'

Or, you could use a generator expression:

In [5]: ",".join(str(i) for i in some_list)

Out[5]: '0,1,2,3,4,5,6,7,8,9'

For more information on using list comprehensions, see the section “Control Flow Statements” in Chapter 4 of Python in a Nutshell (also available online on Safari at http://safari.oreilly.com/0596100469/pythonian-CHP-4-SECT-10).

The last method for creating or modifying strings of text is the replace() method. Replace() takes two arguments: the string that is to be replaced and the string to replace it with, respectively. Here is a simple replace() example:

In [1]: replacable_string = "trancendental hibernational nation"

In [2]: replacable_string.replace("nation", "natty")
Out[2]: 'trancendental hibernattyal natty'

Notice that replace() doesn’t care if the string to replace is in the middle of a word or if it is a full word. So, in cases in which you need to replace only a specific sequence of characters with another specific sequence of characters, replace() is the tool to use.

However, there are times when you need a finer level of control, when replacing one sequence of characters with another sequence of characters isn’t enough. Sometimes you need to be able to specify a pattern of characters to find and replace. Patterns can also help with searching for text from which to extract data. In cases in which using patterns is more helpful, regular expressions can help. We’ll look at regular expressions next.

Note

As slice operations and the strip() methods do, replace() creates a new string rather than modify the string in line.

Unicode strings

So far, all of the examples of strings we’ve looked at have been exclusively of the built-in string types (str), but Python has another string type with which you will want to be familiar: Unicode. When you see any characters on a computer screen, the computer is dealing with those characters internally as numbers. Until Unicode, there were many different sets of number-to-character mappings, depending on the language and platform. Unicode is a standard that provides a single number-to-character mapping regardless of the language, platform, or even the program that is dealing with the text. In this section, we will introduce the concept of Unicode and the way that Python deals with it. For a more in-depth explanation of Unicode, see A. M. Kuchling’s excellent Unicode tutorial at http://www.amk.ca/python/howto/unicode.

Creating a Unicode string is as simple as creating a regular string:

In [1]: unicode_string = u'this is a unicode string'

In [2]: unicode_string
Out[2]: u'this is a unicode string'

In [3]: print unicode_string
this is a unicode string

Or, you can use the built-in unicode() function:

In [4]: unicode('this is a unicode string')
Out[4]: u'this is a unicode string'

This doesn’t seem like it buys us much, particularly as it is just dealing with characters from one language. But what if you have to deal with characters from multiple languages? Unicode will help you here. To create a character in a Unicode string with a specific numerical value, you can use the \uXXXX or \uXXXXXXXX notation. For example, here is a Unicode string that contains Latin, Greek, and Russian characters:

In [1]: unicode_string = u'abc_\u03a0\u03a3\u03a9_\u0414\u0424\u042F'

In [2]: unicode_string
Out[2]: u'abc_\u03a0\u03a3\u03a9_\u0414\u0424\u042f'

Python generates a string (str) dependant on the encoding you use. On the Python that comes standard with Mac, if you attempted to print the string from the previous example, an error would be returned, printing:

In [3]: print unicode_string
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)

/Users/jmjones/<ipython console> in <module>()

UnicodeEncodeError: 'ascii' codec can't encode characters in position 4-6:
ordinal not in range(128)

We have to give it an encoding that knows how to handle all the characters that we give it:

In [4]: print unicode_string.encode('utf-8')
abc_ΠΣΩ_ДФЯ

Here, we encoded the string that contained Latin, Greek, and Russian characters to UTF-8, which is a common encoding for Unicode data.

Unicode strings contain the same facilities, such as the in test, and methods that we’ve already talked about for regular strings:

In [5]: u'abc' in unicode_string
Out[5]: True

In [6]: u'foo' in unicode_string
Out[6]: False

In [7]: unicode_string.split()
Out[7]: [u'abc_\u03a0\u03a3\u03a9_\u0414\u0424\u042f']

In [8]: unicode_string.
unicode_string.__add__           unicode_string.expandtabs
unicode_string.__class__         unicode_string.find
unicode_string.__contains__      unicode_string.index
unicode_string.__delattr__       unicode_string.isalnum
unicode_string.__doc__           unicode_string.isalpha
unicode_string.__eq__            unicode_string.isdecimal
unicode_string.__ge__            unicode_string.isdigit
unicode_string.__getattribute__  unicode_string.islower
unicode_string.__getitem__       unicode_string.isnumeric
unicode_string.__getnewargs__    unicode_string.isspace
unicode_string.__getslice__      unicode_string.istitle
unicode_string.__gt__            unicode_string.isupper
unicode_string.__hash__          unicode_string.join
unicode_string.__init__          unicode_string.ljust
unicode_string.__le__            unicode_string.lower
unicode_string.__len__           unicode_string.lstrip
unicode_string.__lt__            unicode_string.partition
unicode_string.__mod__           unicode_string.replace
unicode_string.__mul__           unicode_string.rfind
unicode_string.__ne__            unicode_string.rindex
unicode_string.__new__           unicode_string.rjust
unicode_string.__reduce__        unicode_string.rpartition
unicode_string.__reduce_ex__     unicode_string.rsplit
unicode_string.__repr__          unicode_string.rstrip
unicode_string.__rmod__          unicode_string.split
unicode_string.__rmul__          unicode_string.splitlines
unicode_string.__setattr__       unicode_string.startswith
unicode_string.__str__           unicode_string.strip
unicode_string.capitalize        unicode_string.swapcase
unicode_string.center            unicode_string.title
unicode_string.count             unicode_string.translate
unicode_string.decode            unicode_string.upper
unicode_string.encode            unicode_string.zfill
unicode_string.endswith

You might not need Unicode right now. But it’s important that you become familiar with it if you want to continue programming with Python.

re

Since Python comes with “batteries included,” you might expect that it would include a regular expression library. You won’t be disappointed. The emphasis in this section will be on using Python to work with regular expressions rather than on the ins and outs of regular expression syntax. So if you aren’t familiar with regular expressions, we recommend that you pick up a copy of Mastering Regular Expressions (O’Reilly) by Jeffrey E. F. Friedl (also available on Safari at http://safari.oreilly.com/0596528124). This section will assume that you are comfortable with regular expressions, but if you’re not, it will be helpful to have Friedl’s text at hand.

If you’re familiar with Perl, you’re probably used to using regular expressions with =~. Python’s inclusion of regular expressions comes by way of a library rather than syntactic features of the language. So, in order to work with regular expressions, you first have to import the regular expression module re. Here is a basic example of the way regular expressions are created and used. See Example 3-13.

Example 3-13. Basic regular expression usage

In [1]: import re

In [2]: re_string = "{{(.*?)}}"

In [3]: some_string = "this is a string with {{words}} embedded in\
...:  {{curly brackets}} to show an {{example}} of {{regular expressions}}"

In [4]: for match in re.findall(re_string, some_string):
...:     print "MATCH->", match
...:
MATCH-> words
MATCH-> curly brackets
MATCH-> example
MATCH-> regular expressions

The first thing we did was to import the re module. As you might have guessed, re stands for “regular expression.” Next, we created a string, re_string, which is the pattern we look for in the example. This pattern will match two consecutive open curly brackets ({{) followed by any text (or no text) followed by two consecutive close curly brackets (}}). Next, we created a string, some_string, which contains a mix of groups of words enclosed in double curly brackets and words not enclosed in curly brackets. Finally, we iterated over the results of the re module’s findall() function as it searched some_string for the pattern found in re_string. And you can see, it printed out words, curly brackets, example, and regular expressions, which are all the words enclosed in double curly brackets.

There are two ways to work with regular expressions in Python. The first is to use the functions in the re module directly, as in the previous example. The second is to create a compiled regular expression object and use the methods on that object.

So what is a compiled regular expression? It is simply an object that was created by passing in a pattern to re.compile(); it contains a number of regular expression methods that were created by passing in a pattern to re.compile(). There are two primary differences between using the compiled and noncompiled examples. First, instead of keeping a reference to the regular expression pattern "{{(.*?)}}", we created a compiled regular expression object and used the pattern to create it. Second, instead of calling findall() on the re module, we called findall() on the compiled regular expression object.

For more information on the re module’s contents, which includes available functions, see the Module Contents section of the Python Library Reference, http://docs.python.org/lib/node46.html. For more information on compiled regular expression objects, see the Regular Expression Objects section of the Python Library Reference, http://docs.python.org/lib/re-objects.html.

Example 3-14 shows our double curly bracket example reworked to show how to use a compiled regular expression object.

Example 3-14. Simple regular expression, compiled pattern

In [1]: import re

In [2]: re_obj = re.compile("{{(.*?)}}")

In [3]: some_string = "this is a string with {{words}} embedded in\
...:  {{curly brackets}} to show an {{example}} of {{regular expressions}}"

In [4]: for match in re_obj.findall(some_string):
...:     print "MATCH->", match
...:
MATCH-> words
MATCH-> curly brackets
MATCH-> example
MATCH-> regular expressions

The method that you choose to work with regular expressions in Python is partially a matter of preference and expression. However, there can be performance implications when you use the functions in the re module rather than creating a compiled regular expression object. Those performance problems can be exacerbated if you are in some kind of a loop that will repeat a lot, such as a loop that applies the regular expression to each line of a text file with hundreds of thousands of lines. In the examples below, we run a simple regex script using both compiled and noncompiled regular expressions, against a file containing 500,000 lines of text. When we run the Unix timeit utility against the results of each script test, you’ll be able to see the difference in performance. See Example 3-15.

Example 3-15. re no compile code performance test

#!/usr/bin/env python

import re

def run_re():
pattern = 'pDq'

infile = open('large_re_file.txt', 'r')
match_count = 0
lines = 0
for line in infile:
match = re.search(pattern, line)
if match:
match_count += 1
lines += 1
return (lines, match_count)

if __name__ == "__main__":
lines, match_count =  run_re()
print 'LINES::', lines
print 'MATCHES::', match_count

The timeit utility executes a piece of code a number of times and reports back the time of the best run. Here are the results from running the Python timeit utility within IPython on this code:

In [1]: import re_loop_nocompile

In [2]: timeit -n 5 re_loop_nocompile.run_re()
5 loops, best of 3: 1.93 s per loop

This example executed the run_re() function in three sets of five iterations each and reported back that the best run took an average of 1.93 seconds per loop. The reason timeit runs the same piece of code a number of times is to reduce the likelihood that other processes running at the same time are affected by the test results.

And here are the results from running the Unix time utility against the same code:

jmjones@dink:~/code$ time python re_loop_nocompile.py
LINES:: 500000 MATCHES:: 242

real    0m2.113s
user    0m1.888s
sys     0m0.163s

Example 3-16 is the same regular expression example, except that we are using re.compile() to create a compiled pattern object.

Example 3-16. re compile code performance test

#!/usr/bin/env python

import re

def run_re():
pattern = 'pDq'
re_obj = re.compile(pattern)

infile = open('large_re_file.txt', 'r')
match_count = 0
lines = 0
for line in infile:
match = re_obj.search(line)
if match:
match_count += 1
lines += 1
return (lines, match_count)

if __name__ == "__main__":
lines, match_count =  run_re()
print 'LINES::', lines
print 'MATCHES::', match_count

Running this script through the Python timeit utility in IPython yields these results:

In [3]: import re_loop_compile

In [4]: timeit -n 5 re_loop_compile.run_re()
5 loops, best of 3: 860 ms per loop

And running the same script through the Unix time utility yields these results:

jmjones@dink:~/code$ time python
re_loop_compile.py LINES:: 500000 MATCHES:: 242

real    0m0.996s
user    0m0.836s
sys     0m0.154s

The clear winner is the compiled version. It took half the time to run as measured by both the Unix time and the Python timeit utilities. So we highly recommend that you get into the habit of creating compiled regular expression objects.

As we discussed earlier in this chapter, raw strings can be used to denote strings that do not interpret escape sequences. Example 3-17 shows raw strings used in regular expressions.

Example 3-17. Raw strings and regular expressions

In [1]: import re

In [2]: raw_pattern = r'\b[a-z]+\b'

In [3]: non_raw_pattern = '\b[a-z]+\b'

In [4]: some_string = 'a few little words'

In [5]: re.findall(raw_pattern, some_string)

Out[5]: ['a', 'few', 'little', 'words']

In [6]: re.findall(non_raw_pattern, some_string)

Out[6]: []

The regular expression pattern \b matches word boundaries. So in both the raw and regular strings, we were looking for individual lowercase words. Notice that raw_pattern matched the word boundaries appropriately on some_string and non_raw_pattern didn’t match anything at all. Raw_pattern recognized \b as two characters rather than interpreting it as an escape sequence for the backspace character. Non_raw_pattern interpreted the \b characters as an escape sequence representing the backspace character. The regular expression function findall() was then able to use the raw string pattern to find words. However, when findall() looked for the non-raw pattern, it didn’t find any backspace characters.

For non_raw_pattern to match a string, we would have to put backspace characters around it, as we did with “little” here:

In [7]: some_other_string = 'a few \blittle\b words'

In [8]: re.findall(non_raw_pattern, some_other_string)
Out[8]: ['\x08little\x08']

Notice that findall() matched the hex notation “\x08” before and after the word “little.” That hex notation corresponds to the backspace character that we inserted with the escape sequence "\b".

So, as you can see, raw strings are helpful when you intend to use some of the backslashed special sequences such as "\b" for word boundaries, "\d" for digits, and "\w" for alpha numeric characters. For a full listing of these backslashed special sequences, see the Regular Expression Syntax section in the Python Library Reference at http://docs.python.org/lib/re-syntax.html.

Examples 3-14 through 3-17 really were quite simple, both in the regular expression used as well as the different methods we applied to it. Sometimes, this limited use of the power of regular expressions is all you need. Other times, you’ll need to make use of more of the power that is contained in the regular expression library.

The four primary regular expression methods (or functions) which are most likely to be used often are findall(), finditer(), match(), and search(). You might also find yourself using split() and sub(), but probably not as often as you will use the others.

Findall() will find all occurrences of the specified pattern in the search string. If findall() matches the pattern, the type of data structure it will return will depend on whether the pattern specified a group.

Note

A quick reminder about regex: grouping allows you to specify text within a regular expression that you want to extract from the result. See “Common Metacharacters and Fields” in Friedl’s Mastering Regular Expressions for more information, or go online to http://safari.oreilly.com/0596528124/regex3-CHP-3-SECT-5?imagepage=137.

If you didn’t specify a group in the regular expression pattern but a match is found, findall() will return a list of strings. For example:

In [1]: import re

In [2]: re_obj = re.compile(r'\bt.*?e\b')

In [3]: re_obj.findall("time tame tune tint tire")
Out[3]: ['time', 'tame', 'tune', 'tint tire']

The pattern doesn’t specify any groups, so findall() returns a list of strings. An interesting side point is that the last element of the returned list contains two words, tint and tire. The regular expression was intended to match words that start with “t” and end with “e”. But the .*? command matches anything, including whitespace. Findall() matched everything it was supposed to. It found a word which started with “t” (tint). It continued looking through the string until it found a word that ended with “e” (tire). So, the match “tint tire” was appropriate. To exclude the whitespace, you would use r'\bt\w*e\b':

In [4]: re_obj = re.compile(r'\bt\w*e\b')

In [5]: re_obj.findall("time tame tune tint tire")
Out[5]: ['time', 'tame', 'tune', 'tire']

The second type of data structure that could be returned is a list of tuples. If you did specify a group and there was a match, then findall() returns a list of tuples. Example 3-18 is a simple example of such a pattern and a string.

Example 3-18. Simple grouped group with findall()

In [1]: import re

In [2]: re_obj = re.compile(r"""(A\W+\b(big|small)\b\W+\b
...: (brown|purple)\b\W+\b(cow|dog)\b\W+\b(ran|jumped)\b\W+\b
...: (to|down)\b\W+\b(the)\b\W+\b(street|moon).*?\.)""",
...: re.VERBOSE)

In [3]: re_obj.findall('A big brown dog ran down the street. \
...: A small purple cow jumped to the moon.')

Out[3]:
[('A big brown dog ran down the street.',
'big',
'brown',
'dog',
'ran',
'down',
'the',
'street'),
('A small purple cow jumped to the moon.',
'small',
'purple',
'cow',
'jumped',
'to',
'the',
'moon')]

Though it is simple, this example shows some important points. First, notice that this simple pattern is ridiculously long and contains enough nonalphanumeric characters to make your eyes bleed if you stare at it for too long. That seems to be a common theme with many regular expressions. Next, notice that the pattern contains explicit nested groups. The outer group should match all the characters beginning with the letter “A” through to the ending period. The characters between the beginning A and the ending period make up inner groups that should match “big or small,” “brown or purple,” and so on. Next, the return value of findall() is a list of tuples. The elements of those tuples are each of the groups we specified in the regular expression. The entire sentence is the first element of the tuple as it is the largest, outermost group. Each of the subgroups is a subsequent element of the tuple. Finally, notice that the last argument to re.compile() was re.VERBOSE. This allowed us to write the regular expression string in verbose mode, which simply means that we were able to split the regular expression across lines without the split interfering with the pattern matching. Whitespace that fell outside of a class grouping was ignored. Though we chose not to do it here, verbose also allows us to insert comments at the end of each line of regex to document what each particular piece of a regular expression does. One of the difficulties of regular expressions in general is that the description of the pattern that you want to match often becomes huge and difficult to read. The re.VERBOSE function lets you write simpler regular expressions, so it is a great tool for improving the maintenance of code that includes regular expressions.

A slight variation of findall() is finditer(). Rather than returning a list of tuples as findall()does, finditer() returns an iterator, as its name implies. Each item of the iterator is a regular expression match object, which we’ll discuss later in this chapter. Example 3-19 is the same simple example using finditer() rather than findall().

Example 3-19. finditer() example

In [4]: re_iter = re_obj.finditer('A big brown dog ran down the street. \
...: A small purple cow jumped to the moon.')

In [5]: re_iter

Out[5]: <callable-iterator object at 0xa17ad0>

In [6]: for item in re_iter:
...:     print item
...:     print item.groups()
...:
<_sre.SRE_Match object at 0x9ff858>
('A big brown dog ran down the street.', 'big', 'brown', 'dog', 'ran',
'down', 'the', 'street')
<_sre.SRE_Match object at 0x9ff940>
('A small purple cow jumped to the moon.', 'small', 'purple', 'cow',
'jumped', 'to', 'the', 'moon')

If you have never encountered iterators before, you can think of them as similar to lists that are built as they are needed. One reason this definition is flawed is that you can’t refer to a specific item in the iterator by its index, as you can some_list[3] for a list. One consequence of this limitation is that you don’t have the ability to slice iterators, as you can some_list[2:6] for a list. Regardless of this limitation, though, iterators are lightweight and powerful, particularly when you only need to iterate over some sequence, because the entire sequence is not loaded up into memory but is retrieved on demand. This allows an iterator to have a smaller memory footprint than its corresponding list counterpart. It also means an iterator will start up with a shorter wait time for accessing the items in the sequence.

Another reason to use finditer() rather than findall() is that each item of finditer() is a match object rather than just a simple list of strings or list of tuples corresponding to the text that matched.

Match() and search() provide similar functionality to one another. Both apply a regular expression to a string; both specify where in the string to start and end looking for the pattern; and both return a match object for the first match of the specified pattern. The difference between them is that match() starts trying to match at the beginning of the string at the place within the string where you specified it should start looking and does not move to random places within the string, but search(), however, will try to match the pattern anywhere in the string or from the place within the string that you tell it to start, ending at the place within the string where you told it to finish. See Example 3-20.

Example 3-20. Comparison of match() and search()

In [1]: import re

In [2]: re_obj = re.compile('FOO')

In [3]: search_string = ' FOO'

In [4]: re_obj.search(search_string)

Out[4]: <_sre.SRE_Match object at 0xa22f38>

In [5]: re_obj.match(search_string)

In [6]:

Even though search_string contains the pattern that match() was looking for, it failed to turn up a match because the substring of search_string that would have turned up a match didn’t start at the beginning of search_string. The search() call turned up a match object.

Search() and match() calls accept start and end parameters that specify the places in a string at which Python should start and end looking for a pattern. See Example 3-21.

Example 3-21. Start and end parameters for search() and match()

In [6]: re_obj.search(search_string, pos=1)

Out[6]: <_sre.SRE_Match object at 0xabe030>

In [7]: re_obj.match(search_string, pos=1)

Out[7]: <_sre.SRE_Match object at 0xabe098>

In [8]: re_obj.search(search_string, pos=1, endpos=3)

In [9]: re_obj.match(search_string, pos=1, endpos=3)

In [10]:

The parameter pos is an index that specifies the place in the string where Python should look for the pattern. Specifying the start parameter pos for search() didn’t change anything; but specifying pos for match() caused it to match the pattern it failed to match without the pos parameter. Setting the end parameter endpos to 3 caused both search() and match() to fail to match the pattern because the pattern begins after the third character position.

As findall() and finditer() answer the question, “What did my pattern match?,” a major question that search() and match() answer is, “Did my pattern match?” Search() and match() also answer the question, “What first thing did my pattern match?,” but often, the thing you really want to know is, “Did my pattern match?” For example, let’s say you are writing code to read in logfiles and wrap each line in HTML so that it displays nicely. You want all “ERROR” lines to display in red, so you would probably loop through each line in the file, check it against a regular expression, and, if search() turned up a hit on its “ERROR” search, you would format the line to display in red.

Search() and match() are beneficial, not only because they indicate whether a pattern matched a piece of text; they also return a match() object. Match() objects contain various pieces of data that can come in handy when you’re walking through pieces of text. Particularly interesting match() methods include start(), end(), span(), groups(), and groupdict().

Start(), end(), and span() specify the places in the searched string that the matched pattern begins and ends. Start() returns an integer that identifies the position in the string at which the pattern match begins. End() returns an integer that identifies the position at which the pattern match ends. And span() returns a tuple containing the beginning and end of the match.

Groups() returns a tuple of the match, each element of which is a group that the pattern specified. This tuple is similar to each tuple in the list that findall() returns. Groupdict() returns a dictionary of named groups in which the names are found in the regular expression itself using the (?P<group_name>pattern) syntax.

In summary, to use regular expressions effectively, it is important to get in to the habit of using compiled regular expression objects. Use findall() and finditer() when you want to see what elements your pattern matched in a piece of text. Remember that finditer() is more flexible than findall() since it returns an iterator of match objects. For a more detailed overview of the regular expression library, see Chapter 9 of Python in a Nutshell by Alex Martelli (O’Reilly). To see regular expressions in action, see Data Crunching by Greg Wilson (The Pragmatic Bookshelf).

Apache Config File Hacking

Now that you’ve been introduced to Python regular expressions, let’s work through an Apache config file:

NameVirtualHost 127.0.0.1:80
<VirtualHost localhost:80>
DocumentRoot /var/www/
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
ErrorLog /var/log/apache2/error.log
LogLevel warn
CustomLog /var/log/apache2/access.log combined
ServerSignature On
</VirtualHost>
<VirtualHost local2:80>
DocumentRoot /var/www2/
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
ErrorLog /var/log/apache2/error2.log
LogLevel warn
CustomLog /var/log/apache2/access2.log combined
ServerSignature On
</VirtualHost>

This is a slightly modified config file from a stock Apache 2 installation on Ubuntu. We created named virtual hosts so that we could have something to work with. We also modified the /etc/hosts file so that it contains this line:

127.0.0.1    local2

This allows us to point a browser on that box at local2 and have it resolve to 127.0.0.1, which is a localhost. So, what is the point of this? If you go to http://local2, your browser will pass the hostname along in an HTTP request. Here is an HTTP request to local2:

GET / HTTP/1.1
Host: local2
User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.8.1.13)
Gecko/20080325 Ubuntu/7.10 (gutsy) Firefox/2.0.0.13
Accept: text/xml,application/xml,application/xhtml+xml,text/html
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
If-Modified-Since: Tue, 15 Apr 2008 17:25:24 GMT
If-None-Match: "ac5ea-53-44aecaf804900"
Cache-Control: max-age=0

Notice the line starting with Host:. When Apache gets this request, it routes it to the virtual host that matches the local2 name.

So, what we want to do is to write a script that parses through an Apache config file, like the one we just presented, finds a specified VirtualHost section, and replaces the DocumentRoot for that VirtualHost. This script does just that:

#!/usr/bin/env python

from cStringIO import StringIO
import re

vhost_start = re.compile(r'<VirtualHost\s+(.*?)>')
vhost_end = re.compile(r'</VirtualHost')
docroot_re = re.compile(r'(DocumentRoot\s+)(\S+)')

def replace_docroot(conf_string, vhost, new_docroot):
'''yield new lines of an httpd.conf file where docroot lines matching
the specified vhost are replaced with the new_docroot
'''
conf_file = StringIO(conf_string)
in_vhost = False
curr_vhost = None
for line in conf_file:
vhost_start_match = vhost_start.search(line)
if vhost_start_match:
curr_vhost = vhost_start_match.groups()[0]
in_vhost = True
if in_vhost and (curr_vhost == vhost):
docroot_match = docroot_re.search(line)
if docroot_match:
sub_line = docroot_re.sub(r'\1%s' % new_docroot, line)
line = sub_line
vhost_end_match = vhost_end.search(line)
if vhost_end_match:
in_vhost = False
yield line

if __name__ == '__main__':
import sys
conf_file = sys.argv[1]
vhost = sys.argv[2]
docroot = sys.argv[3]
conf_string = open(conf_file).read()
for line in replace_docroot(conf_string, vhost, docroot):
print line,

This script initially sets up three compiled regular expression objects: one to match the start of the VirtualHost, one to match the end of the VirtualHost, and one to match the DocumentRoot line. We also created a function to do the dirty work for us. The function is named replace_docroot() and it takes as its arguments the string body of the config file, the name of the VirtualHost to match, and the DocumentRoot to which we want to point the VirtualHost. The function sets up a state machine that checks to see if we are in a VirtualHost section. It also keeps track of the VirtualHost in which it is contained. When it is in the VirtualHost that the calling code specified, this function looks for any occurrence of the DocumentRoot directive and changes that directive’s directory to the one the calling code specified. As replace_docroot() iterates over each line in the config file, it yields either the unmodified input line or the modified DocumentRoot line.

We created a simple command-line interface to this function. It isn’t anything fancy that uses optparse, nor does it do error checking on the number of arguments that you give it, but it’s functional. Now we’ll run the script on the same Apache config file we presented earlier, and change VirtualHost local2:80 to use /tmp as its VirtualHost. This command-line interface prints out the lines from the function replace_docroot() rather than writing them to a file:

jmjones@dinkgutsy:code$ python apache_conf_docroot_replace.py
/etc/apache2/sites-available/psa
local2:80 /tmp
NameVirtualHost 127.0.0.1:80
<VirtualHost localhost:80>
DocumentRoot /var/www/
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
ErrorLog /var/log/apache2/error.log
LogLevel warn
CustomLog /var/log/apache2/access.log combined
ServerSignature On
</VirtualHost>
<VirtualHost local2:80>
DocumentRoot /tmp
<Directory />
Options FollowSymLinks
AllowOverride None
</Directory>
ErrorLog /var/log/apache2/error2.log
LogLevel warn
CustomLog /var/log/apache2/access2.log combined
ServerSignature On
</VirtualHost>

The only line that is different is the DocumentRoot line from the local2:80 VirtualHost section. Here is a difference of the two after we redirected the output of the script to a file:

jmjones@dinkgutsy:code$ diff apache_conf.diff /etc/apache2/sites-available/psa
20c20
<       DocumentRoot /tmp
---
>       DocumentRoot /var/www2/

Modifying an Apache config file to change the DocumentRoot is a very simple task, but if you have to change the document root often, or if you have many virtual hosts that you need to vary, it’s worth writing a script like the one we just wrote. However, this was a pretty simple script to create. It would be pretty simple to modify the script to comment out a VirtualHost section, change the LogLevel directive, or change the place to which the VirtualHost will log.

Working with Files

Learning to deal with files is key to processing textual data. Often, text that you have to process is contained in a text file such as a logfile, config file, or application data file. When you need to consolidate the data that you are analyzing, you often need to create a report file of some sort or put it into a text file for further analysis. Fortunately, Python contains an easy-to-use built-in type called file that can help you do all of those things.

Creating files

It may seem counterintuitive, but in order to read an existing file, you have to create a new file object. But don’t confuse creating a new file object with creating a new file. Writing to a file requires that you create a new file object and might require that you create a new file on disk, so it may be less counterintuitive than creating a file object for reading would be. The reason that you create a file object is so that you can interact with that file on disk.

In order to create a file object, you use the built-in function open(). Here is an example of code that opens a file for reading:

In [1]: infile = open("foo.txt", "r")

In [2]: print infile.read()
Some Random
Lines
Of
Text.

Because open is built-in, you don’t need to import a module. Open() takes three parameters: a filename, the mode in which the file should be opened, and a buffer size. Only the first parameter, filename, is mandatory. The most common values for mode, are “r” (read mode; this is the default), “w” (write mode), and “a” (append mode). A complementary mode that can be added to the other modes is “b,” or binary mode. The third parameter, buffer size, directs the operating the way to buffer the file.

In the previous example, we specified that we would like to open() the “file foo.txt” in read mode and be able to refer to that new readable file object with the variable infile. Once we have infile, we are free to call the read() method on it, which reads the entire contents of the file.

Creating a file for writing is very similar to the way we created the file for reading. Instead of using an "r" flag, you use a "w" flag:

In [1]: outputfile = open("foo_out.txt", "w")

In [2]: outputfile.write("This is\nSome\nRandom\nOutput Text\n")

In [3]: outputfile.close()

In this example, we specified that we would like to open() the file “foo_out.txt” in write mode and be able to refer to that new writable file object with the variable outputfile. Once we have outputfile, we can write() some text to it and close() the file.

While these are the simple ways of creating files, you probably want to get in the habit of creating files in a more error-tolerant way. It is good practice to wrap your file opens with a try/finally block, especially when you are using write() calls. Here is an example of a writeable file wrapped in a try/finally block:

In [1]: try:
...:     f = open('writeable.txt', 'w')
...:     f.write('quick line here\n')
...: finally:
...:     f.close()

This way of writing files causes the close() method to be called when an exception happens somewhere in the try block. Actually, it lets the close() method be closed even when no exception occurred in the try block. Finally blocks are executed after try blocks complete, whether an exception is found or not.

A new idiom in Python 2.5 is the with statement, which lets you use context managers. A context manager is simply an object with an __enter__() and __exit__(). When an object is created in the with expression, the context manager’s __enter__() method is called. When the with block completes, even if an exception occurs, the context manager’s __exit__() is called. File objects have __enter__() and __exit__() methods defined. On __exit__(), the file object’s close() method is called. Here is an example of the with statement:

In [1]: from __future__ import with_statement

In [2]: with open('writeable.txt', 'w') as f:
...:     f.write('this is a writeable file\n')
...:
...:

Even though we didn’t call close() on file object f, the context manager closes it after exiting the with block:

In [3]: f
Out[3]: <closed file 'writeable.txt', mode 'w' at 0x1382770>

In [4]: f.write("this won't work")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/Users/jmjones/<ipython console> in <module>()

ValueError: I/O operation on closed file

As we expected, the file object is closed. While it is a good practice to handle possible exceptions and make sure your file objects are closed when you expect them to be, for the sake of simplicity and clarity, we will not do so for all examples.

For a complete list of the methods available on file objects, see the File Objects section of Python Library Reference at http://docs.python.org/lib/bltin-file-objects.html.

Reading files

Once you have a readable file object, which you opened with the r flag, there are three common file methods that will prove useful for getting data contained in the file: read(), readline(), and readlines(). Read(), not surprisingly, reads data from an open file object, returns the bytes that it has read, and returns a string object of those bytes. Read() takes an optional bytes parameter, which specifies the number of bytes to read. If no bytes are specified, read() tries to read to the end of the file. If more bytes are specified than there are bytes in the file, read() will read until the end of the file and return the bytes which it has read.

Given the following file:

jmjones@dink:~/some_random_directory$ cat foo.txt Some Random
Lines
Of
Text.

Read() works on a file like this.

In [1]: f = open("foo.txt", "r")

In [2]: f.read()
Out[2]: 'Some Random\n    Lines\nOf \n    Text.\n'

Notice that the newlines are shown as a \n character sequence; that is the standard way to refer to a newline.

And if we only wanted the first 5 bytes of the file, we could do something like this:

In [1]: f = open("foo.txt", "r")

In [2]: f.read(5)
Out[2]: 'Some '

The next method for getting text from a file is the readline() method. The purpose of readline() is to read one line of text at a time from a file. Readline() takes one optional parameter: size. Size specifies the maximum number of bytes that readline() will read before returning a string, whether it has reached the end of the line or not. So, in the following example, the program will read the first line of text from the foo.txt, and then it will read the first 7 bytes of text from the second line, followed by the remainder of the second line:

In [1]: f = open("foo.txt", "r")

In [2]: f.readline()
Out[2]: 'Some Random\n'

In [3]: f.readline(7)
Out[3]: '    Lin'

In [4]: f.readline()
Out[4]: 'es\n'

The final file method that we will discuss for getting text out of a file is readlines(). Readlines() is not a typo, nor is it a cut-and-paste error from the previous example. Readlines() reads in all of the lines in a file. Well, that is almost true. Readlines() has a sizehint option that specifies the approximate total number of bytes to read in. In the following example, we created a file, biglines.txt, that contains 10,000 lines, each of which contains 80 characters. We then open the file, state that we want the first N lines in the file, which will total about 1024 bytes (the number of lines and bytes that were read) and then we read the rest of the lines in the file:

In [1]: f = open("biglines.txt", "r")

In [2]: lines = f.readlines(1024)

In [3]: len(lines)
Out[3]: 102

In [4]: len("".join(lines))
Out[4]: 8262

In [5]: lines = f.readlines()

In [6]: len(lines)
Out[6]: 9898

In [7]: len("".join(lines))
Out[7]: 801738

Command [3] shows that we read 102 lines and command [4] shows that those lines totaled 8,262 bytes. How is 1,024 the “approximate” number of bytes read if the actual number of bytes read was 8,262? It rounded up to the internal buffer size, which is about 8 KB. The point is that sizehint does not always do what you think it might, so it’s something to keep in mind.

Writing files

Sometimes you have to do something with files other than just reading data in from them; sometimes you have to create your own file and write data out to it. There are two common file methods that you will need to know in order to write data to files. The first method, which was demonstrated earlier, is write(). write() takes one parameter: the string to write to the file. Here is an example of data being written to a file using the write() method:

In [1]: f = open("some_writable_file.txt", "w")

In [2]: f.write("Test\nFile\n")

In [3]: f.close()

In [4]: g = open("some_writable_file.txt", "r")

In [5]: g.read()
Out[5]: 'Test\nFile\n'

In command [1], we opened the file with the w mode flag, which means writable. Command [2] writes two lines to the file. In command [4], we are using the variable name g this time for the file object to cut down on confusion, although we could have used f again. And command [5] shows that the data we wrote to the file is the same as what comes out when we read() it again.

The next common data writing method is writelines(). Writelines() takes one mandatory parameter: a sequence that writelines() will write to the open file. The sequence can be any type of iterable object such as a list, tuple, list comprehension (which is a list), or a generator. Here is an example of a generator expression writelines() used with writelines to write data to a file:

In [1]: f = open("writelines_outfile.txt", "w")

In [2]: f.writelines("%s\n" % i for i in range(10))

In [3]: f.close()

In [4]: g = open("writelines_outfile.txt", "r")

In [5]: g.read()

Out[5]: '0\n1\n2\n3\n4\n5\n6\n7\n8\n9\n'

And here is an example of a generator function being used to write data to a file (this is functionally equivalent to the previous example, but it uses more code):

In [1]: def myRange(r):
...:     i = 0
...:     while i < r:
...:         yield "%s\n" % i
...:         i += 1
...:
...:

In [2]: f = open("writelines_generator_function_outfile", "w")

In [3]: f.writelines(myRange(10))

In [4]: f.close()

In [5]: g = open("writelines_generator_function_outfile", "r")

In [6]: g.read()
Out[6]: '0\n1\n2\n3\n4\n5\n6\n7\n8\n9\n'

It is important to note that writelines() does not write a newline (\n) for you; you have to supply the \n in the sequence you pass in to it. It’s also important to know you don’t have to use it only to write line-based information to your file. Perhaps a better name would have been something like writeiter(). In the previous examples, we happened to write text that had a newline, but there is no reason that we had to.

Additional resources

For more information on file objects, please see Chapter 7 of Learning Python by David Ascher and Mark Lutz (O’Reilly) (also online in Safari at http://safari.oreilly.com/0596002815/lpython2-chp-7-sect-2) or the File Objects section of the Python Library Reference (available online at http://docs.python.org/lib/bltin-file-objects.html).

For more information on generator expressions, please see the “generator expressions” section of the Python Reference Manual (available online at http://docs.python.org/ref/genexpr.html). For more information on the yield statement, see the “yield statement” section of the Python Reference Manual (available online at http://docs.python.org/ref/yield.html).

Standard Input and Output

Reading text on a process’s standard input and writing to a process’s standard output will be familiar to most system administrators. Standard input is simply data going into a program that the program can read when it runs. Standard output is the output of a program, written there by the program as it is running. A benefit of using standard input and standard output is that it allows commands to be chained together with other utilities.

The Python Standard Library contains a built-in module named sys that provides easy access to standard input and standard output. The standard library provides access to both standard input and output as file-like objects, even though they are not directly connected to a file on disk. Since they are file-like objects, you can use the same methods on them that you can use on files. You can treat them as though they were files on disk and access the appropriate methods for doing so.

Standard input is accessed by importing the sys module and referring to its stdin attribute (sys.stdin). Sys.stdin is a readable file object. Notice what happens when we create a “real” file object by opening a file on disk called foo.txt and then compare that open file object with sys.stdin:

In [1]: import sys

In [2]: f = open("foo.txt", "r")

In [3]: sys.stdin
Out[3]: <open file '<stdin>', mode 'r' at 0x14020>

In [4]: f
Out[4]: <open file 'foo.txt', mode 'r' at 0x12179b0>

In [5]: type(sys.stdin) == type(f)
Out[5]: True

The Python interpreter sees them as the same type, so they use the same methods. While they are technically the same type and use the same methods, some of the methods will behave differently on the file-like objects. For example, sys.stdin.seek() and sys.stdin.tell() are available, but they raise an exception (specifically IOError) when you call them. The main point here, though, is that they are file-like objects and you can pretty much just treat them the same as you would disk-based files.

Accessing sys.stdin is pretty much meaningless at a Python (or IPython) prompt. Importing sys and doing sys.stdin.read() just blocks indefinitely. In order to show you how sys.stdin works, we’ve created a script that reads from sys.stdin() and prints each line back out with a corresponding line number. See Example 3-22.

Example 3-22. Enumerating sys.stdin.readline

#!/usr/bin/env python

import sys

counter = 1
while True:
line = sys.stdin.readline()
if not line:
break
print "%s: %s" % (counter, line)
counter += 1

In this example, we created the variable counter to keep track of the line it is on. It then enters a while loop and begins reading lines from standard input. For each line, it prints out the line number and the line contents. As the program loops, this script deals with all lines that come in, even if they seem to be blank. And blank lines aren’t totally blank, of course; they consist of a newline (\n). When the script hits “end of file,” this script breaks out of the loop.

Here is the output when who is piped through the previous script:

jmjones@dink:~/psabook/code$ who | ./sys_stdin_readline.py
1: jmjones  console  Jul  9 11:01

2: jmjones  ttyp1    Jul  9 19:58

3: jmjones  ttyp2    Jul 10 05:10

4: jmjones  ttyp3    Jul 11 11:51

5: jmjones  ttyp4    Jul 13 06:48

6: jmjones  ttyp5    Jul 11 21:49

7: jmjones  ttyp6    Jul 15 04:38

As a point of interest, the previous example could have been written much more simply and shorter using the enumerate function. See Example 3-23.

Example 3-23. sys.stdin readline example

#!/usr/bin/env python

import sys

for i, line in enumerate(sys.stdin):
print "%s: %s" % (i, line)

Just as you access standard input by importing the sys module and then using the stdin attribute, you access standard output by importing the sys module and referring to the stdout attribute. And just as sys.stdin is a readable file object, sys.stdout is a writable file object. And just as sys.stdin has the same type as a readable file object, so sys.stdout has the same type as a writable file object:

In [1]: import sys

In [2]: f = open('foo.txt', 'w')

In [3]: sys.stdout
Out[3]: <open file '<stdout>', mode 'w' at 0x14068>

In [4]: f
Out[4]: <open file 'foo.txt', mode 'w' at 0x1217968>

In [5]: type(sys.stdout) == type(f)
Out[5]: True

As a relevant aside, this last point is not unexpected since a readable file and a writable file also share the same type:

In [1]: readable_file = open('foo.txt', 'r')

In [2]: writable_file = open('foo_writable.txt', 'w')

In [3]: readable_file
Out[3]: <open file 'foo.txt', mode 'r' at 0x1243530>

In [4]: writable_file
Out[4]: <open file 'foo_writable.txt', mode 'w' at 0x1217968>

In [5]: type(readable_file) == type(writable_file)
Out[5]: True

The important thing to know about the type that sys.stdout has is that it can be treated in pretty much the same way as a writable file can be treated, just as sys.stdin can be treated as a readable file.

StringIO

So, what happens if you have written a text munging function which knows how to deal with a file object, but you stumble across a case in which data that you need to process is available as a text string rather than a file? An easy solution is that you can use import StringIO:

In [1]: from StringIO import StringIO

In [2]: file_like_string = StringIO("This is a\nmultiline string.\n
readline() should see\nmultiple lines of\ninput")

In [3]: file_like_string.readline()
Out[3]: 'This is a\n'

In [4]: file_like_string.readline()
Out[4]: 'multiline string.\n'

In [5]: file_like_string.readline()
Out[5]: 'readline() should see\n'

In [6]: file_like_string.readline()
Out[6]: 'multiple lines of\n'

In [7]: file_like_string.readline()
Out[7]: 'input'

In this example, we created a StringIO object passing the string This is a\nmultiline string. \nreadline() should see\nmultiple lines of\ninput into the constructor. We were then able to call the readline() method on the StringIO object. While readline() was the only method we called, it is by no means the only file method available:

In [8]: dir(file_like_string)
Out[8]:
['__doc__',
'__init__',
'__iter__',
'__module__',
'buf',
'buflist',
'close',
'closed',
'flush',
'getvalue',
'isatty',
'len',
'next',
'pos',
'read',
'readline',
'readlines',
'seek',
'softspace',
'tell',
'truncate',
'write',
'writelines']

To be sure there are differences, but the interface allows an easy transition between files and strings. Here is a comparison of the methods and attributes on a file with the methods and attributes on a StringIO object:

In [9]: f = open("foo.txt", "r")

In [10]: from sets import Set

In [11]: sio_set = Set(dir(file_like_string))

In [12]: file_set = Set(dir(f))

In [13]: sio_set.difference(file_set)
Out[13]: Set(['__module__', 'buflist', 'pos', 'len', 'getvalue', 'buf'])

In [14]: file_set.difference(sio_set)
Out[14]: Set(['fileno', '__setattr__', '__reduce_ex__', '__new__', 'encoding',
'__getattribute__', '__str__', '__reduce__', '__class__', 'name',
'__delattr__', 'mode', '__repr__', 'xreadlines', '__hash__', 'readinto',
'newlines'])

So, as you can see, if you need to treat a string as a file, StringIO can be a huge help.

urllib

What if the file you are interested in reading happens to be on the interweb? Or, what if you want to reuse a piece of code that you wrote which expects a file object? The built-in file type doesn’t know about the interweb, but the urllib module can help.

If all you want to do is read() a file from some web server somewhere, urllib.urlopen() provides an easy solution. Here is a simple example:

In [1]: import urllib

In [2]: url_file = urllib.urlopen("http://docs.python.org/lib/module-urllib.html")

In [3]: urllib_docs = url_file.read()

In [4]: url_file.close()

In [5]: len(urllib_docs)
Out[5]: 28486

In [6]: urllib_docs[:80]
Out[6]: '<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">\
n<html>\n<head>\n<li'

In [7]: urllib_docs[-80:]
Out[7]: 'nt...</a></i> for information on suggesting changes.\
n</address>\n</body>\n</html>\n'

First, we imported urllib. Next, we created a file-like object from urllib and named it url_file. Then, we read the contents of url_file into a string called urllib_docs. And just to show that we actually retrieved something that looks like it might have come from the Internet, we sliced the first and last 80 characters from the retrieved document. Notice that the urllib file object supported the read() and close() methods. It also supports readline(), readlines(), fileno(), info(), and geturl().

If you need more power, such as the ability to use a proxy server, you can find more information about urllib at http://docs.python.org/lib/module-urllib.html. Or if you need even more power like digest authentication and cookies, check out urllib2 at http://docs.python.org/lib/module-urllib2.html.

Log Parsing

No discussion of text processing from a sysadmin’s point of view would be complete without addressing parsing a logfile, so here it is. We have laid the foundation for you to be able to open a logfile, read in each line, and read the data in the way that works best for you. Before we begin coding this example, we have to ask ourselves, “What do we want this logfile reader to do?” Our answer is pretty simple: read in an Apache access log and determine the number of bytes each unique client retrieved.

According to http://httpd.apache.org/docs/1.3/logs.html, the “combined” log format looks something like this:

127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0"
200 2326 "http://www.example.com/start.html" "Mozilla/4.08 [en] (Win98; I
;Nav)"

And this matched the data in our Apache logfile. The two pieces of information from each line of the logfile that we will be interested in are the IP address of the client and the number of bytes that were transferred. The IP address is the first field in the logfile; in this case, the address is 127.0.0.1. The number of bytes that were transferred is the second from the last field, right before the referrer; in this case 2326 bytes were transferred. So how do we get at the fields? See Example 3-24.

Example 3-24. Apache logfile parser—split on whitespace

#!/usr/bin/env python
"""
USAGE:

apache_log_parser_split.py some_log_file

This script takes one command line argument: the name of a log file
to parse. It then parses the log file and generates a report which
associates remote hosts with number of bytes transferred to them.
"""

import sys

def dictify_logline(line):
'''return a dictionary of the pertinent pieces of an apache combined log file

Currently, the only fields we are interested in are remote host and bytes sent,
but we are putting status in there just for good measure.
'''
split_line = line.split()
return {'remote_host': split_line[0],
'status': split_line[8],
'bytes_sent': split_line[9],
}

def generate_log_report(logfile):
'''return a dictionary of format remote_host=>[list of bytes sent]

This function takes a file object, iterates through all the lines in the file,
and generates a report of the number of bytes transferred to each remote host
for each hit on the webserver.
'''
report_dict = {}
for line in logfile:
line_dict = dictify_logline(line)
print line_dict
try:
bytes_sent = int(line_dict['bytes_sent'])
except ValueError:
##totally disregard anything we don't understand
continue
report_dict.setdefault(line_dict['remote_host'], []).append(bytes_sent)
return report_dict

if __name__ == "__main__":
if not len(sys.argv) > 1:
print __doc__
sys.exit(1)
infile_name = sys.argv[1]
try:
infile = open(infile_name, 'r')
except IOError:
print "You must specify a valid file to parse"
print __doc__
sys.exit(1)
log_report = generate_log_report(infile)
print log_report
infile.close()

This example is pretty simple. The __main__ section does only a few things. First, it does minimal checking on the command-line arguments to ensure that at least one argument was passed in. If the user passed in no arguments on the command line, the script prints a usage message and terminates. For a fuller discussion of how to better handle command-line arguments and parameters, see Chapter 13. Next, __main__ attempts to open the specified logfile. If it fails to open the logfile, it prints a usage message and terminates. Next, it passes the logfile to the generate_log_report() function and prints the results.

Generate_log_report() creates a dictionary that serves as the report. It then iterates over all the lines of the logfile and passes each line to dictify_logline(), which returns a dictionary that contains the information we needed. Then, it checks to see if the bytes_sent value is an integer. If it is, it proceeds; if the bytes_sent value is not an integer, it continues to the next line. After that, it updates the report dictionary with the data that dictify_logline() returned to it. Finally, it returns the report dictionary to the __main__ section.

Dictify_logline() simply splits the log line on whitespace, pulls certain items from the resulting list, and returns a dictionary with the data from the split line.

So, does it work? Mostly. Check out the unit test in Example 3-25.

Example 3-25. Unit test for Apache logfile parser—split on whitespace

#!/usr/bin/env python

import unittest
import apache_log_parser_split

class TestApacheLogParser(unittest.TestCase):

def setUp(self):
pass

def testCombinedExample(self):
# test the combined example from apache.org
combined_log_entry = '127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /apache_pb.gif HTTP/1.0" 200 2326 "http://www.example.com/start.html" '\
'"Mozilla/4.08 [en] (Win98; I ;Nav)"'
self.assertEqual(apache_log_parser_split.dictify_logline(combined_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

def testCommonExample(self):
# test the common example from apache.org
common_log_entry = '127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /apache_pb.gif HTTP/1.0" 200 2326'
self.assertEqual(apache_log_parser_split.dictify_logline(common_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

def testExtraWhitespace(self):
# test for extra whitespace between fields
common_log_entry = '127.0.0.1     -     frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /apache_pb.gif HTTP/1.0" 200 2326'
self.assertEqual(apache_log_parser_split.dictify_logline(common_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

def testMalformed(self):
# test for extra whitespace between fields
common_log_entry = '127.0.0.1     -     frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /some/url/with white space.html HTTP/1.0" 200 2326'
self.assertEqual(apache_log_parser_split.dictify_logline(common_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

if __name__ == '__main__':
unittest.main()

It works with the combined and common log formats, but a slight modification of the request field causes the unit test to fail. Here is the result of a test run:

jmjones@dinkgutsy:code$ python test_apache_log_parser_split.py
...F
======================================================================
FAIL: testMalformed (__main__.TestApacheLogParser)
----------------------------------------------------------------------
Traceback (most recent call last):
File "test_apache_log_parser_split.py", line 38, in testMalformed
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})
AssertionError: {'status': 'space.html', 'bytes_sent': 'HTTP/1.0"',
'remote_host': '127.0.0.1'} != {'status': '200', 'bytes_sent': '2326',
'remote_host': '127.0.0.1'}

----------------------------------------------------------------------
Ran 4 tests in 0.001s

FAILED (failures=1)

Because of one colon in the date field converted to a space, all the fields of this logfile were shifted one place to the right. A healthy level of paranoia is a good thing. But based on the specification for the logfile formats, you’re probably pretty safe extracting the remote host and number of bytes that were transferred based on whitespace-separated fields. However, Example 3-26 is the same example using regular expressions.

Example 3-26. Apache logfile parser—regex

#!/usr/bin/env python
"""
USAGE:

apache_log_parser_regex.py some_log_file

This script takes one command line argument: the name of a log file
to parse. It then parses the log file and generates a report which
associates remote hosts with number of bytes transferred to them.
"""

import sys
import re
log_line_re = re.compile(r'''(?P<remote_host>\S+) #IP ADDRESS
\s+ #whitespace
\S+ #remote logname
\s+ #whitespace
\S+ #remote user
\s+ #whitespace
\[[^\[\]]+\] #time
\s+ #whitespace
"[^"]+" #first line of request
\s+ #whitespace
(?P<status>\d+)
\s+ #whitespace
(?P<bytes_sent>-|\d+)
\s* #whitespace
''', re.VERBOSE)

def dictify_logline(line):
'''return a dictionary of the pertinent pieces of an apache combined log file

Currently, the only fields we are interested in are remote host and bytes sent,
but we are putting status in there just for good measure.
'''
m = log_line_re.match(line)
if m:
groupdict = m.groupdict()
if groupdict['bytes_sent'] == '-':
groupdict['bytes_sent'] = '0'
return groupdict
else:
return {'remote_host': None,
'status': None,
'bytes_sent': "0",
}

def generate_log_report(logfile):
'''return a dictionary of format remote_host=>[list of bytes sent]

This function takes a file object, iterates through all the lines in the file,
and generates a report of the number of bytes transferred to each remote host
for each hit on the webserver.
'''
report_dict = {}
for line in logfile:
line_dict = dictify_logline(line)
print line_dict
try:
bytes_sent = int(line_dict['bytes_sent'])
except ValueError:
##totally disregard anything we don't understand
continue
report_dict.setdefault(line_dict['remote_host'], []).append(bytes_sent)
return report_dict

if __name__ == "__main__":
if not len(sys.argv) > 1:
print __doc__
sys.exit(1)
infile_name = sys.argv[1]
try:
infile = open(infile_name, 'r')
except IOError:
print "You must specify a valid file to parse"
print __doc__
sys.exit(1)
log_report = generate_log_report(infile)
print log_report
infile.close()

The only function we changed from the regex example to the “split on whitespace” example was dictify_logline(). This implies that we left the return type for the function exactly as it was in the regex example. Rather than splitting the log line on whitespace, we used a compiled regular expression object, log_line_re, to match() the log line. If it matched, we returned a potentially, slightly modified groupdict() in which bytes_sent was set to 0 when the field contained - (because - means nothing). In the case that nothing matched, we returned a dictionary with the same keys, but with None and 0 for the values.

So, does the regular expression version work work better than the string splitting one? Actually, it does. Here is a unit test for the new regex version of the Apache parsing script:

#!/usr/bin/env python

import unittest
import apache_log_parser_regex

class TestApacheLogParser(unittest.TestCase):

def setUp(self):
pass

def testCombinedExample(self):
# test the combined example from apache.org
combined_log_entry = '127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /apache_pb.gif HTTP/1.0" 200 2326 '\
'"http://www.example.com/start.html" "Mozilla/4.08 [en] (Win98; I ;Nav)"'
self.assertEqual(apache_log_parser_regex.dictify_logline(combined_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

def testCommonExample(self):
# test the common example from apache.org
common_log_entry = '127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /apache_pb.gif HTTP/1.0" 200 2326'
self.assertEqual(apache_log_parser_regex.dictify_logline(common_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

def testMalformedEntry(self):
# test a malformed modification dereived from the example at apache.org
#malformed_log_entry = '127.0.0.1 - frank [10/Oct/2000 13:55:36 -0700] '\
#'"GET /apache_pb.gif HTTP/1.0" 200 2326 '\
#'"http://www.example.com/start.html" "Mozilla/4.08 [en] (Win98; I ;Nav)"'

malformed_log_entry = '127.0.0.1 - frank [10/Oct/2000:13:55:36 -0700] '\
'"GET /some/url/with white space.html HTTP/1.0" 200 2326'
self.assertEqual(apache_log_parser_regex.dictify_logline(malformed_log_entry),
{'remote_host': '127.0.0.1', 'status': '200', 'bytes_sent': '2326'})

if __name__ == '__main__':
unittest.main()

And here is the result of the unit test:

jmjones@dinkgutsy:code$ python test_apache_log_parser_regex.py
...
----------------------------------------------------------------------
Ran 3 tests in 0.001s

OK

ElementTree

If the text that you need to parse is XML, then you probably want to approach things a bit differently than if it were, say, a line-oriented logfile. You probably don’t want to read the file in line by line and look for patterns, and you probably don’t want to rely too much on regular expressions. XML uses a tree-structure, so reading in lines probably isn’t what you want. And using regular expressions to build a tree data structure will be a huge headache for any files larger than trivially tiny.

So, what can you use? There are typically two approaches to handling XML. There is “simple API for XML,” or SAX. The Python Standard Library contains a SAX parser. SAX is typically blazingly fast and doesn’t automatically grab a lot of memory as it is parsing your XML. But it is callback-based, so in certain pieces of data, when it hits sections of your XML document, like start and end tags, it just calls certain methods and passes. This means that you have to set up handlers for your data and maintain your own state, which can be a difficult. These two things make the “simple” in “simple API for XML” seem a bit farfetched. The other approach for handling XML is to use a Document Object Model, or DOM. The Python Standard Library also contains a DOM XML library. DOM is typically slower and consumes more memory than SAX because it reads the whole XML tree into memory and builds objects for each node in the tree. The benefit of using DOM is that you don’t have to keep track of your own state, since each node is aware of who its parents and children are. But the DOM API is cumbersome at best.

A third option is ElementTree. ElementTree is an XML parsing library that has been included in the standard library since Python 2.5. ElementTree feels like a lighter weight DOM with a usable, indeed a friendly, API. In addition to the code usability, it is fast and consumes little memory. We give ElementTree a hearty recommendation. If you have XML parsing to do, try ElementTree first.

To start parsing an XML file using ElementTree, simply import the library and parse() a file:

In [1]: from xml.etree import ElementTree as ET

In [2]: tcusers = ET.parse('/etc/tomcat5.5/tomcat-users.xml')

In [3]: tcusers

Out[3]: <xml.etree.ElementTree.ElementTree instance at 0xabb4d0></xml>

So that we could save keystrokes as we use the library, we imported the ElementTree module under the name ET so that we could save on keystrokes as we use the library. Then, we told ElementTree to parse the users XML file from an installed Tomcat servlet engine. We called the ElementTree object tcusers. The type of tcusers is xml.etree.ElementTree.ElementTree.

We removed the license and a usage note, and the Tomcat users file that we just parsed has the following content:

<?xml version="1.0" encoding="UTF-8"?>
<tomcat-users>
<user name="tomcat" password="tomcat" roles="tomcat" />
<user name="role1"  password="tomcat" roles="role1"  />
<user name="both"   password="tomcat" roles="tomcat,role1" />
</tomcat-users>

When ElementTree parsed the Tomcat XML file, it created a tree object, which we referred to as tcusers, that we could use to get at the various nodes in the XML file. Two of the most interesting methods on this tree object are find() and findall(). Find() finds the first node that matches the query that you pass it and returns an Element object based on that node. Findall() finds all occurrences of nodes matching the query that you pass it and returns a list of Element objects based on those matching nodes.

The type of pattern that both find() and findall() look for is a limited subset of XPath expressions. Valid search criteria for ElementTree includes the tagname * to match all child elements, . to match the current element, and // to match all nodes that are descendents of the search starting point. The slash (/) character can be used to separate the match criteria. Using the Tomcat user file, we’ll use find() and a tagname to pull out the first user node:

In [4]: first_user = tcusers.find('/user')

In [5]: first_user

Out[5]: <Element user at abdd88>

We fed find() the search criteria '/user'. The leading slash character specified the absolute path starting at the root node. The text 'user' specified the tagname to look for. So, find() returned the first node with a tag of user. You can see that the object we referred to as first_user is of type Element.

Some of the more interesting Element methods and attributes include Attrib, find(), findall(), get(), tag, and text. attrib is a dictionary of the attributes of the Element that it belongs to. find() and findall() work here the same way they do on ElementTree objects. Get() is a dictionary method that retrieves the specified attribute or, if the attribute is not defined, returns None. Both attrib and get() access the same dictionary of attributes for the current XML tag. tag is an attribute that contains the tag name of the current Element. text is an attribute that contains the text contained as a text node in the current Element.

Here is the XML element ElementTree created for the first_user Element object:

<user name="tomcat" password="tomcat" roles="tomcat" />

Now we are going to call the methods of and reference the attributes of the tcusers object:

In [6]: first_user.attrib

Out[6]: {'name': 'tomcat', 'password': 'tomcat', 'roles': 'tomcat'}

In [7]: first_user.get('name')

Out[7]: 'tomcat'

In [8]: first_user.get('foo')

In [9]: first_user.tag

Out[9]: 'user'

In [10]: first_user.text

Now that you’ve seen some of the basics of what ElementTree will do, we’ll look at a slightly more involved, more formal example. We will parse the Tomcat users file and look for any user nodes where the name attribute matches the one we specify (in this case, 'tomcat'). See Example 3-27.

Example 3-27. ElementTree parse of Tomcat users file

#!/usr/bin/env python

from xml.etree import ElementTree as ET

if __name__ == '__main__':
infile = '/etc/tomcat5.5/tomcat-users.xml'
tomcat_users = ET.parse(infile)
for user in [e for e in tomcat_users.findall('/user') if
e.get('name') == 'tomcat']:
print user.attrib

The only trick in this example is that we’ve used a list comprehension to match on the name attribute. Running this example returns the following result:

jmjones@dinkgutsy:code$ python elementtree_tomcat_users.py
{'password': 'tomcat', 'name': 'tomcat', 'roles': 'tomcat'}

Finally, here is an example of ElementTree used to extract some information from a poorly written piece of XML. Mac OS X has a utility called system_profiler that will display a bulk of information about your system. XML is one of two output formats that system_profiler supports, but it appears that XML was an afterthought. The piece of information that we want to extract is the operating system version and is contained in a portion of the XML file that looks like this:

<dict>
<key>_dataType</key>
<string>SPSoftwareDataType</string>
<key>_detailLevel</key>
<integer>-2</integer>
<key>_items</key>
<array>
<dict>
<key>_name</key>
<string>os_overview</string>
<key>kernel_version</key>
<string>Darwin 8.11.1</string>
<key>os_version</key>
<string>Mac OS X 10.4.11 (8S2167)</string>
</dict>
</array>

So, why do we think this XML format is poorly written? There are no attributes on any XML tags. The tag types are mainly data types. And elements such as alternating key and string tags are under the same parent. See Example 3-28.

Example 3-28. Mac OS X system_profiler output parser

#!/usr/bin/env python
import sys

from xml.etree import ElementTree as ET
e = ET.parse('system_profiler.xml')

if __name__ == '__main__':
for d in e.findall('/array/dict'):
if d.find('string').text == 'SPSoftwareDataType':
sp_data = d.find('array').find('dict')
break
else:
print "SPSoftwareDataType NOT FOUND"
sys.exit(1)

record = []
for child in sp_data.getchildren():
record.append(child.text)
if child.tag == 'string':
print "%-15s -> %s" % tuple(record)
record = []

Basically, the script looks for any dict tag that has a string child element whose text value is 'SPSoftwareDataType'. The information that the script is looking for is under that node. The only thing that we used in this example that we didn’t discuss previously was the getchildren() method. This method simply returns a list of the child nodes of a particular element. Other than that, this example should be pretty clear, even though the XML might have been written better. Here is the output the script generates when it is run on a laptop running Mac OS X Tiger:

dink:~/code jmjones$ python elementtree_system_profile.py
_name           -> os_overview
kernel_version  -> Darwin 8.11.1
os_version      -> Mac OS X 10.4.11 (8S2167)

ElementTree has been a great addition to the Python Standard Library. We have been using it for quite a while now and have been happy with what it has done for us. You can try out the SAX and DOM libraries in the Python Standard Library, but we think you will come back to ElementTree.

Summary

This chapter outlined some of the fundamentals of handling text in Python. We dealt with the built-in string type, regular expressions, standard input and output, StringIO, and the urllib module from the standard library. We then pulled many of these things together into two examples that parsed Apache logfiles. Finally, we discussed some of the essentials of the ElementTree library and showed two examples of the way to use it in the real world.

It seems that when a lot of Unix folks think of wrangling text in a way that is beyond what is comfortable to do with grep or awk, the only advanced alternative that they consider is Perl. While Perl is an extremely powerful language, particularly in the area of dealing with text, we think that Python has just as much to offer as Perl does. In fact, if you look at the clean syntax and the ease with which you can go from procedural code to object-oriented code, we think that Python has a distinct advantage over Perl, even in the text handling arena. So, we hope that the next time you have a text handling task to work on, you’ll reach for Python first.

Chapter 4. Documentation and Reporting

As we do, you may find that one of the most tedious, least desirable aspects of your job is to document various pieces of information for the sake of your users. This can either be for the direct benefit of your users who will read the documentation, or perhaps it may be for the indirect benefit of your users because you or your replacement might refer to it when making changes in the future. In either case, creating documentation is often a critical aspect of your job. But if it is not a task that you find yourself longing to do, it might be rather neglected. Python can help here. No, Python cannot write your documentation for you, but it can help you gather, format, and distribute the information to the intended parties.

In this chapter, we are going to focus on: gathering, formatting, and distributing information about the programs you write. The information that you are interested in sharing exists somewhere; it may be in a logfile somewhere; it may be in your head; it may be accessible as a result of some shell command that you execute; it may even be in a database somewhere. The first thing you have to do is to gather that information. The next step in effectively sharing this information is to format the data in a way that makes it meaningful. The format could be a PDF, PNG, JPG, HTML, or even plain text. Finally, you need to get this information to the people who are interested in it. Is it most convenient for the interested parties to receive an email, or visit a website, or look at the files directly on a shared drive?

Automated Information Gathering

The first step of information sharing is gathering the information. There are two other chapters in this book dedicated to gathering data: Text Processing (Chapter 3) and SNMP (Chapter 7). Text processing contains examples of the ways to parse and extract various pieces of data from a larger body of text. One specific example in that chapter is parsing the client IP address, number of bytes transmitted, and HTTP status code out of each line in an Apache web server log. And SNMP contains examples of system queries for information ranging from amount of installed RAM to the speed of network interfaces.

Gathering information can be more involved than just locating and extracting certain pieces of data. Often, it can be a process that involves taking information from one format, such as an Apache logfile, and storing it in some intermediate format to be used at a later time. For example, if you wanted to create a chart that showed the number of bytes that each unique IP address downloaded from a specific Apache web server over the course of a month, the information gathering part of the process could involve parsing the Apache logfile each night, extracting the necessary information (in this case, it would be the IP address and “bytes sent” for each request), and appending the data to some data store that you can open up later. Examples of such data stores include relational databases, object databases, pickle files, CSV files, and plain-text files.

The remainder of this section will attempt to bring together some of the concepts from the chapters on text processing and data persistence. Specifically, it will show how to build on the techniques of data extraction from Chapter 3 and data storage from Chapter 12. We will use the same library from the text processing. We will also use the shelve module, introduced in Chapter 12, to store data about HTTP requests from each unique HTTP client.

Here is a simple module that uses both the Apache log parsing module created in the previous chapter and the shelve module:

#!/usr/bin/env python

import shelve
import apache_log_parser_regex

logfile = open('access.log', 'r')
shelve_file = shelve.open('access.s')

for line in logfile:
d_line = apache_log_parser_regex.dictify_logline(line)
shelve_file[d_line['remote_host']] = \
shelve_file.setdefault(d_line['remote_host'], 0) + \
int(d_line['bytes_sent'])

logfile.close()
shelve_file.close()

This example first imports shelve and apache_log_parser_regex. Shelve is a module from the Python Standard Library. Apache_log_parser_regex is a module we wrote in Chapter 3. We then open the Apache logfile, access.log, and a shelve file, access.s. We iterate over each line in the logfile and use the Apache log parsing module to create a dictionary from each line. The dictionary consists of the HTTP status code for the request, the client’s IP address, and the number of bytes transferred to the client. We then add the number of bytes for this specific request to the total number of bytes already tallied in the shelve object for this client IP address. If there is no entry in the shelve object for this client IP address, the total is automatically set to zero. After iterating through all the lines in the logfile, we close the logfile and the shelve object. We’ll use this example later in this chapter when we get into formatting information.

Receiving Email

You may not think of receiving email as a means of information gathering, but it really can be. Imagine that you have a number of servers, none of which can easily connect to the other, but each of which has email capabilities. If you have a script that monitors web applications on these servers by logging in and out every few minutes, you could use email as an information passing mechanism. Whether the login/logout succeeds or fails, you can send an email with the pass/fail information in it. And you can gather these email messages for reporting or for the purpose of alerting someone if it’s down.

The two most commonly available protocols for retrieving email server are IMAP and POP3. In Python’s standard “batteries included” fashion, there are modules to support both of these protocols in the standard library.

POP3 is perhaps the more common of these two protocols, and accessing your email over POP3 using poplib is quite simple. Example 4-1 shows code that uses poplib to retrieve all of the email that is stored on the specified server and writes it to a set of files on disk.

Example 4-1. Retrieving email using POP3

#!/usr/bin/env python

import poplib

username = 'someuser'
password = 'S3Cr37'

mail_server = 'mail.somedomain.com'

p = poplib.POP3(mail_server)
p.user(username)
p.pass_(password)
for msg_id in  p.list()[1]:
print msg_id
outf = open('%s.eml' % msg_id, 'w')
outf.write('\n'.join(p.retr(msg_id)[1]))
outf.close()
p.quit()

As you can see, we defined the username, password, and mail_server first. Then, we connected to the mail server and gave it the defined username and password. Assuming that all is well and we actually have permission to look at the email for this account, we then iterate over the list of email files, retrieve them, and write them to a disk. One thing this script doesn’t do is delete each email after retrieving it. All it would take to delete the email is a call to dele() after retr().

IMAP is nearly as easy as POP3, but it’s not as well documented in the Python Standard Library documents. Example 4-2 shows IMAP code that does the same thing as the code did in the POP3 example.

Example 4-2. Retrieving email using IMAP

#!/usr/bin/env python

import imaplib

username = 'some_user'
password = '70P53Cr37'

mail_server = 'mail_server'

i = imaplib.IMAP4_SSL(mail_server)
print i.login(username, password)
print i.select('INBOX')
for msg_id in  i.search(None, 'ALL')[1][0].split():
print msg_id
outf = open('%s.eml' % msg_id, 'w')
outf.write(i.fetch(msg_id, '(RFC822)')[1][0][1])
outf.close()
i.logout()

As we did in the POP3 example, we defined the username, password, and mail_server at the top of the script. Then, we connected to the IMAP server over SSL. Next, we logged in and set the email directory to INBOX. Then we started iterating over a search of the entire directory. The search() method is poorly documented in the Python Standard Library documentation. The two mandatory parameters for search() are character set and search criterion. What is a valid character set? What format should we put in there? What are the choices for search criteria? What format is required? We suspect that a reading of the IMAP RFC could be helpful, but fortunately there is enough documentation in the example for IMAP to retrieve all messages in the folder. For each iteration of the loop, we write the contents of the email to disk. A small word of warning is in order here: this will mark all email in that folder as “read.” This may not be a problem for you, and it’s not a big problem as it may be if this deleted the messages, but it’s something that you should be aware of.

Manual Information Gathering

Let’s also look at the more complicated path of manually gathering information. By this, we mean information that you gather with your own eyes and key in with your own hands. Examples include a list of servers with corresponding IP addresses and functions, a list of contacts with email addresses, phone numbers, and IM screen names, or the dates that members of your team are planning to be on vacation. There are certainly tools available that can manage most, if not, all of these types of information. There is Excel or OpenOffice Spreadsheet for managing the server list. There is Outlook or Address Book.app for managing contacts. And either Excel/OpenOffice Spreadsheet or Outlook can manage vacations. This may be the solution for the situations that arise when technologies are freely available and use an editing data format that is plain text and which provides output that is configurable and supports HTML (or preferably XHTML).

Celebrity Profile: ReSTless: Aaron Hillegass

Aaron Hillegass, who has worked for NeXT and Apple, is an expert on developing applications for the Mac. He is the author of Cocoa Programming for Mac OS X (Big Nerd Ranch) and teaches classes on Cocoa programming at Big Nerd Ranch.

Please download the full source for ReSTless from the book’s code repository at http://www.oreilly.com/9780596515829. Here is how to call a Python script from a fancy Cocoa application:

#import "MyDocument.h"

@implementation MyDocument

- (id)init
{
if (![super init]) {
return nil;
}

// What you see for a new document
textStorage = [[NSTextStorage alloc] init];
return self;
}

- (NSString *)windowNibName
{
return @"MyDocument";
}

- (void)prepareEditView
{
// The layout manager monitors the text storage and
// layout the text in the text view
NSLayoutManager *lm = [editView layoutManager];

// Detach the old text storage
[[editView textStorage] removeLayoutManager:lm];

// Attach the new text storage
[textStorage addLayoutManager:lm];
}

- (void)windowControllerDidLoadNib:(NSWindowController *) aController
{
[super windowControllerDidLoadNib:aController];

// Show the text storage in the text view
[self prepareEditView];
}

#pragma mark Saving and Loading

// Saves (the URL is always a file:)
- (BOOL)writeToURL:(NSURL *)absoluteURL
ofType:(NSString *)typeName
error:(NSError **)outError;
{
return [[textStorage string] writeToURL:absoluteURL
atomically:NO
encoding:NSUTF8StringEncoding
error:outError];
}

// Reading (the URL is always a file:)
- (BOOL)readFromURL:(NSURL *)absoluteURL
ofType:(NSString *)typeName
error:(NSError **)outError
{
NSString *string = [NSString stringWithContentsOfURL:absoluteURL
encoding:NSUTF8StringEncoding
error:outError];
// Read failed?
if (!string) {
return NO;
}
[textStorage release];
textStorage = [[NSTextStorage alloc] initWithString:string
attributes:nil];
// Is this a revert?
if (editView) {
[self prepareEditView];
}

return YES;
}

#pragma mark Generating and Saving HTML

- (NSData *)dataForHTML
{
// Create a task to run rst2html.py
NSTask *task = [[NSTask alloc] init];

// Guess the location of the executable
NSString *path = @"/usr/local/bin/rst2html.py";

// Is that file missing? Try inside the python framework
if (![[NSFileManager defaultManager] fileExistsAtPath:path]) {
path = @"/Library/Frameworks/Python.framework/Versions/Current/bin/rst2html.py";
}
[task setLaunchPath:path];

// Connect a pipe where the ReST will go in
NSPipe *inPipe = [[NSPipe alloc] init];
[task setStandardInput:inPipe];
[inPipe release];

// Connect a pipe where the HMTL will come out
NSPipe *outPipe = [[NSPipe alloc] init];
[task setStandardOutput:outPipe];
[outPipe release];

// Start the process
[task launch];

// Get the data from the text view
NSData *inData = [[textStorage string] dataUsingEncoding:NSUTF8StringEncoding];

// Put the data in the pipe and close it
[[inPipe fileHandleForWriting] writeData:inData];
[[inPipe fileHandleForWriting] closeFile];

// Read the data out of the pipe
NSData *outData = [[outPipe fileHandleForReading] readDataToEndOfFile];

// All done with the task
[task release];

return outData;
}

- (IBAction)renderRest:(id)sender
{
// Start the spinning so the user feels like waiting
[progressIndicator startAnimation:nil];

// Get the html as an NSData
NSData *htmlData = [self dataForHTML];

// Put the html in the main WebFrame
WebFrame *wf = [webView mainFrame];
[wf loadData:htmlData
MIMEType:@"text/html"
textEncodingName:@"utf-8"
baseURL:nil];

// Stop the spinning so the user feels done
[progressIndicator stopAnimation:nil];

}

// Triggered by menu item
- (IBAction)startSavePanelForHTML:(id)sender
{
// Where does it save by default?
NSString *restPath = [self fileName];
NSString *directory = [restPath stringByDeletingLastPathComponent];
NSString *filename = [[[restPath lastPathComponent]
stringByDeletingPathExtension]
stringByAppendingPathExtension:@"html"];

// Start the save panel
NSSavePanel *sp = [NSSavePanel savePanel];
[sp setRequiredFileType:@"html"];
[sp setCanSelectHiddenExtension:YES];
[sp beginSheetForDirectory:directory
file:filename
modalForWindow:[editView window]
modalDelegate:self
didEndSelector:@selector(htmlSavePanel:endedWithCode:context:)
contextInfo:NULL];
}

// Called when the save panel is dismissed
- (void)htmlSavePanel:(NSSavePanel *)sp
endedWithCode:(int)returnCode
context:(void *)context
{
// Did the user hit Cancel?
if (returnCode != NSOKButton) {
return;
}

// Get the chosen filename
NSString *savePath = [sp filename];

// Get the HTML data
NSData *htmlData = [self dataForHTML];

// Write it to the file
NSError *writeError;
BOOL success = [htmlData writeToFile:savePath
options:NSAtomicWrite
error:&writeError];

// Did the write fail?
if (!success) {

// Show the user why
NSAlert *alert = [NSAlert alertWithError:writeError];
[alert beginSheetModalForWindow:[editView window]
modalDelegate:nil
didEndSelector:NULL
contextInfo:NULL];
return;
}

}

#pragma mark Printing Support

- (NSPrintOperation *)printOperationWithSettings:(NSDictionary *)printSettings
error:(NSError **)outError
{
// Get the information from Page Setup
NSPrintInfo *printInfo = [self printInfo];

// Get the view that displays the whole HTML document
NSView *docView = [[[webView mainFrame] frameView] documentView];

// Create a print operation
return [NSPrintOperation printOperationWithView:docView
printInfo:printInfo];
}

@end

While there are a number of alternatives, the specific plain-text format that we’re going to suggest here is reStructuredText (also referred to as reST). Here is how the reStructuredText website describes it:

reStructuredText is an easy-to-read, what-you-see-is-what-you-get plaintext markup syntax and parser system. It is useful for in-line program documentation (such as Python docstrings), for quickly creating simple web pages, and for standalone documents. reStructuredText is designed for extensibility for specific application domains. The reStructuredText parser is a component of Docutils. reStructuredText is a revision and reinterpretation of the StructuredText and Setext lightweight markup systems.

ReST is the preferred format for Python documentation. If you create a Python package of your code and decide to upload it to the PyPI, reStructuredText is the expected documentation format. Many individual Python projects are also using ReST as the primary format for their documentation needs.

So why would you want to use ReST as a documentation format? First, because the format is uncomplicated. Second, there is an almost immediate familiarity with the markup. When you see the structure of a document, you quickly understand what the author intended. Here is an example of a very simple ReST file:

=======
Heading
=======
SubHeading
----------
This is just a simple
little subsection. Now,
we'll show a bulleted list:

- item one
- item two
- item three

That probably makes some sort of structured sense to you without having to read the documentation about what constitutes a valid reStructuredText file. You might not be able to write a ReST text file, but you can probably follow along enough to read one.

Third, converting from ReST to HTML is simple. And it’s that third point that we’re going to focus on in this section. We won’t try to give a tutorial on reStructuredText here. If you want a quick overview of the markup syntax, visit http://docutils.sourceforge.net/docs/user/rst/quickref.html.

Using the document that we just showed you as an example, we’ll walk through the steps converting ReST to HTML:

In [2]: import docutils.core

In [3]: rest = '''=======
...: Heading
...: =======
...: SubHeading
...: ----------
...: This is just a simple
...: little subsection. Now,
...: we'll show a bulleted list:
...:
...: - item one
...: - item two
...: - item three
...: '''

In [4]: html =  docutils.core.publish_string(source=rest, writer_name='html')

In [5]: print html[html.find('<body>') + 6:html.find('</body>')]

<div class="document" id="heading">
<h1 class="title">Heading</h1>
<h2 class="subtitle" id="subheading">SubHeading</h2>
<p>This is just a simple
little subsection. Now,
we'll show a bulleted list:</p>
<ul class="simple">
<li>item one</li>
<li>item two</li>
<li>item three</li>
</ul>
</div>

This was a simple process. We imported docutils.core. Then we defined a string that contained our reStructuredText, and ran the string through docutils.core.publish_string(), and then told it to format it as HTML. Then we did a string slice and extracted the text between the <body> and </body> tags. The reason we sliced this div area is because docutils, the library we used to convert to HTML, puts an embedded stylesheet in the generated HTML page so that it doesn’t look too plain.

Now that you see how simple it is, let’s take an example that is slightly more in the realm of system administration. Every good sysadmin needs to keep track of the servers they have and the tasks those servers are being used for. So, here’s an example of the way to create a plain-text server list table and convert it to HTML:

In [6]: server_list = '''==============  ============  ================
...:   Server Name    IP Address    Function
...: ==============  ============  ================
...: card            192.168.1.2   mail server
...: vinge           192.168.1.4   web server
...: asimov          192.168.1.8   database server
...: stephenson      192.168.1.16  file server
...: gibson          192.168.1.32  print server
...: ==============  ============  ================'''

In [7]: print server_list
==============  ============  ================
Server Name    IP Address    Function
==============  ============  ================
card            192.168.1.2   mail server
vinge           192.168.1.4   web server
asimov          192.168.1.8   database server
stephenson      192.168.1.16  file server
gibson          192.168.1.32  print server
==============  ============  ================

In [8]: html = docutils.core.publish_string(source=server_list,
writer_name='html')

In [9]: print html[html.find('<body>') + 6:html.find('</body>')]

<div class="document">
<table border="1" class="docutils">
<colgroup>
<col width="33%" />
<col width="29%" />
<col width="38%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Server Name</th>
<th class="head">IP Address</th>
<th class="head">Function</th>
</tr>
</thead>
<tbody valign="top">
<tr><td>card</td>
<td>192.168.1.2</td>
<td>mail server</td>
</tr>
<tr><td>vinge</td>
<td>192.168.1.4</td>
<td>web server</td>
</tr>
<tr><td>asimov</td>
<td>192.168.1.8</td>
<td>database server</td>
</tr>
<tr><td>stephenson</td>
<td>192.168.1.16</td>
<td>file server</td>
</tr>
<tr><td>gibson</td>
<td>192.168.1.32</td>
<td>print server</td>
</tr>
</tbody>
</table>
</div>

Another excellent choice for a plain text markup format is Textile. According to its website, “Textile takes plain text with *simple* markup and produces valid XHTML. It’s used in web applications, content management systems, blogging software and online forums.” So if Textile is a markup language, why are we writing about it in a book about Python? The reason is that a Python library exists that allows you to process Textile markup and convert it to XHTML. You can write command-line utilities to call the Python library and convert Textile files and redirect the output into XHTML files. Or you can call the Textile conversion module from within some script and programmatically deal with the XHTML that is returned. Either way, the Textile markup and the Textile processing module can be hugely beneficial to your documenting needs.

You can install the Textile Python module, with easy_install textile. Or you can install it using your system’s packaging system if it’s included. For Ubuntu, the package name is python-textile, and you can install it with apt-get install python-textile. Once Textile is installed, you can start using it by simply importing it, creating a Textiler object, and calling a single method on that object. Here is an example of code that converts a Textile bulleted list to XHTML:

In [1]: import textile

In [2]: t = textile.Textiler('''* item one
...: * item two
...: * item three''')

In [3]: print t.process()
<ul>
<li>item one</li>
<li>item two</li>
<li>item three</li>
</ul>

We won’t try to present a Textile tutorial here. There are plenty of resources on the Web for that. For example, http://hobix.com/textile/ provides a good reference for using Textile. While we won’t get too in-depth into the ins and outs of Textile, we will look at the way Textile works for one of the examples of manually gathered information we described earlier—a server list with corresponding IP addresses and functions:

In [1]: import textile

In [2]: server_list = '''|_. Server Name|_. IP Address|_. Function|
...: |card|192.168.1.2|mail server|
...: |vinge|192.168.1.4|web server|
...: |asimov|192.168.1.8|database server|
...: |stephenson|192.168.1.16|file server|
...: |gibson|192.168.1.32|print server|'''

In [3]: print server_list
|_. Server Name|_. IP Address|_. Function|
|card|192.168.1.2|mail server|
|vinge|192.168.1.4|web server|
|asimov|192.168.1.8|database server|
|stephenson|192.168.1.16|file server|
|gibson|192.168.1.32|print server|

In [4]: t = textile.Textiler(server_list)

In [5]: print t.process()
<table>
<tr>
<th>Server Name</th>
<th>IP Address</th>
<th>Function</th>
</tr>
<tr>
<td>card</td>
<td>192.168.1.2</td>
<td>mail server</td>
</tr>
<tr>
<td>vinge</td>
<td>192.168.1.4</td>
<td>web server</td>
</tr>
<tr>
<td>asimov</td>
<td>192.168.1.8</td>
<td>database server</td>
</tr>
<tr>
<td>stephenson</td>
<td>192.168.1.16</td>
<td>file server</td>
</tr>
<tr>
<td>gibson</td>
<td>192.168.1.32</td>
<td>print server</td>
</tr>
</table>

So you can see that ReST and Textile can both be used effectively to integrate the conversion of plain text data into a Python script. If you do have data, such as server lists and contact lists, that needs to be converted into HTML and then have some action (such as emailing the HTML to a list of recipients or FTPing the HTML to a web server somewhere) taken upon it, then either the docutils or the Textile library could be a useful tool for you.

Information Formatting

The next step in getting your information into the hands of your audience is formatting the data into a medium that is easily read and understood. We think of that medium as being something at least comprehensible to the user, but better yet, it can be something attractive. Technically, ReST and Textile encompass both the data gathering and the data formatting steps of information sharing, but the following examples will focus specifically on converting data that we’ve already gathered into a more presentable medium.

Graphical Images

The following two examples will continue the example of parsing an Apache logfile for the client IP address and the number of bytes that were transferred. In the previous section, our example generated a shelve file that contained some information that we want to share with other users. So, now, we will create a chart object from the shelve file to make the data easy to read:

#!/usr/bin/env python

import gdchart
import shelve

shelve_file = shelve.open('access.s')
items_list = [(i[1], i[0]) for i in shelve_file.items()]
items_list.sort()
bytes_sent = [i[0] for i in items_list]
#ip_addresses = [i[1] for i in items_list]
ip_addresses = ['XXX.XXX.XXX.XXX' for i in items_list]

chart = gdchart.Bar()
chart.width = 400
chart.height = 400
chart.bg_color = 'white'
chart.plot_color = 'black'
chart.xtitle = "IP Address"
chart.ytitle = "Bytes Sent"
chart.title = "Usage By IP Address"
chart.setData(bytes_sent)
chart.setLabels(ip_addresses)
chart.draw("bytes_ip_bar.png")

shelve_file.close()

In this example, we imported two modules, gdchart and shelve. We then opened the shelve file we created in the previous example. Since the shelve object shares the same interface as the builtin dictionary object, we were able to call the Items() method on it. items() returns a list of tuples in which the first element of the tuple is the dictionary key and the second element of the tuple is the value for that key. We are able to use the items() method to help sort the data in a way that will make more sense when it is plotted. We use a list comprehension to reverse the order of the previous tuple. Instead of being tuples of (ip_address, bytes_sent), it is now (bytes_sent, ip_addresses). We then sort this list and since the bytes_sent element is first, the list.sort() method will sort by that field first. We then use list comprehensions again to pull the bytes_sent and the ip_addresses fields. You may notice that we’re inserting an obfuscated XXX.XXX.XXX.XXX for the IP addresses because we’ve taken these logfiles from a production web server.

After getting the data that is going to feed the chart out of the way, we can actually start using gdchart to make a graphical representation of the data. We first create a gdchart.Bar object. This is simply a chart object for which we’ll be setting some attributes and then we’ll render a PNG file. We then define the size of the chart, in pixels; we assign colons to use for the background and foreground; and we create titles. We set the data and labels for the chart, both of which we are pulling from the Apache log parsing module. Finally, we draw() the chart out to a file and then close our shelve object. Figure 4-1 shows the chart image.

Figure 4-1. Bar chart of bytes requested per IP address

Here is another example of a script for visually formatting the shelve data, but this time, rather than a bar chart, the program creates a pie chart:

#!/usr/bin/env python

import gdchart
import shelve
import itertools

shelve_file = shelve.open('access.s')
items_list = [(i[1], i[0]) for i in shelve_file.items() if i[1] > 0]
items_list.sort()
bytes_sent = [i[0] for i in items_list]
#ip_addresses = [i[1] for i in items_list]
ip_addresses = ['XXX.XXX.XXX.XXX' for i in items_list]

chart = gdchart.Pie()
chart.width = 800
chart.height = 800
chart.bg_color = 'white'
color_cycle = itertools.cycle([0xDDDDDD, 0x111111, 0x777777])
color_list = []
for i in bytes_sent:
color_list.append(color_cycle.next())
chart.color = color_list

chart.plot_color = 'black'
chart.title = "Usage By IP Address"
chart.setData(*bytes_sent)
chart.setLabels(ip_addresses)
chart.draw("bytes_ip_pie.png")

shelve_file.close()

This script is nearly identical to the bar chart example, but we did have to make a few variations. First, this script creates an instance of gdchart.Pie rather than gdchart.Bar. Second, we set the colors for the individual data points rather than just using black for all of them. Since this is a pie chart, having all data pieces black would make the chart impossible to read, so we decided to alternate among three shades of grey. We were able to alternate among these three choices by using the cycle() function from the itertools module. We recommend having a look at the itertools module. There are lots of fun functions in there to help you deal with iterable objects (such as lists). Figure 4-2 is the result of our pie graph script.

Figure 4-2. Pie chart of the number of bytes requested for each IP address

The only real problem with the pie chart is that the (obfuscated) IP addresses get mingled together toward the lower end of the bytes transferred. Both the bar chart and the pie chart make the data in the shelve file much easier to read, and creating each chart was surprisingly simple. And plugging in the information was startlingly simple.

PDFs

Another way to format information from a data file is to save it in a PDF file. PDF has gone mainstream, and we almost expect all documents to be able to convert to PDF. As a sysadmin, knowing how to generate easy-to-read PDF documents can make your life easier. After reading this section, you should be able to apply your knowledge to creating PDF reports of network utilization, user accounts, and so on. We will also describe the way to embed a PDF automatically in multipart MIME emails with Python.

The 800 pound gorilla in PDF libraries is ReportLab. There is a free version and a commercial version of the software. There are quite a few examples you can look at in the ReportLab PDF library at http://www.reportlab.com/docs/userguide.pdf. In addition to reading this section, we highly recommend that you read ReportLab’s official documentation. To install ReportLab on Ubuntu, you can simply apt-get install python-reportlab. If you’re not on Ubuntu, you can seek out a package for your operating system. Or, there is always the source distribution to rely on.

Example 4-3 is an example of a “Hello World” PDF created with ReportLab.

Example 4-3. “Hello World” PDF

#!/usr/bin/env python
from reportlab.pdfgen import canvas

def hello():
c = canvas.Canvas("helloworld.pdf")
c.drawString(100,100,"Hello World")
c.showPage()
c.save()
hello()

There are a few things you should notice about our “Hello World” PDF creation. First, we creat a canvas object. Next, we use the drawString() method to do the equivalent of file_obj.write() to a text file. Finally, showPage() stops the drawing, and save() actually creates the PDF. If you run this code, you will get a big blank PDF with the words “Hello World” at the bottom.

If you’ve downloaded the source distribution for ReportLab, you can use the tests they’ve included as example-driven documentation. That is, when you run the tests, they’ll generate a set of PDFs for you, and you can compare the test code with the PDFs to see how to accomplish various visual effects with the ReportLab library.

Now that you’ve seen how to create a PDF with ReportLab, let’s see how you can use ReportLab to create a custom disk usage report. Creating a custom disk usage report could be useful. See Example 4-4.

Example 4-4. Disk report PDF

#!/usr/bin/env python
import subprocess
import datetime
from reportlab.pdfgen import canvas
from reportlab.lib.units import inch

def disk_report():
p = subprocess.Popen("df -h", shell=True,
stdout=subprocess.PIPE)
return p.stdout.readlines()

def create_pdf(input,output="disk_report.pdf"):
now = datetime.datetime.today()
date = now.strftime("%h %d %Y %H:%M:%S")
c = canvas.Canvas(output)
textobject = c.beginText()
textobject.setTextOrigin(inch, 11*inch)
textobject.textLines('''
Disk Capacity Report: %s
''' % date)
for line in input:
textobject.textLine(line.strip())
c.drawText(textobject)
c.showPage()
c.save()
report = disk_report()
create_pdf(report)

This code will generate a report that displays the current disk usage, with a datestamp and the words, “Disk Capacity Report.” For such a small handful of lines of codes, this is quite impressive. Let’s look at some of the highlights of this example. First, the disk_report() function that simply takes the output of df -h and returns it as a list. Next in the create_pdf() function, let’s create a formatted datestamp. The most important part of this example is the textobject.

The textobject function is used to create the object that you will place in a PDF. We create a textobject by calling beginText(). Then we define the way we want the data to pack into the page. Our PDF approximates an 8.5×11–inch document, so to pack our text near the top of the page, we told the text object to set the text origin at 11 inches. After that we created a title by writing out a string to the text object, and then we finished by iterating over our list of lines from the df command. Notice that we used line.strip() to remove the newline characters. If we didn’t do this, we would have seen blobs of black squares where the newline characters were.

You can create much more complex PDFs by adding colors and pictures, but you can figure that out by reading the excellent userguide associated with the ReportLab PDF library. The main thing to take away from these examples is that the text is the core object that holds the data that ultimately gets rendered out.

Information Distribution

After you’ve gathered and formatted your data, you need to get it to the people who are interested in it. In this chapter, we’ll mainly focus on ways to email the documentation to your recipients. If you need to post some documentation to a web server for your users to look at, you can use FTP. We discuss using the Python standard FTP module in the next chapter.

Sending Email

Dealing with email is a significant part of being a sysadmin. Not only do we have to manage email servers, but we often to need come up with ways to generate warning messages and alerts via email. The Python Standard Library has terrific support for sending email, but very little has been written about it. Because all sysadmins should take pride in a carefully crafted automated email, this section will show you how to use Python to perform various email tasks.

Sending basic messages

There are two different packages in Python that allow you to send email. One low level package, smtplib, is an interface that corresponds to the various RFC’s for the SMTP protocol. It sends email. The other package, email, assists with parsing and generating emails. Example 4-5 uses smtplib to build a string that represents the body of an email message and then uses the email package to send it to an email server.

Example 4-5. Sending messages with SMTP

#!/usr/bin/env python

import smtplib
mail_server = 'localhost'
mail_server_port = 25

from_addr = 'sender@example.com'
to_addr = 'receiver@example.com'

from_header = 'From: %s\r\n' % from_addr
to_header = 'To: %s\r\n\r\n' % to_addr
subject_header = 'Subject: nothing interesting'

body = 'This is a not-very-interesting email.'

email_message = '%s\n%s\n%s\n\n%s' % (from_header, to_header, subject_header, body)

s = smtplib.SMTP(mail_server, mail_server_port)
s.sendmail(from_addr, to_addr, email_message)
s.quit()

Basically, we defined the host and port for the email server along with the “to” and “from” addresses. Then we built up the email message by concatenating the header portions together with the email body portion. Finally, we connected to the SMTP server and sent it to to_addr and from from_addr. We should also note that we specifically formatted the From: and To: with \r\n to conform to the RFC specification.

See Chapter 10, specifically the section Scheduling Python Processes,” for an example of code that creates a cron job that sends mail with Python. For now, let’s move from this basic example onto some of the fun things Python can do with mail.

Using SMTP authentication

Our last example was pretty simple, as it is trivial to send email from Python, but unfortunately, quite a few SMTP servers will force you to use authentication, so it won’t work in many situations. Example 4-6 is an example of including SMTP authentication.

Example 4-6. SMTP authentication

#!/usr/bin/env python
import smtplib
mail_server = 'smtp.example.com'
mail_server_port = 465

from_addr = 'foo@example.com'
to_addr = 'bar@exmaple.com'

from_header = 'From: %s\r\n' % from_addr
to_header = 'To: %s\r\n\r\n' % to_addr
subject_header = 'Subject: Testing SMTP Authentication'

body = 'This mail tests SMTP Authentication'

email_message = '%s\n%s\n%s\n\n%s' % (from_header, to_header, subject_header, body)

s = smtplib.SMTP(mail_server, mail_server_port)
s.set_debuglevel(1)
s.starttls()
s.login("fatalbert", "mysecretpassword")
s.sendmail(from_addr, to_addr, email_message)
s.quit()

The main difference with this example is that we specified a username and password, enabled a debuglevel, and then started SSL by using the starttls() method. Enabling debugging when authentication is involved is an excellent idea. If we take a look at a failed debug session, it will look like this:

$ python2.5 mail.py
send: 'ehlo example.com\r\n'
reply: '250-example.com Hello example.com [127.0.0.1], pleased to meet you\r\n'
reply: '250-ENHANCEDSTATUSCODES\r\n'
reply: '250-PIPELINING\r\n'
reply: '250-8BITMIME\r\n'
reply: '250-SIZE\r\n'
reply: '250-DSN\r\n'
reply: '250-ETRN\r\n'
reply: '250-DELIVERBY\r\n'
reply: '250 HELP\r\n'
reply: retcode (250); Msg: example.com example.com [127.0.0.1], pleased to meet you
ENHANCEDSTATUSCODES
PIPELINING
8BITMIME
SIZE
DSN
ETRN
DELIVERBY
HELP
send: 'STARTTLS\r\n'
reply: '454 4.3.3 TLS not available after start\r\n'
reply: retcode (454); Msg: 4.3.3 TLS not available after start

In this example, the server with which we attempted to initiate SSL did not support it and sent us out. It would be quite simple to work around this and many other potential issues by writing scripts that included some error handle code to send mail using a cascading system of server attempts, finally finishing at localhost attempt to send mail.

Sending attachments with Python

Sending text-only email is so passé. With Python we can send messages using the MIME standard, which lets us encode attachments in the outgoing message. In a previous section of this chapter, we covered creating PDF reports. Because sysadmins are impatient, we are going to skip a boring diatribe on the origin of MIME and jump straight into sending an email with an attachment. See Example 4-7.

Example 4-7. Sending a PDF attachment email

import email
from email.MIMEText import MIMEText
from email.MIMEMultipart import MIMEMultipart
from email.MIMEBase import MIMEBase
from email import encoders
import smtplib
import mimetypes

from_addr = 'noah.gift@gmail.com'
to_addr = 'jjinux@gmail.com'
subject_header = 'Subject: Sending PDF Attachemt'
attachment = 'disk_usage.pdf'
body = '''
This message sends a PDF attachment created with Report
Lab.
'''

m = MIMEMultipart()
m["To"] = to_addr
m["From"] = from_addr
m["Subject"] = subject_header

ctype, encoding = mimetypes.guess_type(attachment)
print ctype, encoding
maintype, subtype = ctype.split('/', 1)
print maintype, subtype

m.attach(MIMEText(body))
fp = open(attachment, 'rb')
msg = MIMEBase(maintype, subtype)
msg.set_payload(fp.read())
fp.close()
encoders.encode_base64(msg)
msg.add_header("Content-Disposition", "attachment", filename=attachment)
m.attach(msg)

s = smtplib.SMTP("localhost")
s.set_debuglevel(1)
s.sendmail(from_addr, to_addr, m.as_string())
s.quit()

So, we used a little magic and encoded our disk report PDF we created earlier and emailed it out.

Trac

Trac is a wiki and issue tracking system. It is typically used for software development, but can really be used for anything that you would want to use a wiki or ticketing system for, and it is written in Python. You can find the latest copy of the Trac documentation and package here: http://trac.edgewall.org/. It is beyond the scope of this book to get into too much detail about Trac, but it is a good tool for general trouble tickets as well. One of the other interesting aspects of Trac is that it can be extended via plug-ins.

We’re mentioning it last because it really fits into all three of the categories that we’ve been discussing: information gathering, formatting, and distribution. The wiki portion allows users to create web pages through browsers. The information they put into those passages is rendered in HTML for other users to view through browsers. This is the full cycle of what we’ve been discussing in this chapter.

Similarly, the ticket tracking system allows users to put in requests for work or to report problems they encounter. You can report on the tickets that have been entered via the web interface and can even generate CSV reports. Once again, Trac spans the full cycle of what we’ve discussed in this chapter.

We recommend that you explore Trac to see if it meets your needs. You might need something with more features and capabilities or you might want something simpler, but it’s worth finding out more about.

Summary

In this chapter, we looked at ways to gather data, in both an automated and a manual way. We also looked at ways to put that data together into a few different, more distributable formats, namely HTML, PDF, and PNG. Finally, we looked at how to get the information out to people who are interested in it. As we said at the beginning of this chapter, documentation might not be the most glamorous part of your job. You might not have even realized that you were signing up to document things when you started. But clear and precise documentation is a critical element of system administration. We hope the tips in this chapter can make the sometimes mundane task of documentation a little more fun.

Chapter 5. Networking

Networking often refers to connecting multiple computers together for the purpose of allowing some communication among them. But, for our purposes, we are less interested in allowing computers to communicate with one another and more interested in allowing processes to communicate with one another. Whether the processes are on the same computer or different computers is irrelevant for the techniques that we’re going to show.

This chapter will focus on writing Python programs that connect to other processes using the standard socket library (as well as libraries built on top of socket) and then interacting with those other processes.

Network Clients

While servers sit and wait for a client to connect to them, clients initiate connections. The Python Standard Library contains implementations of many used network clients. This section will discuss some of the more common and frequently useful clients.

socket

The socket module provides a Python interface to your operating system’s socket implementation. This means that you can do whatever can be done to or with sockets, using Python. In case you have never done any network programming before, this chapter does provide a brief overview of networking. It should give you a flavor of what kinds of things you can do with the Python networking libraries.

The socket module provides the factory function, socket(). The socket() function, in turn, returns a socket object. While there are a number of arguments to pass to socket() for specifying the kind of socket to create, calling the socket() factory function with no arguments returns a socket object with sensible defaults—a TCP/IP socket:

In [1]: import socket

In [2]: s = socket.socket()

In [3]: s.connect(('192.168.1.15', 80))

In [4]: s.send("GET / HTTP/1.0\n\n")
Out[4]: 16

In [5]: s.recv(200)
Out[5]: 'HTTP/1.1 200 OK\r\n\
Date: Mon, 03 Sep 2007 18:25:45 GMT\r\n\
Server: Apache/2.0.55 (Ubuntu) DAV/2 PHP/5.1.6\r\n\
Content-Length: 691\r\n\
Connection: close\r\n\
Content-Type: text/html; charset=UTF-8\r\n\
\r\n\
<!DOCTYPE HTML P'
In [6]: s.close()

This example created a socket object called s from the socket() factory function. It then connected to a local default web server, indicated by port 80, which is the default port for HTTP. Then, it sent the server the text string "GET / HTTP/1.0\n\n" (which is simply an HTTP request). Following the send, it received the first 200 bytes of the server’s response, which is a 200 OK status message and HTTP headers. Finally, we closed the connection.

The socket methods demonstrated in this example represent the methods that you are likely to find yourself using most often. Connect() establishes a communication channel between your socket object and the remote (specifically meaning “not this socket object”). Send() transmits data from your socket object to the remote end. Recv() receives any data that the remote end has sent back. And close() terminates the communication channel between the two sockets. This is a really simple example that shows the ease with which you can create socket objects and then send and receive data over them.

Now we’ll look at a slightly more useful example. Suppose you have a server that is running some sort of network application, such as a web server. And suppose that you are interested in watching this server to be sure that, over the course of a day, you can make a socket connection to the web server. This sort of monitoring is minimal, but it proves that the server itself is still up and that the web server is still listening on some port. See Example 5-1.

Example 5-1. TCP port checker

#!/usr/bin/env python

import socket
import re
import sys

def check_server(address, port):
#create a TCP socket
s = socket.socket()
print "Attempting to connect to %s on port %s" % (address, port)
try:
s.connect((address, port))
print "Connected to %s on port %s" % (address, port)
return True
except socket.error, e:
print "Connection to %s on port %s failed: %s" % (address, port, e)
return False

if __name__ == '__main__':
from optparse import OptionParser
parser = OptionParser()

parser.add_option("-a", "--address", dest="address", default='localhost',
help="ADDRESS for server", metavar="ADDRESS")

parser.add_option("-p", "--port", dest="port", type="int", default=80,
help="PORT for server", metavar="PORT")

(options, args) = parser.parse_args()
print 'options: %s, args: %s' % (options, args)
check = check_server(options.address, options.port)
print 'check_server returned %s' % check
sys.exit(not check)

All of the work occurs in the check_server() function. Check_server() creates a socket object. Then, it tries to connect to the specified address and port number. If it succeeds, it returns True. If it fails, the socket.connect() call will throw an exception, which is handled, and the function returns False. The main section of the code calls check_server(). This “main” section parses the arguments from the user and puts the user requested arguments into an appropriate format to pass in to check_server(). This whole script prints out status messages as it goes along. The last thing it prints out is the return value of check_server(). The script returns the opposite of the check_server() return code to the shell. The reason that we return the opposite of this return code is to make this script a useful scriptable utility. Typically, utilities like this return 0 to the shell on success and something other than 0 on failure (typically something positive). Here is an example of the piece of code successfully connecting to the web server we connected to earlier:

jmjones@dinkgutsy:code$ python port_checker_tcp.py -a 192.168.1.15 -p 80
options: {'port': 80, 'address': '192.168.1.15'}, args: []
Attempting to connect to 192.168.1.15 on port 80
Connected to 192.168.1.15 on port 80
check_server returned True

The last output line, which contains check_server returned True, means that the connection was a success.

Here is an example of a connection call that failed:

jmjones@dinkgutsy:code$ python port_checker_tcp.py -a 192.168.1.15 -p 81
options: {'port': 81, 'address': '192.168.1.15'}, args: []
Attempting to connect to 192.168.1.15 on port 81
Connection to 192.168.1.15 on port 81 failed: (111, 'Connection refused')
check_server returned False

The last log line, which contains check_server returned False, means that the connection was a failure. In the penultimate output line, which contains Connection to 192.168.1.15 on port 81 failed, we also see the reason, 'Connection refused'. Just a wild guess here, but it may have something to do with there being nothing running on port 81 of this particular server.

We’ve created three examples to demonstrate how you can use this utility in shell scripts. First, we give a shell command to run the script and to print out SUCCESS if the script succeeds. We use the && operator in place of an if-then statement:

$ python port_checker_tcp.py -a 192.168.1.15 -p 80 && echo "SUCCESS"
options: {'port': 80, 'address': '192.168.1.15'}, args: []
Attempting to connect to 192.168.1.15 on port 80
Connected to 192.168.1.15 on port 80
check_server returned True
SUCCESS

This script succeeded, so after executing and printing status results, the shell prints SUCCESS:

$ python port_checker_tcp.py -a 192.168.1.15 -p 81 && echo "FAILURE"
options: {'port': 81, 'address': '192.168.1.15'}, args: []
Attempting to connect to 192.168.1.15 on port 81
Connection to 192.168.1.15 on port 81 failed: (111, 'Connection refused')
check_server returned False

This script failed, so it never printed FAILURE:

$ python port_checker_tcp.py -a 192.168.1.15 -p 81 || echo "FAILURE"
options: {'port': 81, 'address': '192.168.1.15'}, args: []
Attempting to connect to 192.168.1.15 on port 81
Connection to 192.168.1.15 on port 81 failed: (111, 'Connection refused')
check_server returned False
FAILURE

This script failed, but we changed the && to ||. This just means if the script returns a failure result, print FAILURE. So it did.

The fact that a web server allows a connection on port 80 doesn’t mean that there is an HTTP server available for the connection. A test that will help us better determine the status of a web server is whether the web server generates HTTP headers with the expected status code for some specific URL. Example 5-2 does just that.

Example 5-2. Socket-based web server checker

#!/usr/bin/env python

import socket
import re
import sys

def check_webserver(address, port, resource):
#build up HTTP request string
if not resource.startswith('/'):
resource = '/' + resource
request_string = "GET %s HTTP/1.1\r\nHost: %s\r\n\r\n" % (resource, address)
print 'HTTP request:'
print '|||%s|||' % request_string

#create a TCP socket
s = socket.socket()
print "Attempting to connect to %s on port %s" % (address, port)
try:
s.connect((address, port))
print "Connected to %s on port %s" % (address, port)
s.send(request_string)
#we should only need the first 100 bytes or so
rsp = s.recv(100)
print 'Received 100 bytes of HTTP response'
print '|||%s|||' % rsp
except socket.error, e:
print "Connection to %s on port %s failed: %s" % (address, port, e)
return False
finally:
#be a good citizen and close your connection
print "Closing the connection"
s.close()
lines = rsp.splitlines()
print 'First line of HTTP response: %s' % lines[0]
try:
version, status, message = re.split(r'\s+', lines[0], 2)
print 'Version: %s, Status: %s, Message: %s' % (version, status, message)
except ValueError:
print 'Failed to split status line'
return False
if status in ['200', '301']:
print 'Success - status was %s' % status
return True
else:
print 'Status was %s' % status
return False

if __name__ == '__main__':
from optparse import OptionParser
parser = OptionParser()
parser.add_option("-a", "--address", dest="address", default='localhost',
help="ADDRESS for webserver", metavar="ADDRESS")

parser.add_option("-p", "--port", dest="port", type="int", default=80,
help="PORT for webserver", metavar="PORT")

parser.add_option("-r", "--resource", dest="resource", default='index.html',
help="RESOURCE to check", metavar="RESOURCE")

(options, args) = parser.parse_args()
print 'options: %s, args: %s' % (options, args)
check = check_webserver(options.address, options.port, options.resource)
print 'check_webserver returned %s' % check
sys.exit(not check)

Similar to the previous example where check_server() did all the work, check_webserver() does all the work in this example, too. First, check_webserver() builds up the HTTP request string. The HTTP protocol, in case you don’t know, is a well-defined way that HTTP clients and servers communicate. The HTTP request that check_webserver() builds is nearly the simplest HTTP request possible. Next, check_webserver() creates a socket object, connects to the server, and sends the HTTP request to the server. Then, it reads back the response from the server and closes the connection. When there is a socket error, check_webserver() returns False, indicating that the check failed. It then takes what it read from the server, and extracts the status code from it. If the status code is either 200 meaning “OK,” or 301, meaning “Moved Permanently,” check_webserver() returns True, otherwise, it returns False. The main portion of the script parses the input from the user and calls check_webserver(). After it gets the result back from check_webserver(), it returns the opposite of the return value from check_webserver() to the shell. The concept here is similar to what we did with the plain socket checker. We want to be able to call this from a shell script and see if it succeeded or failed. Here is the code in action:

$ python web_server_checker_tcp.py -a 192.168.1.15 -p 80 -r apache2-default
options: {'resource': 'apache2-default', 'port': 80, 'address':
'192.168.1.15'}, args: []
HTTP request:
|||GET /apache2-default HTTP/1.1
Host: 192.168.1.15

|||
Attempting to connect to 192.168.1.15 on port 80
Connected to 192.168.1.15 on port 80
Received 100 bytes of HTTP response
|||HTTP/1.1 301 Moved Permanently
Date: Wed, 16 Apr 2008 23:31:24 GMT
Server: Apache/2.0.55 (Ubuntu) |||
Closing the connection
First line of HTTP response: HTTP/1.1 301 Moved Permanently
Version: HTTP/1.1, Status: 301, Message: Moved Permanently
Success - status was 301
check_webserver returned True

The last four output lines show that the HTTP status code for /apache2-default on this web server was 301, so this run was successful.

Here is another run. This time, we’ll intentionally specify a resource that isn’t there to show what happens when the HTTP call is False:

$ python web_server_checker_tcp.py -a 192.168.1.15 -p 80 -r foo
options: {'resource': 'foo', 'port': 80, 'address': '192.168.1.15'}, args: []
HTTP request:
|||GET /foo HTTP/1.1
Host: 192.168.1.15

|||
Attempting to connect to 192.168.1.15 on port 80
Connected to 192.168.1.15 on port 80
Received 100 bytes of HTTP response
|||HTTP/1.1 404 Not Found
Date: Wed, 16 Apr 2008 23:58:55 GMT
Server: Apache/2.0.55 (Ubuntu) DAV/2 PH|||
Closing the connection
First line of HTTP response: HTTP/1.1 404 Not Found
Version: HTTP/1.1, Status: 404, Message: Not Found
Status was 404
check_webserver returned False

Just as the last four lines of the previous example showed that the run was successful, the last four lines of this example show that it was unsuccessful. Because there is no /foo on this web server, this checker returned False.

This section showed how to construct low-level utilities to connect to network servers and perform basic checks on them. The purpose of these examples was to introduce you to what happens behind the scenes when clients and servers communicate with one another. If you have an opportunity to write a network component using a higher library than the socket module, you should take it. It is not desirable to spend your time writing network components using a low-level library such as socket.

httplib

The previous example showed how to make an HTTP request using the socket module directly. This example will show how to use the httplib module. When should you consider using the httplib module rather than the socket module? Or more generically, when should you consider using a higher level library rather than a lower level library? A good rule of thumb is any chance you get. Sometimes using a lower level library makes sense. You might need to accomplish something that isn’t already in an available library, for example, or you might need to have finer-grained control of something already in a library, or there might be a performance advantage. But in this case, there is no reason not to use a higher-level library such as httplib over a lower-level library such as socket.

Example 5-3 accomplishes the same functionality as the previous example did with the httplib module.

Example 5-3. httplib-based web server checker

#!/usr/bin/env python

import httplib
import sys

def check_webserver(address, port, resource):
#create connection
if not resource.startswith('/'):
resource = '/' + resource
try:
conn = httplib.HTTPConnection(address, port)
print 'HTTP connection created successfully'
#make request
req = conn.request('GET', resource)
print 'request for %s successful' % resource
#get response
response = conn.getresponse()
print 'response status: %s' % response.status
except sock.error, e:
print 'HTTP connection failed: %s' % e
return False
finally:
conn.close()
print 'HTTP connection closed successfully'
if response.status in [200, 301]:
return True
else:
return False

if __name__ == '__main__':
from optparse import OptionParser
parser = OptionParser()
parser.add_option("-a", "--address", dest="address", default='localhost',
help="ADDRESS for webserver", metavar="ADDRESS")
parser.add_option("-p", "--port", dest="port", type="int", default=80,
help="PORT for webserver", metavar="PORT")
parser.add_option("-r", "--resource", dest="resource", default='index.html',
help="RESOURCE to check", metavar="RESOURCE")
(options, args) = parser.parse_args()
print 'options: %s, args: %s' % (options, args)
check = check_webserver(options.address, options.port, options.resource)
print 'check_webserver returned %s' % check
sys.exit(not check)

In its conception, this example follows the socket example pretty closely. Two of the biggest differences are that you don’t have to manually create the HTTP request and that you don’t have to manually parse the HTTP response. The httplib connection object has a request() method that builds and sends the HTTP request for you. The connection object also has a getresponse() method that creates a response object for you. We were able to access the HTTP status by referring to the status attribute on the response object. Even if it isn’t that much less code to write, it is nice to not have to go through the trouble of keeping up with creating, sending, and receiving the HTTP request and response. This code just feels more tidy.

Here is a run that uses the same command-line parameters the previous successful scenario used. We’re looking for / on our web server, and we find it:

$ python web_server_checker_httplib.py -a 192.168.1.15 -r /
options: {'resource': '/', 'port': 80, 'address': '192.168.1.15'}, args: []
HTTP connection created successfully
request for / successful
response status: 200
HTTP connection closed successfully
check_webserver returned True

And here is a run with the same command-line parameters as the failure scenario earlier. We’re looking for /foo, and we don’t find it:

$ python web_server_checker_httplib.py -a 192.168.1.15 -r /foo
options: {'resource': '/foo', 'port': 80, 'address': '192.168.1.15'}, args: []
HTTP connection created successfully
request for /foo successful
response status: 404
HTTP connection closed successfully
check_webserver returned False

As we said earlier, any time you have a chance to use a higher-level library, you should use it. Using httplib rather than using the socket module alone was a simpler, cleaner process. And the simpler you can make your code, the fewer bugs you’ll have.

ftplib

In addition to the socket and httplib modules, the Python Standard Library also contains an FTP client module named ftplib. ftplib is a full-featured FTP client library that will allow you to programmatically perform any tasks you would normally use an FTP client application to perform. For example, you can log in to an FTP server, list files in a particular directory, retrieve files, put files, change directories, and logout, all from within a Python script. You can even use one of the many GUI frameworks available in Python and build your own GUI FTP application.

Rather than give a full overview of this library, we’ll show you Example 5-4 and then explain how it works.

Example 5-4. FTP URL retriever using ftplib

#!/usr/bin/env python

from ftplib import FTP
import ftplib
import sys
from optparse import OptionParser

parser = OptionParser()

parser.add_option("-a", "--remote_host_address", dest="remote_host_address",
help="REMOTE FTP HOST.",
metavar="REMOTE FTP HOST")

parser.add_option("-r", "--remote_file", dest="remote_file",
help="REMOTE FILE NAME to download.",
metavar="REMOTE FILE NAME")

parser.add_option("-l", "--local_file", dest="local_file",
help="LOCAL FILE NAME to save remote file to", metavar="LOCAL FILE NAME")

parser.add_option("-u", "--username", dest="username",
help="USERNAME for ftp server", metavar="USERNAME")

parser.add_option("-p", "--password", dest="password",
help="PASSWORD for ftp server", metavar="PASSWORD")

(options, args) = parser.parse_args()

if not (options.remote_file and
options.local_file and
options.remote_host_address):
parser.error('REMOTE HOST, LOCAL FILE NAME, ' \
'and REMOTE FILE NAME are mandatory')

if options.username and not options.password:
parser.error('PASSWORD is mandatory if USERNAME is present')

ftp = FTP(options.remote_host_address)
if options.username:
try:
ftp.login(options.username, options.password)
except ftplib.error_perm, e:
print "Login failed: %s" % e
sys.exit(1)
else:
try:
ftp.login()
except ftplib.error_perm, e:
print "Anonymous login failed: %s" % e
sys.exit(1)
try:
local_file = open(options.local_file, 'wb')
ftp.retrbinary('RETR %s' % options.remote_file, local_file.write)
finally:
local_file.close()
ftp.close()

The first part of the working code (past all the command-line parsing) creates an FTP object by passing the FTP server’s address to FTP’s constructor. Alternatively, we could have created an FTP object by passing nothing to the constructor and then calling the connect() method with the FTP server’s address. The code then logs into the FTP server, using the username and password if they were provided, or anonymous authentication if they were not. Next, it creates a file object to store the data from the file on the FTP server. Then it calls the retrbinary() method on the FTP object. Retrbinary(), as the name implies, retrieves a binary file from an FTP server. It takes two parameters: the FTP retrieve command and a callback function. You might notice that our callback function is the write method on the file object we created in the previous step. It is important to note that we are not calling the write() method in this case. We are passing the write method in to the retrbinary() method so that retrbinary() can call write(). Retrbinary() will call whatever callback function we pass it with each chunk of data that it receives from the FTP server. This callback function could do anything with the data. The callback function could just log that it received N number of bytes from the FTP server. Passing in a file object’s write method causes the script to write the contents of the file from the FTP server to the file object. Finally, it closes the file object and the FTP connection. We did a little error handling in the process: we set up a try block around retrieving the binary file from the FTP server and a finally block around the call to close the local file and FTP connection. If anything bad happens, we want to clean up our files before the script terminates. For a brief discussion of callbacks, see the Appendix.

urllib

Moving up the standard library modules to a higher-level library, we arrive at urllib. When you think of urllib, it’s easy to think of HTTP libraries only and forget that FTP resources also can be identified by URLs. Consequently, you might not have considered using urllib to retrieve FTP resources, but the functionality is there. Example 5-5 is the same as the ftplib example earlier, except it uses urllib.

Example 5-5. FTP URL retriever using urllib

#!/usr/bin/env python
"""
url retriever

Usage:

url_retrieve_urllib.py URL FILENAME

URL:
If the URL is an FTP URL the format should be:
ftp://[username[:password]@]hostname/filename
If you want to use absolute paths to the file to download,
you should make the URL look something like this:
ftp://user:password@host/%2Fpath/to/myfile.txt
Notice the '%2F' at the beginning of the path to the file.

FILENAME:
absolute or relative path to the filename to save downloaded file as
"""

import urllib
import sys

if '-h' in sys.argv or '--help' in sys.argv:
print __doc__
sys.exit(1)

if not len(sys.argv) == 3:
print 'URL and FILENAME are mandatory'
print __doc__
sys.exit(1)
url = sys.argv[1]
filename = sys.argv[2]
urllib.urlretrieve(url, filename)

This script is short and sweet. It really shows off the power of urllib. There are actually more lines of usage documentation than code in it. There is even more argument parsing than code, which says a lot because there isn’t much of that, either. We decided to go with a very simple argument parsing routine with this script. Since both of the “options” were mandatory, we decided to use positional arguments rather than option switches. Effectively, the only line of code in this example that performs work is this one:

urllib.urlretrieve(url, filename)

After retrieving the options with sys.argv, this line of code pulls down the specified URL and saves it to the specified local filename. It works with HTTP URLs and FTP URLs, and will even work when the username and password are included in the URL.

A point worth emphasizing here is that if you think that something should be easier than the way you are doing it with another language, it probably is. There is probably some higher-level library out there somewhere that will do what you need to do frequently, and that library will be in the Python Standard Library. In this case, urllib did exactly what we wanted to do, and we didn’t have to go anywhere beyond the standard library docs to find out about it. Sometimes, you might have to go outside the Python Standard Library, but you will find other Python resources such as the Python Package Index (PyPI) at http://pypi.python.org/pypi.

urllib2

Another high level library is urllib2. Urllib2 contains pretty much the same functionality as urllib, but expands on it. For example, urllib2 contains better authentication support and better cookie support. So if you start using urllib and think it isn’t doing everything for you that it should, take a look at urllib2 to see if it meets your needs.

Remote Procedure Call Facilities

Typically, the reason for writing networking code is that you need interprocess communication (IPC). Often, plain IPC, such as HTTP or a plain socket, is good enough. However, there are times when it would be even more useful to execute code in a different process or even on a different computer, as though it were in the same process that the code you are working on is in. If you could, in fact, execute code remotely in some other process from your Python program, you might expect that the return values from the remote calls would be Python objects which you could deal more easily with than chunks of text through which you have to manually parse. The good news is that there are several tools for remote procedure call (RPC) functionality.

XML-RPC

XML-RPC exchanges a specifically formatted XML document between two processes to perform a remote procedure call. But you don’t need to worry about XML part; you’ll probably never have to know the format of the document that is being exchanged between the two processes. The only thing you really need to know to get started using XML-RPC is that there is an implementation of both the client and the server portions in the Python Standard Library. Two things that might be useful to know are XML-RPC is available for most programming languages, and it is very simple to use.

Example 5-6 is a simple XML-RPC server.

Example 5-6. Simple XML-RPC server

#!/usr/bin/env python

import SimpleXMLRPCServer
import os

def ls(directory):
try:
return os.listdir(directory)
except OSError:
return []

def ls_boom(directory):
return os.listdir(directory)

def cb(obj):
print "OBJECT::", obj
print "OBJECT.__class__::", obj.__class__
return obj.cb()

if __name__ == '__main__':
s = SimpleXMLRPCServer.SimpleXMLRPCServer(('127.0.0.1', 8765))
s.register_function(ls)
s.register_function(ls_boom)
s.register_function(cb)
s.serve_forever()

This code creates a new SimpleXMLRPCServer object and binds it to port 8765 on 127.0.0.1, the loop back interface, which makes this accessible to processes only on this particular machine. It then registers the functions ls(), ls_boom(), and cb(), which we defined in the code. We’ll explain the cb() function in a few moments. The Ls() function will list the contents of the directory passed in using os.listdir() and return those results as a list. ls() masks any OSError exceptions that we may get. ls_boom() lets any exception that we hit find its way back to the XML-RPC client. Then, the code enters into the serve_forever() loop, which waits for a connection it can handle. Here is an example of this code used in an IPython shell:

In [1]: import xmlrpclib

In [2]: x = xmlrpclib.ServerProxy('http://localhost:8765')

In [3]: x.ls('.')
Out[3]:
['.svn',
'web_server_checker_httplib.py',
....
'subprocess_arp.py',
'web_server_checker_tcp.py']

In [4]: x.ls_boom('.')
Out[4]:
['.svn',
'web_server_checker_httplib.py',
....
'subprocess_arp.py',
'web_server_checker_tcp.py']

In [5]: x.ls('/foo')
Out[5]: []

In [6]: x.ls_boom('/foo')
---------------------------------------------------------------------------
<class 'xmlrpclib.Fault'>                 Traceback (most recent call last)
...
.
.
<<big nasty traceback>>
.
.
...
786         if self._type == "fault":
--> 787             raise Fault(**self._stack[0])
788         return tuple(self._stack)
789

<class 'xmlrpclib.Fault'>: <Fault 1: "<type 'exceptions.OSError'>
:[Errno 2] No such file or directory: '/foo'">

First, we created a ServerProxy() object by passing in the address of the XML-RPC server. Then, we called .ls('.') to see which files were in the server’s current working directory. The server was running in a directory that contains example code from this book, so those are the files you see from the directory listing. The really interesting thing is that on the client side, x.ls('.') returned a Python list. Had this server been implemented in Java, Perl, Ruby, or C#, you could expect the same thing. The language that implements the server would have done a directory listing; created a list, array, or collection of filenames; and the XML-RPC server code would have then created an XML representation of that list or array and sent it back over the wire to your client. We also tried out ls_boom(). Since ls_boom() lacks the exception handling of ls(), we can see that the exception passes from the server back to the client. We even see a traceback on the client.

The interoperability possibilities that XML-RPC opens up to you are certainly interesting. But perhaps more interesting is the fact that you can write a piece of code to run on any number of machines and be able to execute that code remotely whenever you wish.

XML-RPC is not without its limitations, though. Whether you think these limitations are problematic or not is a matter of engineering taste. For example, if you pass in a custom Python object, the XML-RPC library will convert that object to a Python dictionary, serialize it to XML, and pass it across the wire. You can certainly work around this, but it would require writing code to extract your data from the XML version of the dictionary so that you could pass it back into the original object that was dictified. Rather than go through that trouble, why not use your objects directly on your RPC server? You can’t with XML-RPC, but there are other options.

Pyro

Pyro is one framework that alleviates XML-RPC shortcomings. Pyro stands for Python Remote Objects (capitalization intentional). It lets you do everything you could do with XML-RPC, but rather than dictifying your objects, it maintains their types when you pass them across. If you do want to use Pyro, you will have to install it separately. It doesn’t come with Python. Also be aware that Pyro only works with Python, whereas XML-RPC can work between Python and other languages. Example 5-7 is an implementation of the same ls() functionality from the XML-RPC example.

Example 5-7. Simple Pyro server

#!/usr/bin/env python

import Pyro.core
import os
from xmlrpc_pyro_diff import PSACB

class PSAExample(Pyro.core.ObjBase):

def ls(self, directory):
try:
return os.listdir(directory)
except OSError:
return []

def ls_boom(self, directory):
return os.listdir(directory)

def cb(self, obj):
print "OBJECT:", obj
print "OBJECT.__class__:", obj.__class__
return obj.cb()

if __name__ == '__main__':
Pyro.core.initServer()
daemon=Pyro.core.Daemon()
uri=daemon.connect(PSAExample(),"psaexample")

print "The daemon runs on port:",daemon.port
print "The object's uri is:",uri

daemon.requestLoop()

The Pyro example is similar to the XML-RPC example. First, we created a PSAExample class with ls(), ls_boom(), and cb() methods on it. We then created a daemon from Pyro’s internal plumbing. Then, we associated the PSAExample with the daemon. Finally, we told the daemon to start serving requests.

Here we access the Pyro server from an IPython prompt:

In [1]: import Pyro.core
/usr/lib/python2.5/site-packages/Pyro/core.py:11: DeprecationWarning:
The sre module is deprecated, please import re.
import sys, time, sre, os, weakref

In [2]: psa = Pyro.core.getProxyForURI("PYROLOC://localhost:7766/psaexample")
Pyro Client Initialized. Using Pyro V3.5

In [3]: psa.ls(".")
Out[3]:
['pyro_server.py',
....
'subprocess_arp.py',
'web_server_checker_tcp.py']

In [4]: psa.ls_boom('.')
Out[4]:
['pyro_server.py',
....
'subprocess_arp.py',
'web_server_checker_tcp.py']

In [5]: psa.ls("/foo")
Out[5]: []

In [6]: psa.ls_boom("/foo")
---------------------------------------------------------------------------
<type 'exceptions.OSError'>               Traceback (most recent call last)

/home/jmjones/local/Projects/psabook/oreilly/<ipython console> in <module>()
.
.
...
<<big nasty traceback>>
...
.
.
--> 115                 raise self.excObj
116         def __str__(self):
117                 s=self.excObj.__class__.__name__

<type 'exceptions.OSError'>: [Errno 2] No such file or directory: '/foo'

Nifty. It returned the same output as the XML-RPC example. We expected as much. But what happens when we pass in a custom object? We’re going to define a new class, create an object from it, and then pass it to the XML-RPC cb() function and the Pyro cb() method from the examples above. Example 5-8 shows the piece of code that we are going to execute.

Example 5-8. Differences between XML-RPC and Pyro

import Pyro.core
import xmlrpclib

class PSACB:
def __init__(self):
self.some_attribute = 1

def cb(self):
return "PSA callback"

if __name__ == '__main__':
cb = PSACB()

print "PYRO SECTION"
print "*" * 20
psapyro = Pyro.core.getProxyForURI("PYROLOC://localhost:7766/psaexample")
print "-->>", psapyro.cb(cb)
print "*" * 20

print "XML-RPC SECTION"
print "*" * 20
psaxmlrpc = xmlrpclib.ServerProxy('http://localhost:8765')
print "-->>", psaxmlrpc.cb(cb)
print "*" * 20

The call to the Pyro and XML-RPC implementation of the cb() function should both call cb() on the object passed in to it. And in both instances, it should return the string PSA callback. And here is what happens when we run it:

jmjones@dinkgutsy:code$ python xmlrpc_pyro_diff.py
/usr/lib/python2.5/site-packages/Pyro/core.py:11: DeprecationWarning:
The sre module is deprecated, please import re.
import sys, time, sre, os, weakref
PYRO SECTION
********************
Pyro Client Initialized. Using Pyro V3.5
-->> PSA callback
********************
XML-RPC SECTION
********************
-->>
Traceback (most recent call last):
File "xmlrpc_pyro_diff.py", line 23, in <module>
print "-->>", psaxmlrpc.cb(cb)
File "/usr/lib/python2.5/xmlrpclib.py", line 1147, in __call__
return self.__send(self.__name, args)
File "/usr/lib/python2.5/xmlrpclib.py", line 1437, in __request
verbose=self.__verbose
File "/usr/lib/python2.5/xmlrpclib.py", line 1201, in request
return self._parse_response(h.getfile(), sock)
File "/usr/lib/python2.5/xmlrpclib.py", line 1340, in _parse_response
return u.close()
File "/usr/lib/python2.5/xmlrpclib.py", line 787, in close
raise Fault(**self._stack[0])
xmlrpclib.Fault: <Fault 1: "<type 'exceptions.AttributeError'>:'dict' object
has no attribute 'cb'">

The Pyro implementation worked, but the XML-RPC implementation failed and left us a traceback. The last line of the traceback says that a dict object has no attribute of cb. This will make more sense when we show you the output from the XML-RPC server. Remember that the cb() function had some print statements in it to show some information about what was going on. Here is the XML-RPC server output:

OBJECT:: {'some_attribute': 1}
OBJECT.__class__:: <type 'dict'>
localhost - - [17/Apr/2008 16:39:02] "POST /RPC2 HTTP/1.0" 200 -

In dictifying the object that we created in the XML-RPC client, some_attribute was converted to a dictionary key. While this one attribute was preserved, the cb() method was not.

Here is the Pyro server output:

OBJECT: <xmlrpc_pyro_diff.PSACB instance at 0x9595a8>
OBJECT.__class__: xmlrpc_pyro_diff.PSACB

Notice that the class of the object is a PSACB, which is how it was created. On the Pyro server side, we had to include code that imported the same code that the client was using. It makes sense that the Pyro server needs to import the client’s code. Pyro uses the Python standard pickle to serialize objects, so it makes sense that Pyro behaves similarly.

In summary, if you want a simple RPC solution, don’t want external dependencies, can live with the limitations of XML-RPC, and think that interoperability with other languages could come in handy, then XML-RPC is probably a good choice. On the other hand, if the limitations of XML-RPC are too constraining, you don’t mind installing external libraries, and you don’t mind being limited to using only Python, then Pyro is probably a better option for you.

SSH

SSH is an incredibly powerful, widely used protocol. You can also think of it as a tool since the most common implementation includes the same name. SSH allows you to securely connect to a remote server, execute shell commands, transfer files, and forward ports in both directions across the connection.

If you have the command-line ssh utility, why would you ever want to script using the SSH protocol? The main reason is that using the SSH protocol gives you the full power of SSH combined with the full power of Python.

The SSH2 protocol is implemented using the Python library called paramkio. From within a Python script, writing nothing but Python code, you can connect to an SSH server and accomplish those pressing SSH tasks. Example 5-9 is an example of connecting to an SSH server and executing a simple command.

Example 5-9. Connecting to an SSH server and remotely executing a command

#!/usr/bin/env python

import paramiko

hostname = '192.168.1.15'
port = 22
username = 'jmjones'
password = 'xxxYYYxxx'

if __name__ == "__main__":
paramiko.util.log_to_file('paramiko.log')
s = paramiko.SSHClient()
s.load_system_host_keys()
s.connect(hostname, port, username, password)
stdin, stdout, stderr = s.exec_command('ifconfig')
print stdout.read()
s.close()

As you can see, we import the paramiko module and define three variables. Next, we create an SSHClient object. Then we tell it to load the host keys, which, on Linux, come from the “known_hosts” file. After that we connect to the SSH server. None of these steps is particularly complicated, especially if you’re already familiar with SSH.

Now we’re ready to execute a command remotely. The call to exec_command() executes the command that you pass in and returns three file handles associated with the execution of the command: standard input, standard output, and standard error. And to show that this is being executed on a machine with the same IP address as the address we connected to with the SSH call, we print out the results of ifconfig on the remote server:

jmjones@dinkbuntu:~/code$ python paramiko_exec.py
eth0      Link encap:Ethernet  HWaddr XX:XX:XX:XX:XX:XX
inet addr:192.168.1.15  Bcast:192.168.1.255  Mask:255.255.255.0
inet6 addr: xx00::000:x0xx:xx0x:0x00/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:9667336 errors:0 dropped:0 overruns:0 frame:0
TX packets:11643909 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1427939179 (1.3 GiB)  TX bytes:2940899219 (2.7 GiB)

lo        Link encap:Local Loopback
inet addr:127.0.0.1  Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING  MTU:16436  Metric:1
RX packets:123571 errors:0 dropped:0 overruns:0 frame:0
TX packets:123571 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:94585734 (90.2 MiB)  TX bytes:94585734 (90.2 MiB)

It looks exactly as if we had run ifconfig on our local machine, except the IP address is different.

Example 5-10 shows you how to use paramiko to SFTP files between a remote machine and your local machine. This particular example only retrieves files from the remote machine using the get() method. If you want to send files to the remote machine, use the put() method.

Example 5-10. Retrieving files from an SSH server

#!/usr/bin/env python

import paramiko
import os

hostname = '192.168.1.15'
port = 22
username = 'jmjones'
password = 'xxxYYYxxx'
dir_path = '/home/jmjones/logs'

if __name__ == "__main__":
t = paramiko.Transport((hostname, port))
t.connect(username=username, password=password)
sftp = paramiko.SFTPClient.from_transport(t)
files = sftp.listdir(dir_path)
for f in files:
print 'Retrieving', f
sftp.get(os.path.join(dir_path, f), f)
t.close()

In case you want to use public/private keys rather than passwords, Example 5-11 is a modification of the remote execution example using an RSA key.

Example 5-11. Connecting to an SSH server and remotely executing a command—private keys enabled

#!/usr/bin/env python

import paramiko

hostname = '192.168.1.15'
port = 22
username = 'jmjones'
pkey_file = '/home/jmjones/.ssh/id_rsa'

if __name__ == "__main__":
key = paramiko.RSAKey.from_private_key_file(pkey_file)
s = paramiko.SSHClient()
s.load_system_host_keys()
s.connect(hostname, port, pkey=key)
stdin, stdout, stderr = s.exec_command('ifconfig')
print stdout.read()
s.close()

And Example 5-12 is a modification of the sftp script using an RSA key.

Example 5-12. Retrieving files from an SSH server

#!/usr/bin/env python

import paramiko
import os

hostname = '192.168.1.15'
port = 22
username = 'jmjones'
dir_path = '/home/jmjones/logs'
pkey_file = '/home/jmjones/.ssh/id_rsa'

if __name__ == "__main__":
key = paramiko.RSAKey.from_private_key_file(pkey_file)
t = paramiko.Transport((hostname, port))
t.connect(username=username, pkey=key)
sftp = paramiko.SFTPClient.from_transport(t)
files = sftp.listdir(dir_path)
for f in files:
print 'Retrieving', f
sftp.get(os.path.join(dir_path, f), f)
t.close()

Twisted

Twisted is an event-driven networking framework for Python that can tackle pretty much any type of network-related task you need it to. A comprehensive single solution has a price of complexity. Twisted will begin to make sense after you’ve used it a few times, but understanding it initially can be difficult. Further, learning Twisted is such a large project that finding a beginning point to solve a specific problem can often be daunting.

Despite that, though, we highly recommend that you become familiar with it and see if it fits the way you think. If you can easily tailor your thinking to “the Twisted way,” then learning Twisted will be likely to be a valuable investment. Twisted Network Programming Essentials by Abe Fettig (O’Reilly) is a good place to get started. This book helps to reduce the negative points we have mentioned.

Twisted is an event-driven network, meaning that rather than focusing on writing code that initiates connections being made and dropped and low-level details of data reception, you focus on writing code that handles those happenings.

What advantage would you gain by using Twisted? The framework encourages, and at times nearly requires, that you break your problems into small pieces. The network connection is decoupled from the logic of what occurs when connections are made. These two facts gain you some level of automatic re-usability from your code. Another thing that Twisted gains for you is that you won’t have to worry so much about lower level connection and error handling with network connections. Your part in writing network code is deciding what happens when certain events transpire.

Example 5-13 is a port checker that we’ve implemented in Twisted. It is very basic, but will demonstrate the event-driven nature of Twisted as we go through the code. But before we do that, we’ll go over a few basic concepts that you’ll need to know. The basics include reactors, factory, protocols, and deferreds. Reactors are the heart of a Twisted application’s main event loop. Reactors handle event dispatching, network communications, and threading. Factories are responsible for spawning new protocol instances. Each factory instance can spawn one type of protocol. Protocols define what to do with a specific connection. At runtime, a protocol instance is created for each connection. And deferreds are a way of chaining actions together.

Twisted

Most folks who write code have a very strong intuition about the logical flow of a program or script: it’s like water running down hill, complete with damns, shunts, etc. As a result, such code is fairly easy to think about, both the writing and the debugging. Twisted code is quite different. Being asynchronous, one might say it’s more like droplets of water in a low-g environment than a river flowing downhill, but there the analogy really breaks down. A new component has been introduced: the event listener (reactor) and friends. To create and debug Twisted code, one must abandon preconceptions with a Zen-like attitude and begin building an intuition for a different logical flow.

Example 5-13. Port checker implemented in Twisted

#!/usr/bin/env python

from twisted.internet import reactor, protocol
import sys

class PortCheckerProtocol(protocol.Protocol):
def __init__(self):
print "Created a new protocol"
def connectionMade(self):
print "Connection made"
reactor.stop()

class PortCheckerClientFactory(protocol.ClientFactory):
protocol = PortCheckerProtocol
def clientConnectionFailed(self, connector, reason):
print "Connection failed because", reason
reactor.stop()

if __name__ == '__main__':
host, port = sys.argv[1].split(':')
factory = PortCheckerClientFactory()
print "Testing %s" % sys.argv[1]
reactor.connectTCP(host, int(port), factory)
reactor.run()

Notice that we defined two classes (PortCheckerProtocol and PortCheckerClientFactory), both of which inherit from Twisted classes. We tied our factory, PortCheckerClientFactory, to PortCheckerProtocol by assigning PortCheckerProtocol to PortCheckerClientFactory’s class attribute protocol. If a factory attempts to make a connection but fails, the factory’s clientConnectionFailed() method will be called. ClientConnectionFailed() is a method that is common to all Twisted factories and is the only method we defined for our factory. By defining a method that “comes with” the factory class, we are overriding the default behavior of the class. When a client connection fails, we want to print out a message to that effect and stop the reactor.

PortCheckerProtocol is one of the protocols we discussed earlier. An instance of this class will be created once we have established a connection to the server whose port we are checking. We have only defined one method on PortCheckerProtocol: connectionMade(). This is a method that is common to all Twisted protocol classes. By defining this method ourselves, we are overriding the default behavior. When a connection is successfully made, Twisted will call this protocol’s connectionMade() method. As you can see, it prints out a simple message and stops the reactor. (We’ll get to the reactor shortly.)

In this example, both connectionMade() and clientConnectionFailed() demonstrate the “event-driven” nature of Twisted. A connection being made is an event. So also is when a client connection fails to be made. When these events occur, Twisted calls the appropriate methods to handle the events, which are referred to as event handlers.

In the main section of this example, we create an instance of PortCheckerClientFactory. We then tell the Twisted reactor to connect to the hostname and port number, which were passed in as command-line arguments, using the specified factory. After telling the reactor to connect to a certain port on a certain host, we tell the reactor to run. If we had not told the reactor to run, nothing would have happened.

To summarize the flow chronologically, we start the reactor after giving it a directive. In this case, the directive was to connect to a server and port and use PortCheckerClientFactory to help dispatch events. If the connection to the given host and port fails, the event loop will call clientConnectionFailed() on PortCheckerClientFactory. If the connection succeeds, the factory creates an instance of the protocol, PortCheckerProtocol, and calls connectionMade() on that instance. Whether the connection succeeds or fails, the respective event handlers will shut the reactor down and the program will stop running.

That was a very basic example, but it showed the basics of Twisted’s event handling nature. A key concept of Twisted programming that we did not cover in this example is the idea of deferreds and callbacks. A deferred represents a promise to execute the requested action. A callback is a way of specifying an action to accomplish. Deferreds can be chained together and pass their results on from one to the next. This point is often difficult to really understand in Twisted. (Example 5-14 will elaborate on deferreds.)

Example 5-14 is an example of using Perspective Broker, an RPC mechanism that is unique to Twisted. This example is another implementation of the remote “ls” server that we implemented in XML-RPC and Pyro, earlier in this chapter. First, we will walk you through the server.

Example 5-14. Twisted Perspective Broker server

import os
from twisted.spread import pb
from twisted.internet import reactor

class PBDirLister(pb.Root):
def remote_ls(self, directory):
try:
return os.listdir(directory)
except OSError:
return []

def remote_ls_boom(self, directory):
return os.listdir(directory)

if __name__ == '__main__':
reactor.listenTCP(9876, pb.PBServerFactory(PBDirLister()))
reactor.run()

This example defines one class, PBDirLister. This is the Perspective Broker (PB) class that will act as a remote object when the client connects to it. This example defines only two methods on this class: remote_ls() and remote_ls_boom(). Remote_ls() is, not surprisingly, one of the remote methods that the client will call. This remote_ls() method will simply return a listing of the specified directory. And remote_ls_boom() will do the same thing that remote_ls()will do, except that it won’t perform exception handling. In the main section of the example, we tell the Perspective Broker to bind to port 9876 and then run the reactor.

Example 5-15 is not as straightforward; it calls remote_ls().

Example 5-15. Twisted Perspective Broker client

#!/usr/bin/python

from twisted.spread import pb
from twisted.internet import reactor

def handle_err(reason):
print "an error occurred", reason
reactor.stop()

def call_ls(def_call_obj):
return def_call_obj.callRemote('ls', '/home/jmjones/logs')

def print_ls(print_result):
print print_result
reactor.stop()

if __name__ == '__main__':
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 9876, factory)
d = factory.getRootObject()
d.addCallback(call_ls)
d.addCallback(print_ls)
d.addErrback(handle_err)
reactor.run()

This client example defines three functions, handle_err(), call_ls(), and print_ls(). Handle_err() will handle any errors that occur along the way. Call_ls() will initiate the calling of the remote “ls” method. Print_ls() will print the results of the “ls” call. This seems a bit odd that there is one function to initiate a remote call and another to print the results of the call. But because Twisted is an asynchronous, event-driven network framework, it makes sense in this case. The framework intentionally encourages writing code that breaks work up into many small pieces.

The main section of this example shows how the reactor knows when to call which callback function. First, we create a client Perspective Broker factory and tell the reactor to connect to localhost:9876, using the PB client factory to handle requests. Next, we get a placeholder for the remote object by calling factory.getRootObject(). This is actually a deferred, so we can pipeline activity together by calling addCallback() to it.

The first callback that we add is the call_ls() function call. Call_ls() calls the callRemote() method on the deferred object from the previous step. CallRemote() returns a deferred as well. The second callback in the processing chain is print_ls(). When the reactor calls print_ls(), print_ls() prints the result of the remote call to remote_ls() in the previous step. In fact, the reactor passes in the results of the remote call into print_ls(). The third callback in the processing chain is handle_err(), which is simply an error handler that lets us know if an error occurred along the way. When either an error occurs or the pipeline reaches print_ls(), the respective methods shut the reactor down.

Here is what running this client code looks like:

jmjones@dinkgutsy:code$ python twisted_perspective_broker_client.py
['test.log']

The output is a list of files in the directory we specified, exactly as we would have expected.

This example seems a bit complicated for the simple RPC example we laid out here. The server side seems pretty comparable. Creating the client seemed to be quite a bit more work with the pipeline of callbacks, deferreds, reactors, and factories. But this was a very simple example. The structure that Twisted provides really shines when the task at hand is of a higher level of complexity.

Example 5-16 is a slight modification to the Perspective Broker client code that we just demonstrated. Rather than calling ls on the remote side, it calls ls_boom. This will show us how the client and server deal with exceptions.

Example 5-16. Twisted Perspective Broker client—error

#!/usr/bin/python

from twisted.spread import pb
from twisted.internet import reactor

def handle_err(reason):
print "an error occurred", reason
reactor.stop()

def call_ls(def_call_obj):
return def_call_obj.callRemote('ls_boom', '/foo')

def print_ls(print_result):
print print_result
reactor.stop()

if __name__ == '__main__':
factory = pb.PBClientFactory()
reactor.connectTCP("localhost", 9876, factory)
d = factory.getRootObject()
d.addCallback(call_ls)
d.addCallback(print_ls)
d.addErrback(handle_err)
reactor.run()

Here is what happens when we run this code:

jmjones@dinkgutsy:code$ python twisted_perspective_broker_client_boom.py an
error occurred [Failure instance: Traceback from remote host -- Traceback
unavailable
]

And on the server:

Peer will receive following PB traceback:
Traceback (most recent call last):
...
<more traceback>
...
state = method(*args, **kw)
File "twisted_perspective_broker_server.py", line 13, in remote_ls_boom
return os.listdir(directory)
exceptions.OSError: [Errno 2] No such file or directory: '/foo'

The specifics of the error were in the server code rather than the client. In the client, we only knew that an error had occurred. If Pyro or XML-RPC had behaved like this, we would have considered that to be a bad thing. However, in the Twisted client code, our error handler was called. Since this is a different model of programming from Pyro and XML-RPC (event-based), we expect to have to handle our errors differently, and the Perspective Broker code did what we would have expected it to do.

We gave a less-than-tip-of-the-iceberg introduction to Twisted here. Twisted can be a bit difficult to get started with because it is such a comprehensive project and takes such a different approach than what most of us are accustomed to. Twisted is definitely worth investigating further and having in your toolbox when you need it.

Scapy

If you like writing network code, you are going to love Scapy. Scapy is an incredibly handy interactive packet manipulation program and library. Scapy can discover networks, perform scans, traceroutes, and probes. There is also excellent documentation available for Scapy. If you like this intro, you should buy the book for even more details on Scapy.

The first thing to figure out about Scapy is that, as of this writing, it is kept in a single file. You will need to download the latest copy of Scapy here: http://hg.secdev.org/scapy/raw-file/tip/scapy.py. Once you download Scapy, you can run it as a standalone tool or import it and use it as a library. Let’s get started by using it as an interactive tool. Please keep in mind that you will need to run Scapy with root privileges, as it needs privileged control of your network interfaces.

Once you download and install Scapy, you will see something like this:

Welcome to Scapy (1.2.0.2)
>>>

You can do anything you would normally do with a Python interpreter,and there are special Scapy commands as well. The first thing we are going to do is run a Scapy ls() function, which lists all available layers:

>>> ls()
ARP        : ARP
ASN1_Packet : None
BOOTP      : BOOTP
CookedLinux : cooked linux
DHCP       : DHCP options
DNS        : DNS
DNSQR      : DNS Question Record
DNSRR      : DNS Resource Record
Dot11      : 802.11
Dot11ATIM  : 802.11 ATIM
Dot11AssoReq : 802.11 Association Request
Dot11AssoResp : 802.11 Association Response
Dot11Auth  : 802.11 Authentication
[snip]

We truncated the output as it is quite verbose. Now, we’ll perform a recursive DNS query of http://www.oreilly.com using Caltech University’s public DNS server:

>>> sr1(IP(dst="131.215.9.49")/UDP()/DNS(rd=1,qd=DNSQR(qname="www.oreilly.com")))
Begin emission:
Finished to send 1 packets.
...*
Received 4 packets, got 1 answers, remaining 0 packets
IP  version=4L ihl=5L tos=0x0 len=223 id=59364 flags=DF
frag=0L ttl=239 proto=udp chksum=0xb1e src=131.215.9.49 dst=10.0.1.3 options=''
|UDP  sport=domain dport=domain len=203 chksum=0x843 |
DNS  id=0 qr=1L opcode=QUERY aa=0L tc=0L rd=1L ra=1L z=0L
rcode=ok qdcount=1 ancount=2 nscount=4 arcount=3 qd=
DNSQR  qname='www.oreilly.com.' qtype=A qclass=IN |>
an=DNSRR  rrname='www.oreilly.com.' type=A rclass=IN ttl=21600 rdata='208.201.239.36'
[snip]

Next, let’s perform a traceroute:

>>> ans,unans=sr(IP(dst="oreilly.com",
>>> ttl=(4,25),id=RandShort())/TCP(flags=0x2))
Begin emission:
..............*Finished to send 22 packets.
*...........*********.***.***.*.*.*.*.*
Received 54 packets, got 22 answers, remaining 0 packets
>>> for snd, rcv in ans:
...    print snd.ttl, rcv.src, isinstance(rcv.payload, TCP)
...
[snip]
20 208.201.239.37 True
21 208.201.239.37 True
22 208.201.239.37 True
23 208.201.239.37 True
24 208.201.239.37 True
25 208.201.239.37 True

Scapy can even do pure packet dumps like tcpdump:

>>> sniff(iface="en0", prn=lambda x: x.show())
###[ Ethernet ]###
dst= ff:ff:ff:ff:ff:ff
src= 00:16:cb:07:e4:58
type= IPv4
###[ IP ]###
version= 4L
ihl= 5L
tos= 0x0
len= 78
id= 27957
flags=
frag= 0L
ttl= 64
proto= udp
chksum= 0xf668
src= 10.0.1.3
dst= 10.0.1.255
options= ''
[snip]

You can also do some very slick network visualization of traceroutes if you install graphviz and imagemagic. This example is borrowed from the official Scapy documentation:

>>> res,unans = traceroute(["www.microsoft.com","www.cisco.com","www.yahoo.com",
"www.wanadoo.fr","www.pacsec.com"],dport=[80,443],maxttl=20,retry=-2)
Begin emission:
************************************************************************
Finished to send 200 packets.
******************Begin emission:
*******************************************Finished to send 110 packets.
**************************************************************Begin emission:
Finished to send 5 packets.
Begin emission:
Finished to send 5 packets.

Received 195 packets, got 195 answers, remaining 5 packets
193.252.122.103:tcp443 193.252.122.103:tcp80 198.133.219.25:tcp443 198.133.219.25:tcp80
207.46.193.254:tcp443 207.46.193.254:tcp80 69.147.114.210:tcp443 69.147.114.210:tcp80
72.9.236.58:tcp443 72.9.236.58:tcp80

You can now create a fancy graph from those results:

>>> res.graph()
>>> res.graph(type="ps",target="| lp")
>>> res.graph(target="> /tmp/graph.svg")

Now that you’ve installed graphviz and imagemagic, the network visualization will blow your mind!

The real fun in using Scapy, though, is when you create custom command-line tools and scripts. In the next section, we will take a look at Scapy the library.

Creating Scripts with Scapy

Now that we can build something permanent with Scapy, one interesting thing to show right off the bat is an arping tool. Let’s look at a platform-specific arping tool first:

#!/usr/bin/env python
import subprocess
import re
import sys

def arping(ipaddress="10.0.1.1"):
"""Arping function takes IP Address or Network, returns nested mac/ip list"""

#Assuming use of arping on Red Hat Linux
p = subprocess.Popen("/usr/sbin/arping -c 2 %s" % ipaddress, shell=True,
stdout=subprocess.PIPE)
out = p.stdout.read()
result = out.split()
#pattern = re.compile(":")
for item in result:
if ':' in item:
print item

if __name__ == '__main__':
if len(sys.argv) > 1:
for ip in sys.argv[1:]:
print "arping", ip
arping(ip)
else:
arping()

Now let’s look at how we can create that exact same thing using Scapy, but in a platform-neutral way:

#!/usr/bin/env python
from scapy import srp,Ether,ARP,conf
import sys

def arping(iprange="10.0.1.0/24"):
"""Arping function takes IP Address or Network, returns nested mac/ip list"""

conf.verb=0
ans,unans=srp(Ether(dst="ff:ff:ff:ff:ff:ff")/ARP(pdst=iprange),
timeout=2)

collection = []
for snd, rcv in ans:
result = rcv.sprintf(r"%ARP.psrc% %Ether.src%").split()
collection.append(result)
return collection

if __name__ == '__main__':
if len(sys.argv) > 1:
for ip in sys.argv[1:]:
print "arping", ip
print arping(ip)
else:
print arping()

As you can see, the information contained in the output is quite handy, as it gives us the Mac and IP addresses of everyone on the subnet:

# sudo python scapy_arp.py
[['10.0.1.1', '00:00:00:00:00:10'], ['10.0.1.7', '00:00:00:00:00:12'],
['10.0.1.30', '00:00:00:00:00:11'], ['10.0.1.200', '00:00:00:00:00:13']]

From these examples, you should get the impression of how handy Scapy is and how easy it is to use.

Chapter 6. Data

Introduction

The need to control dealing with data, files, and directories is one of the reasons IT organizations need sysadmins. What sysadmin hasn’t had the need to process all of the files in a directory tree and parse and replace text? And if you haven’t written a script yet that renames all of the files in a directory tree, you probably will at some point in the future. These abilities are the essence of what it means to be a sysadmin, or at least to be a really good sysadmin. For the rest of this chapter, we’re going to focus on data, files, and directories.

Sysadmins need to constantly wrangle data from one location to the next. The movement of data on a daily basis is more prevelant in some sysadmin jobs than others. In the animation industry, constantly “wrangling” data from one location to the next is required because digital film production requires terabytes upon terabytes of storage. Also, there are different disk I/O requirements based on the quality and resolution of the image being viewed at any given time. If data needs to be “wrangled” to an HD preview room so that it can be inspected during a digital daily, then the “fresh” uncompressed, or slightly compressed, HD image files will need to be moved. Files need to be moved because there are generally two types of storage in animation. There is cheap, large, slow, safe, storage, and there is fast, expensive storage that is oftentimes a JBOD, or “just a bunch of disks,” striped together RAID 0 for speed. A sysadmin in the film industry who primarily deals with data is called a “data wrangler.”

A data wrangler needs to be constantly moving and migrating fresh data from location to location. Often the workhorse of moving data is rsync, scp, cp, or mv. These simple and powerful tools can be scripted with Python to do some incredible things.

Using the standard library, it is possible to do some amazing things without shelling out once. The advantage of using the standard library is that your data moving script will work just about anywhere, without having to depend on a platform-specific version of, say, tar.

Let’s also not forget backups. There are many custom backup scripts and applications that can be written with a trivial amount of Python code. We will caution that writing extra tests for your backup code is not only wise, but necessary. You should make sure you have both unit, and functional testing in place when you are depending on backup scripts you have written yourself.

In addition, it is often necessary to process data at some point before, after, or during a move. Of course, Python is great for this as well. Creating a deduplication tool, a tool that finds duplicate files, and performs actions upon them can be very helpful for this, so we’ll show you how to do it. This is one example of dealing with the constant flow of data that a sysadmin often encounters.

Using the OS Module to Interact with Data

If you have ever struggled with writing cross-platform shell scripts, you will appreciate the fact that the OS module is a portable application programming interface (API) to system services. In Python 2.5, the OS module contains over 200 methods, and many of those methods deal with data. In this section, we will go over many of the methods in that module that systems administrators care about when dealing with data.

Whenever you find yourself needing to explore a new module, IPython is often the right tool for the job, so let’s start our journey through the OS module using IPython to execute a sequence of actions that are fairly commonly encountered. Example 6-1 shows you how to do that.

Example 6-1. Exploring common OS module data methods

In [1]: import os

In [2]: os.getcwd()
Out[2]: '/private/tmp'

In [3]: os.mkdir("/tmp/os_mod_explore")

In [4]: os.listdir("/tmp/os_mod_explore")
Out[4]: []

In [5]: os.mkdir("/tmp/os_mod_explore/test_dir1")

In [6]: os.listdir("/tmp/os_mod_explore")
Out[6]: ['test_dir1']

In [7]: os.stat("/tmp/os_mod_explore")
Out[7]: (16877, 6029306L, 234881026L, 3, 501, 0, 102L,
1207014425, 1207014398, 1207014398)

In [8]: os.rename("/tmp/os_mod_explore/test_dir1",
"/tmp/os_mod_explore/test_dir1_renamed")

In [9]: os.listdir("/tmp/os_mod_explore")
Out[9]: ['test_dir1_renamed']

In [10]: os.rmdir("/tmp/os_mod_explore/test_dir1_renamed")

In [11]: os.rmdir("/tmp/os_mod_explore/")

As you can see, after we imported the OS module, in line [2] we get the current working directory, then proceed to make a directory in line [3]. We then use os.listdir in line [4] to list the contents of our newly created directory. Next, we do an os.stat, which is very similar to the stat command in Bash, and then rename a directory in line [8]. In line [9], we verify that the directory was created and then we proceed to delete what we created by using the os.rmdir method.

This is by no means an exhaustive look at the OS module. There are methods to do just about anything you would need to do to the data, including changing permissions and creating symbolic links. Please refer to the documentation for the version of Python you are using, or alternately, use IPython with tab completion to view the available methods for the OS module.

Copying, Moving, Renaming, and Deleting Data

Since we talked about data wrangling in the introduction, and you now also have a bit of an idea about how to use the OS module, we can jump right into a higher-level module, called shutil that deals with data on a larger scale. The shutil module has methods for copying, moving, renaming, and deleting data just as the OS module does, but it can perform actions on an entire data tree.

Exploring the shutil module with IPython is a fun way to get aquainted with it. In the example below, we will be using shutil.copytree, but shutil has many other methods that do slightly different things. Please refer to the Python Standard Library documentation to see the differences between shutil copy methods. See Example 6-2.

Example 6-2. Using the shutil module to copy a data tree

In [1]: import os

In [2]: os.chdir("/tmp")
In [3]: os.makedirs("test/test_subdir1/test_subdir2")

In [4]: ls -lR
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test/

./test:
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test_subdir1/

./test/test_subdir1:
total 0
drwxr-xr-x  2 ngift  wheel  68 Mar 31 22:27 test_subdir2/

./test/test_subdir1/test_subdir2:

In [5]: import shutil

In [6]: shutil.copytree("test", "test-copy")

In [19]: ls -lR
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test/
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test-copy/

./test:
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test_subdir1/

./test/test_subdir1:
total 0
drwxr-xr-x  2 ngift  wheel  68 Mar 31 22:27 test_subdir2/

./test/test_subdir1/test_subdir2:

./test-copy:
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test_subdir1/

./test-copy/test_subdir1:
total 0
drwxr-xr-x  2 ngift  wheel  68 Mar 31 22:27 test_subdir2/

./test-copy/test_subdir1/test_subdir2:

Obviously, this is quite simple, yet incredibly useful, as you can quite easily wrap this type of code into a more sophisticated cross-platform, data mover script. The immediate use for this kind of code sequence that pops into our heads is to move data from one filesystem to another on an event. In an animation environment, it is often necessary to wait for the latest frames to be finished to convert them into a sequence to edit.

We could write a script to watch a directory for “x” number of frames in a cron job. When that cron job sees that the correct number of frames has been reached, it could then migrate that directory into another directory where the frames could be processed, or even just moved so that they are on a faster disk with I/O quick enough to handle playback of uncompressed HD footage.

The shutil module doesn’t just copy files though, it also has methods for moving and deleting trees of data as well. Example 6-3 shows a move of our tree, and Example 6-4 shows how to delete it.

Example 6-3. Moving a data tree with shutil

In [20]: shutil.move("test-copy", "test-copy-moved")

In [21]: ls -lR
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test/
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test-copy-moved/

./test:
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test_subdir1/

./test/test_subdir1:
total 0
drwxr-xr-x  2 ngift  wheel  68 Mar 31 22:27 test_subdir2/

./test/test_subdir1/test_subdir2:

./test-copy-moved:
total 0
drwxr-xr-x  3 ngift  wheel  102 Mar 31 22:27 test_subdir1/

./test-copy-moved/test_subdir1:
total 0
drwxr-xr-x  2 ngift  wheel  68 Mar 31 22:27 test_subdir2/

./test-copy-moved/test_subdir1/test_subdir2:

Example 6-4. Deleting a data tree with shutil

In [22]: shutil.rmtree("test-copy-moved")

In [23]: shutil.rmtree("test-copy")
In [24]: ll

Moving a data tree is a bit more exciting than deleting a data tree, as there is nothing to show after a delete. Many of these simple examples could be combined with other actions in more sophisticated scripts. One kind of script that might be useful is to write a backup tool that copies a directory tree to cheap network storage and then creates a datestamped archive. Fortunately, we have an example of doing just that in pure Python in the backup section of this chapter.

Working with Paths, Directories, and Files

One can’t talk about dealing with data without taking into account paths, directories, and files. Every sysadmin needs to be able to, at the very least, write a tool that walks a directory, searches for a condition, and then does something with the result. We are going to cover some interesting ways to do just that.

As always, the Standard Library in Python has some killer tools to get the job done. Python doesn’t have a reputation for being “batteries included” for nothing. Example 6-5 shows how to create an extra verbose directory walking script with functions that explicitly return files, directories, and paths.

Example 6-5. Verbose directory walking script

import os
path = "/tmp"

def enumeratepaths(path=path):
"""Returns the path to all the files in a directory recursively"""
path_collection = []
for dirpath, dirnames, filenames in os.walk(path):
for file in filenames:
fullpath = os.path.join(dirpath, file)
path_collection.append(fullpath)

return path_collection

def enumeratefiles(path=path):
"""Returns all the files in a directory as a list"""
file_collection = []
for dirpath, dirnames, filenames in os.walk(path):
for file in filenames:
file_collection.append(file)

return file_collection

def enumeratedir(path=path):
"""Returns all the directories in a directory as a list"""
dir_collection = []
for dirpath, dirnames, filenames in os.walk(path):
for dir in dirnames:
dir_collection.append(dir)

return dir_collection

if __name__ == "__main__":
print "\nRecursive listing of all paths in a dir:"
for path in enumeratepaths():
print path
print "\nRecursive listing of all files in dir:"
for file in enumeratefiles():
print file
print "\nRecursive listing of all dirs in dir:"
for dir in enumeratedir():
print dir

On a Mac laptop, the output of this script looks like this:

[ngift@Macintosh-7][H:12022][J:0]# python enumarate_file_dir_path.py

Recursive listing of all paths in a dir:
/tmp/.aksusb
/tmp/ARD_ABJMMRT
/tmp/com.hp.launchport
/tmp/error.txt
/tmp/liten.py
/tmp/LitenDeplicationReport.csv
/tmp/ngift.liten.log
/tmp/hsperfdata_ngift/58920
/tmp/launch-h36okI/Render
/tmp/launch-qy1S9C/Listeners
/tmp/launch-RTJzTw/:0
/tmp/launchd-150.wDvODl/sock

Recursive listing of all files in dir:
.aksusb
ARD_ABJMMRT
com.hp.launchport
error.txt
liten.py
LitenDeplicationReport.csv
ngift.liten.log
58920
Render
Listeners
:0
sock

Recursive listing of all dirs in dir:
.X11-unix
hsperfdata_ngift
launch-h36okI
launch-qy1S9C
launch-RTJzTw
launchd-150.wDvODl
ssh-YcE2t6PfnO

A note about the previous code snippet—os.walk returns a generator object, so if you call pass a value to os.walk, you can walk a tree yourself:

In [2]: import os

In [3]: os.walk("/tmp")
Out[3]: [generator object at 0x508e18]

This is what it looks like when it is run from IPython. You will notice using a generator gives us the ability to call path.next(). We won’t get into the nitty gritty details about generators, but it is important to know that os.walk returns a generator object. Generators are tremendously useful for systems programming. Visit David Beazely’s website (http://www.dabeaz.com/generators/) to find out all you need to know about them.

In [2]: import os

In [3]: os.walk("/tmp")
Out[3]: [generator object at 0x508e18]

In [4]: path = os.walk("/tmp")

In [5]: path.
path.__class__         path.__init__          path.__repr__          path.gi_running
path.__delattr__       path.__iter__          path.__setattr__       path.next
path.__doc__           path.__new__           path.__str__           path.send
path.__getattribute__  path.__reduce__        path.close             path.throw
path.__hash__          path.__reduce_ex__     path.gi_frame

In [5]: path.next()
Out[5]:
('/tmp',
['.X11-unix',
'hsperfdata_ngift',
'launch-h36okI',
'launch-qy1S9C',
'launch-RTJzTw',
'launchd-150.wDvODl',
'ssh-YcE2t6PfnO'],
['.aksusb',
'ARD_ABJMMRT',
'com.hp.launchport',
'error.txt',
'liten.py',
'LitenDeplicationReport.csv',
'ngift.liten.log'])

In a bit, we will look at generators in more detail, but let’s first make a cleaner module that gives us files, directories, and paths in a clean API.

Now that we have walked a very basic directory, let’s make this an object-oriented module so that we can easily import and reuse it again. It will take a small amount of work to make a hardcoded module, but a generic module that we can reuse later will certainly make our lives easier. See Example 6-6.

Example 6-6. Creating reusable directory walking module

import os

class diskwalk(object):
"""API for getting directory walking collections"""
def __init__(self, path):
self.path = path

def enumeratePaths(self):
"""Returns the path to all the files in a directory as a list"""
path_collection = []
for dirpath, dirnames, filenames in os.walk(self.path):
for file in filenames:
fullpath = os.path.join(dirpath, file)
path_collection.append(fullpath)

return path_collection

def enumerateFiles(self):
"""Returns all the files in a directory as a list"""
file_collection = []
for dirpath, dirnames, filenames in os.walk(self.path):
for file in filenames:
file_collection.append(file)

return file_collection

def enumerateDir(self):
"""Returns all the directories in a directory as a list"""
dir_collection = []
for dirpath, dirnames, filenames in os.walk(self.path):
for dir in dirnames:
dir_collection.append(dir)

return dir_collection

As you can see, with a few small modifications, we were able to make a very nice interface for future modifications. One of the nice things about this new module is that we can import it into another script.

Comparing Data

Comparing data is quite important to a sysadmin. Questions you might often ask yourself are, “What files are different between these two directories? How many copies of this same file exist on my system?” In this section, you will find the ways to answer those questions and more.

When dealing with massive quantities of important data, it often is necessary to compare directory trees and files to see what changes have been made. This becomes critical if you start writing large data mover scripts. The absolute doomsday scenario is to write a large data move script that damages critical production data.

In this section, we will first explore a few lightweight methods to compare files and directories and then move on to eventually exploring doing checksum comparisons of files. The Python Standard Library has several modules that assist with comparisons and we will be covering filecmp and os.listdir.

Using the filecmp Module

The filecmp module contains functions for doing fast and efficient comparisons of files and directories. The filecmp module will perform a os.stat on two files and return a True if the results of os.stat are the same for both files or a False if the results are not. Typically, os.stat is used to determine whether or not two files use the same inodes on a disk and whether they are the same size, but it does not actually compare the contents.

In order to fully understand how filecmp works, we need to create three files from scratch. To do this on computer, change into the /tmp directory, make a file called file0.txt, and place a “0” in the file. Next, create a file called file1.txt, and place a “1” in that file. Finally, create a file called file00.txt, and place a “0” in it. We will use these files as examples in the following code:

In [1]: import filecmp

In [2]: filecmp.cmp("file0.txt", "file1.txt")
Out[2]: False

In [3]: filecmp.cmp("file0.txt", "file00.txt")
Out[3]: True

As you can see, the cmp function returned True in the case of file0.txt and file00.txt, and False when file1.txt was compared with file0.txt.

The dircmp function has a number of attributes that report differences between directory trees. We won’t go over every attribute, but we have created a few examples of useful things you can do. For this example, we created two subdirectories in the /tmp directory and copied the files from our previous example into each directory. In dirB, we created one extra file named file11.txt, into which we put “11”:

In [1]: import filecmp

In [2]: pwd
Out[2]: '/private/tmp'

In [3]: filecmp.dircmp("dirA", "dirB").diff_files
Out[3]: []

In [4]: filecmp.dircmp("dirA", "dirB").same_files
Out[4]: ['file1.txt', 'file00.txt', 'file0.txt']

In [5]: filecmp.dircmp("dirA", "dirB").report()
diff dirA dirB
Only in dirB : ['file11.txt']
Identical files : ['file0.txt', 'file00.txt', 'file1.txt']

You might be a bit surprised to see here that there were no matches for diff_files even though we created a file11.txt that has unique information in it. The reason is that diff_files compares only the differences between files with the same name.

Next, look at the output of same_files, and notice that it only reports back files that are identical in two directories. Finally, we can generate a report as shown in the last example. It has a handy output that includes a breakdown of the differences between the two directories. This brief overview is just a bit of what the filecmp module can do, so we recommend taking a look at the Python Standard Library documentation to get a full overview of the features we did not have space to cover.

Using os.list

Another lightweight method of comparing directories is to use os.listdir. You can think of os.listdir as an ls command that returns a Python list of the files found. Because Python supports many interesting ways to deal with lists, you can use os.listdir to determine differences in a directory yourself, quite simply by converting your list into a set and then subtracting one set from another. Here is an example of what this looks like in IPython:

In [1]: import os

In [2]: dirA = set(os.listdir("/tmp/dirA"))

In [3]: dirA
Out[3]: set(['file1.txt', 'file00.txt', 'file0.txt'])

In [4]: dirB = set(os.listdir("/tmp/dirB"))

In [5]: dirB
Out[5]: set(['file1.txt', 'file00.txt', 'file11.txt', 'file0.txt'])

In [6]: dirA - dirB
Out[6]: set([])

In [7]: dirB-dirA
Out[7]: set(['file11.txt'])

From this example, you can see that we used a neat trick of converting two lists into sets and then subtracting the sets to find the differences. Notice that line [7] returns file11.txt because dirB is a superset of dirA, but in line [6] the results are empty because dirA contains all of the same items as dirB. Using sets makes it easy to create a simple merge of two data structures as well, by subtracting the full paths of one directory against another and then copying the difference. We will discuss merging data in the next section.

This approach has some very large limitations though. The actual name of a file is often misleading, as it is possible to have a file that is 0k that has the same name as a file with 200 GBs. In the next section, we cover a better approach to finding the differences between two directories and merging the contents together.

Merging Data

What can you do when you don’t want to simply compare data files, but you would like to merge two directory trees together? A problem often can occur when you want to merge the contents of one tree into another without creating any duplicates.

You could just blindly copy the files from one directory into your target directory, and then deduplicate the directory, but it would be more efficient to prevent the duplicates in the first place. One reasonably simple solution would be to use the filecmp module’s dircmp function to compare two directories, and then copy the unique results using the os.listdir technique described earlier. A better choice would be to use MD5 checksums, which we explain in the next section.

MD5 Checksum Comparisons

Performing a MD5 checksum on a file and comparing it to another file is like going target shooting with a bazooka. It is the big weapon you pull out when you want to be sure of what you are doing, although a byte-by-byte comparison is truly 100 percent accurate. Example 6-7 shows how the function takes in a path to a file and returns a checksum.

Example 6-7. Performing an MD5 checksum on files

import hashlib

def create_checksum(path):
"""
Reads in file. Creates checksum of file line by line.
Returns complete checksum total for file.
"""
fp = open(path)
checksum = hashlib.md5()
while True:
buffer = fp.read(8192)
if not buffer:break
checksum.update(buffer)
fp.close()
checksum = checksum.digest()
return checksum

Here is an iterative example that uses this function with IPython to compare two files:

In [2]: from checksum import createChecksum

In [3]: if createChecksum("image1") == createChecksum("image2"):
...:     print "True"
...:
...:
True

In [5]: if createChecksum("image1") == createChecksum("image_unique"):
print "True"
...:
...:

In that example, the checksums of the files were manually compared, but we can use the code we wrote earlier that returns a list of paths to recursively compare a directory tree full of files and gives us duplicates. One of the other nice things about creating a reasonable API is that we can now use IPython to interactively test our solution. Then, if it works, we can create another module. Example 6-8 shows the code for finding the duplicates.

Example 6-8. Performing an MD5 checksum on a directory tree to find duplicates

In [1]: from checksum import createChecksum

In [2]: from diskwalk_api import diskwalk

In [3]: d = diskwalk('/tmp/duplicates_directory')

In [4]: files = d.enumeratePaths()

In [5]: len(files)
Out[5]: 12

In [6]: dup = []

In [7]: record = {}

In [8]: for file in files:
compound_key = (getsize(file),create_checksum(file))
if compound_key in record:
dup.append(file)
else:

record[compound_key] = file

....:
....:
In [9]: print dup
['/tmp/duplicates_directory/image2']

The only portion of this code that we haven’t looked at in previous examples is found on line [8]. We create an empty dictionary and then use a key to store the checksum we generate. This can serve as a simple way to determine whether or not that checksum has been seen before. If it has, then we toss the file into a dup list. Now, let’s separate this into a piece of code we can use again. After all that is quite useful. Example 6-9 shows how to do that.

Example 6-9. Finding duplicates

from checksum import create_checksum
from diskwalk_api import diskwalk
from os.path import getsize

def findDupes(path = '/tmp'):
dup = []
record = {}
d = diskwalk(path)
files = d.enumeratePaths()
for file in files:
compound_key = (getsize(file),create_checksum(file))
if compound_key in record:
dup.append(file)
else:
#print "Creating compound key record:", compound_key
record[compound_key] = file
return dup

if __name__ == "__main__":
dupes = findDupes()
for dup in dupes:
print “Duplicate: %s” % dup

When we run that script, we get the following output:

[ngift@Macintosh-7][H:10157][J:0]# python find_dupes.py
Duplicate: /tmp/duplicates_directory/image2

We hope you can see that this shows what even a little bit of code reuse can accomplish. We now have a generic module that will take a directory tree and return a list of all the duplicate files. This is quite handy in and of itself, but next we can take this one step further and automatically delete the duplicates.

Deleting files in Python is simple, as you can use os.remove (file). In this example, we have a number of 10 MB files in our /tmp directory; let’s try to delete one of them using os.remove (file):

In [1]: import os

In [2]: os.remove("10
10mbfile.0  10mbfile.1  10mbfile.2  10mbfile.3  10mbfile.4
10mbfile.5  10mbfile.6  10mbfile.7  10mbfile.8

In [2]: os.remove("10mbfile.1")

In [3]: os.remove("10
10mbfile.0  10mbfile.2  10mbfile.3  10mbfile.4  10mbfile.5
10mbfile.6  10mbfile.7  10mbfile.8

Notice that tab completion in IPython allows us to see the matches and fills out the names of the image files for us. Be aware that the os.remove (file) method is silent and permanent, so this might or might not be what you want to do. With that in mind, we can implement an easy method to delete our duplicates, and then enhance it after the fact. Because it is so easy to test interactive code with IPython, we are going to write a test function on the fly and try it:

In [1]: from find_dupes import findDupes

In [2]: dupes = findDupes("/tmp")

In [3]: def delete(file):
import os
...:     print "deleting %s" % file
...:     os.remove(file)
...:
...:

In [4]: for dupe in dupes:
...:     delete(dupe)
...:
...:
In [5]: for dupe in dupes:
delete(dupe)
...:
...:
deleting /tmp/10mbfile.2
deleting /tmp/10mbfile.3
deleting /tmp/10mbfile.4
deleting /tmp/10mbfile.5
deleting /tmp/10mbfile.6
deleting /tmp/10mbfile.7
deleting /tmp/10mbfile.8

In this example, we added some complexity to our delete method by including a print statement of the files we automatically deleted. Just because we created a whole slew of reusable code, it doesn’t mean we need to stop now. We can create another module that does fancy delete-related things when it is a file object. The module doesn’t even need to be tied to duplicates, it can be used to delete anything. See Example 6-10.

Example 6-10. Delete module

#!/usr/bin/env python
import os

class Delete(object):
"""Delete Methods For File Objects"""

def __init__(self, file):
self.file = file

def interactive(self):
"""interactive deletion mode"""

input = raw_input("Do you really want to delete %s [N]/Y" % self.file)
if input.upper():
print "DELETING:  %s" % self.file
status = os.remove(self.file)
else:
print "Skipping:  %s" % self.file
return
def dryrun(self):
"""simulation mode for deletion"""

print "Dry Run:  %s [NOT DELETED]" % self.file
return

def delete(self):

"""Performs a delete on a file, with additional conditions
"""

print "DELETING:  %s" % self.file
try:
status = os.remove(self.file)
except Exception, err:
print err
return status

if __name__ == "__main__":
from find_dupes import findDupes
dupes = findDupes('/tmp')

for dupe in dupes:
delete = Delete(dupe)
#delete.dryrun()
#delete.delete()
#delete.interactive()

In this module, you will see three different types of deletes. The interactive delete method prompts the user to confirm each file he is going to delete. This can seem a bit annoying, but it is good protection when other programmers will be maintaining and updating the code.

The dry run method simulates a deletion. And, finally, there is an actual delete method that will permanently delete your files. At the bottom of the module, you can see that there is a commented example of the ways to use each of these three different methods. Here is an example of each method in action:

- Dry run
ngift@Macintosh-7][H:10197][J:0]# python delete.py
Dry Run:  /tmp/10mbfile.1 [NOT DELETED]
Dry Run:  /tmp/10mbfile.2 [NOT DELETED]
Dry Run:  /tmp/10mbfile.3 [NOT DELETED]
Dry Run:  /tmp/10mbfile.4 [NOT DELETED]
Dry Run:  /tmp/10mbfile.5 [NOT DELETED]
Dry Run:  /tmp/10mbfile.6 [NOT DELETED]
Dry Run:  /tmp/10mbfile.7 [NOT DELETED]
Dry Run:  /tmp/10mbfile.8 [NOT DELETED]

- Interactive
ngift@Macintosh-7][H:10201][J:0]# python delete.py
Do you really want to delete /tmp/10mbfile.1 [N]/YY
DELETING:  /tmp/10mbfile.1
Do you really want to delete /tmp/10mbfile.2 [N]/Y
Skipping:  /tmp/10mbfile.2
Do you really want to delete /tmp/10mbfile.3 [N]/Y

- Delete
[ngift@Macintosh-7][H:10203][J:0]# python delete.py
DELETING:  /tmp/10mbfile.1
DELETING:  /tmp/10mbfile.2
DELETING:  /tmp/10mbfile.3
DELETING:  /tmp/10mbfile.4
DELETING:  /tmp/10mbfile.5
DELETING:  /tmp/10mbfile.6
DELETING:  /tmp/10mbfile.7
DELETING:  /tmp/10mbfile.8

You might find using encapsulation techniques like this very handy when dealing with data because you can prevent a future problem by abstracting what you are working on enough to make it nonspecific to your problem. In this situation, we wanted to automatically delete duplicate files, so we created a module that generically finds filenames and deletes them. We could make another tool that generically takes file objects and applies some form of compression as well. We are actually going to get to that example in just a bit.

Pattern Matching Files and Directories

So far you have seen how to process directories and files, and perform actions such as finding duplicates, deleting directories, moving directories, and so on. The next step in mastering the directory tree is to use pattern matching, either alone or in combination with these previous techniques. As just about everything else in Python, performing a pattern match for a file extension or filename is simple. In this section, we will demonstrate a few common pattern matching problems and apply the techniques used earlier to create simple, yet powerful reusable tools.

A fairly common problem sysadmins need to solve is that they need to track down and delete, move, rename, or copy a certain file type. The most straightforward approach to doing this in Python is to use either the fnmatch module or the glob module. The main difference between these two modules is that fnmatch returns a True or False for a Unix wildcard, and glob returns a list of pathnames that match a pattern. Alternatively, regular expressions can be used to create more sophisticated pattern matching tools. Please refer to Chapter 3 to get more detailed instructions on using regular expressions to match patterns.

Example 6-11 will look at how fnmatch and glob can be used. We will reuse the code we’ve been working on by importing diskwalk from the diskwalk_api module.

Example 6-11. Interactively using fnmatch and glob to search for file matches

In [1]: from diskwalk_api import diskwalk

In [2]: files = diskwalk("/tmp")

In [3]: from fnmatch import fnmatch

In [4]: for file in files:
...:     if fnmatch(file,"*.txt"):
...:         print file
...:
...:
/tmp/file.txt

In [5]: from glob import glob

In [6]: import os

In [7]: os.chdir("/tmp")

In [8]: glob("*")
Out[8]: ['file.txt', 'image.iso', 'music.mp3']

In the previous example, after we reused our previous diskwalk module, we received a list of all of the full paths located in the /tmp directory. We then used fnmatch to determine whether each file matched the pattern “*.txt”. The glob module is a bit different, in that it will literally “glob,” or match a pattern, and return the full path. Glob is a much higher-level function than fnmatch, but both are very useful tools for slightly different jobs.

The fnmatch function is particularly useful when it is combined with other code to create a filter to search for data in a directory tree. Often, when dealing with directory trees, you will want to work with files that match certain patterns. To see this in action, we will solve a classic sysadmin problem by renaming all of the files that match a pattern in a directory tree. Keep in mind that it is just as simple to rename files as it is to delete, compress, or process them. There is a simple pattern here:

1. Get the path to a file in a directory.

2. Perform some optional layer of filtering; this could involve many filters, such as filename, extension, size, uniqueness, and so on.

3. Perform an action on them; this could be copying, deleting, compressing, reading, and so on. Example 6-12 shows how to do this.

Example 6-12. Renaming a tree full of MP3 files to text files

In [1]: from diskwalk_api import diskwalk

In [2]: from shutil import move

In [3]: from fnmatch import fnmatch

In [4]: files = diskwalk("/tmp")

In [5]: for file in files:
if fnmatch(file, "*.mp3"):
#here we can do anything we want, delete, move, rename...hmmm rename
move(file, "%s.txt" % file)
In [6]: ls -l /tmp/
total 0
-rw-r--r--  1 ngift  wheel  0 Apr  1 21:50 file.txt
-rw-r--r--  1 ngift  wheel  0 Apr  1 21:50 image.iso
-rw-r--r--  1 ngift  wheel  0 Apr  1 21:50 music.mp3.txt
-rw-r--r--  1 ngift  wheel  0 Apr  1 22:45 music1.mp3.txt
-rw-r--r--  1 ngift  wheel  0 Apr  1 22:45 music2.mp3.txt
-rw-r--r--  1 ngift  wheel  0 Apr  1 22:45 music3.mp3.txt

Using code we already wrote, we used four lines of very readable Python code to rename a tree full of mp2 files to text files. If you are one of the few sysadmins who has not read at least one episode of BOFH, or Bastard Operator From Hell, it might not be immediately obvious what we could do next with our bit of code.

Imagine you have a production file server that is strictly for high-performance disk I/O storage, and it has a limited capacity. You have noticed that it often gets full because one or two abusers place hundreds of GBs of MP3 files on it. You could put a quota on the amount of file space each user can access, of course, but often that is more trouble than it is worth. One solution would be to create a cron job every night that finds these MP3 files, and does “random” things to them. On Monday it could rename them to text files, on Tuesday it could compress them into ZIP files, on Wednesday it could move them all into the /tmp directory, and on Thursday it could delete them, and send the owner of the file an emailed list of all the MP3 files it deleted. We would not suggest doing this unless you own the company you work for, but for the right BOFH, the earlier code example is a dream come true.

Wrapping Up rsync

As you might well already know, rsync is a command-line tool that was originally written by Andrew Tridgell and Paul Mackerra. Late in 2007, rsync version 3 was released for testing and it includes an even greater assortment of options than the original version.

Over the years, we have found ourselves using rsync as the primary tool to move data from point A to point B. The manpage and options are staggering works, so we recommend that you read through them in detail. Rsync may just be the single most useful command-line tool ever written for systems administrators.

With that being said, there are some ways that Python can help control, or glue rsync’s behavior. One problem that we have encountered is ensuring that data gets copied at a scheduled time. We have been in many situations in which we needed to synchronize TBs of data as quickly as possible between one file server and another, but we did not want to monitor the process manually. This is a situation in which Python really makes a lot of sense.

With Python you can add a degree of artificial intelligence to rsync and customize it to your particular needs. The point of using Python as glue code is that you make Unix utilities do things they were never intended to do, and so you make highly flexible and customizable tools. The limit is truly only your imagination. Example 6-13 shows a very simple example of how to wrap rsync.

Example 6-13. Simple wrap of rsync

#!/usr/bin/env python
#wraps up rsync to synchronize two directories
from subprocess import call
import sys

source = "/tmp/sync_dir_A/" #Note the trailing slash
target = "/tmp/sync_dir_B"
rsync = "rsync"
arguments = "-a"
cmd = "%s %s %s %s" % (rsync, arguments, source, target)

def sync():

ret = call(cmd, shell=True)
if ret !=0:
print "rsync failed"
sys.exit(1)
sync()

This example is hardcoded to synchronize two directories and to print out a failure message if the command does not work. We could do something a bit more interesting, though, and solve a problem that we have frequently run into. We have often found that we are called upon to synchronize two very large directories, and we don’t want to monitor data synchronization overnight. But if you don’t monitor the synchronization, you can find that it disrupted partway through the process, and quite often the data, along with a whole night, is wasted, and the process needs to start again the next day. Using Python, you can create a more aggressive, highly motivated rsync command.

What would a highly motivated rsync command do exactly? Well, it would do what you would do if you were monitoring the synchronization of two directories: it would continue trying to synchronize the directories until it finished, and then it would send an email saying it was done. Example 6-14 shows the rsync code of our little over achiever in action.

Example 6-14. An rsync command that doesn’t quit until the job is finished

#!/usr/bin/env python
#wraps up rsync to synchronize two directories
from subprocess import call
import sys
import time

"""this motivated rsync tries to synchronize forever"""

source = "/tmp/sync_dir_A/" #Note the trailing slash
target = "/tmp/sync_dir_B"
rsync = "rsync"
arguments = "-av"
cmd = "%s %s %s %s" % (rsync, arguments, source, target)

def sync():
while True:
ret = call(cmd, shell=True)
if ret !=0:
print "resubmitting rsync"
time.sleep(30)
else:
print "rsync was succesful"
subprocess.call("mail -s 'jobs done' bofh@example.com", shell=True)
sys.exit(0)
sync()

</literallayout>
</example>

This is overly simplified and contains hardcoded data, but it is an example of the kind of useful tool you can develop to automate something you normally need to monitor manually. There are some other features you can include, such as the ability to set the retry interval and limit as well as the ability to check for disk usage on the machine to which you are connecting and so on.

Metadata: Data About Data

Most systems administrators get to the point where they start to be concerned, not just about data, but about the data about the data. Metadata, or data about data, can often be more important than the data itself. To give an example, in film and television, the same data often exists in multiple locations on a filesystem or even on several filesystems. Keeping track of the data often involves creating some type of metadata management system.

It is the data about how those files are organized and used, though, that can be the most critical to an application, to an animation pipeline, or to restore a backup. Python can help here, too, as it is easy to both use metadata and write metadata with Python.

Let’s look at using a popular ORM, SQLAlchemy, to create some metadata about a filesystem. Fortunately, the documentation for SQLAlchemy is very good, and SQLAlchemy works with SQLite. We think this is a killer combination for creating custom metadata solutions.

In the examples above, we walked a filesystem in real time and performed actions and queries on paths that we found. While this is incredibly useful, it is also time-consuming to search a large filesystem consisting of millions of files to do just one thing. In Example 6-15, we show what a very basic metadata system could look like by combining the directory walking techniques we have just mastered with an ORM.

Example 6-15. Creating metadata about a filesystem with SQLAlchemy

#!/usr/bin/env python
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, String, MetaData, ForeignKey
from sqlalchemy.orm import mapper, sessionmaker
import os

#path
path = " /tmp"

#Part 1:  create engine
engine = create_engine('sqlite:///:memory:', echo=False)

#Part 2:  metadata
metadata = MetaData()

filesystem_table = Table('filesystem', metadata,
Column('id', Integer, primary_key=True),
Column('path', String(500)),
Column('file', String(255)),
)

metadata.create_all(engine)

#Part 3:  mapped class
class Filesystem(object):

def __init__(self, path, file):
self.path = path
self.file = file

def __repr__(self):
return "[Filesystem('%s','%s')]" % (self.path, self.file)

#Part 4:  mapper function
mapper(Filesystem,filesystem_table)

#Part 5:  create session
Session = sessionmaker(bind=engine, autoflush=True, transactional=True)
session = Session()

#Part 6:  crawl file system and populate database with results
for dirpath, dirnames, filenames in os.walk(path):
for file in filenames:
fullpath = os.path.join(dirpath, file)
record = Filesystem(fullpath, file)
session.save(record)

#Part 7:  commit to the database
session.commit()

#Part 8:  query
for record in session.query(Filesystem):
print "Database Record Number: %s, Path: %s , File: %s " \
% (record.id,record.path, record.file)

It would be best to think about this code as a set of procedures that are followed one after another. In part one, we create an engine, which is really just a fancy way of defining the database we are going to use. In part two, we define a metadata instance, and create our database tables. In part three, we create a class that will map to the tables in the database that we created. In part four, we call a mapper function that puts the ORM; it actually maps this class to the tables. In part five, we create a session to our database. Notice that there are a few keyword parameters that we set, including autoflush and transactional.

Now that we have the very explicit ORM setup completed, in part six, we do our usual song and dance, and grab the filenames and complete paths while we walk a directory tree. There are a couple of twists this time, though. Notice that we create a record in the database for each fullpath and file we encounter, and that we then save each newly created record as it is created. We then commit this transaction to our “in memory” SQLite database in part seven.

Finally, in part eight, we perform a query, in Python, of course, that returns the results of the records we placed in the database. This example could potentially be a fun way to experiment with creating custom SQLAlchemy metadata solutions for your company or clients. You could expand this code to do something interesting, such as perform relational queries or write results out to a file, and so on.

Archiving, Compressing, Imaging, and Restoring

Dealing with data in big chunks is a problem that sysadmins have to face every day. They often use tar, dd, gzip, bzip, bzip2, hdiutil, asr, and other utilities to get their jobs done.

Believe it or not, the “batteries included” Python Standard Library has built-in support for TAR files, zlib files, and gzip files. If compression and archiving is your goal, then you will not have any problem with the rich tools Python has to offer. Let’s look at the grandaddy of all archive packages: tar; and we we’ll see how the standard library implements tar.

Using tarfile Module to Create TAR Archives

Creating a TAR archive is quite easy, almost too easy in fact. In Example 6-16, we create a very large file as an example. Note, the syntax is much more user friendly than even the tar command itself.

Example 6-16. Create big text file

In [1]: f = open("largeFile.txt", "w")

In [2]: statement = "This is a big line that I intend to write over and over again."
ln [3]: x = 0
In [4]: for x in xrange(20000):
....:     x += 1
....:     f.write("%s\n" % statement)
....:
....:
In [4]: ls -l
-rw-r--r-- 1 root root 1236992 Oct 25 23:13 largeFile.txt

OK, now that we have a big file full of junk, let’s TAR that baby up. See Example 6-17.

Example 6-17. TAR up contents of file

In [1]: import tarfile

In [2]: tar = tarfile.open("largefile.tar", "w")

In [3]: tar.add("largeFile.txt")

In [4]: tar.close()

In [5]: ll

-rw-r--r-- 1 root root 1236992 Oct 25 23:15 largeFile.txt
-rw-r--r-- 1 root root 1236992 Oct 26 00:39 largefile.tar

So, as you can see, this makes a vanilla TAR archive in a much easier syntax than the regular tar command. This certainly makes the case for using the IPython shell to do all of your daily systems administration work.

While it is handy to be able to create a TAR file using Python, it is almost useless to TAR up only one file. Using the same directory walking pattern we have used numerous times in this chapter, we can create a TAR file of the whole /tmp directory by walking the tree and then adding each file to the contents of the /tmp directory TAR. See Example 6-18.

Example 6-18. TAR up contents of a directory tree

In [27]: import tarfile

In [28]: tar = tarfile.open("temp.tar", "w")

In [29]: import os

In [30]: for root, dir, files in os.walk("/tmp"):
....:     for file in filenames:
....:
KeyboardInterrupt

In [30]: for root, dir, files in os.walk("/tmp"):
for file in files:
....:         fullpath = os.path.join(root,file)
....:         tar.add(fullpath)
....:
....:

In [33]: tar.close()

It is quite simple to add the contents of a directory tree by walking a directory, and it is a good pattern to use, because it can be combined with some of the other techniques we have covered in this chapter. Perhaps you are archiving a directory full of media files. It seems silly to archive exact duplicates, so perhaps you want to replace duplicates with symbolic links before you create a TAR file. With the information in this chapter, you can easily build the code that will do just that and save quite a bit of space.

Since doing a generic TAR archive is a little bit boring, let’s spice it up a bit and add bzip2 compression, which will make your CPU whine and complain at how much you are making it work. The bzip2 compression algorithm can do some really funky stuff. Let’s look at an example of how impressive it can truly be.

Then get real funky and make a 60 MB text file shrink down to 10 K! See Example 6-19.

Example 6-19. Creating bzip2 TAR archive

In [1: tar = tarfile.open("largefilecompressed.tar.bzip2", "w|bz2")

In [2]: tar.add("largeFile.txt")

In [3]: ls -h
foo1.txt fooDir1/ largeFile.txt largefilecompressed.tar.bzip2*
foo2.txt fooDir2/ largefile.tar

ln [4]: tar.close()

In [5]: ls -lh

-rw-r--r-- 1 root root 61M Oct 25 23:15 largeFile.txt
-rw-r--r-- 1 root root 61M Oct 26 00:39 largefile.tar
-rwxr-xr-x 1 root root 10K Oct 26 01:02 largefilecompressed.tar.bzip2*

What is amazing is that bzip2 was able to compress our 61 M text file into 10 K, although we did cheat quite a bit using the same data over and over again. This didn’t come at zero cost of course, as it took a few minutes to compress this file on a dual core AMD system.

Let’s go the whole nine yards and do a compressed archive with the rest of the available options, starting with gzip next. The syntax is only slightly different. See Example 6-20.

Example 6-20. Creating a gzip TAR archive

In [10]: tar = tarfile.open("largefile.tar.gzip", "w|gz")

In [11]: tar.add("largeFile.txt")

ln [12]: tar.close()

In [13]: ls -lh

-rw-r--r-- 1 root root  61M Oct 26 01:20 largeFile.txt
-rw-r--r-- 1 root root  61M Oct 26 00:39 largefile.tar
-rwxr-xr-x 1 root root 160K Oct 26 01:24 largefile.tar.gzip*

A gzip archive is still incredibly small, coming in at 160 K, but on my machine it was able to create this compressed TAR file in seconds. This seems like a good trade-off in most situations.

Using a tarfile Module to Examine the Contents of TAR Files

Now that we have a tool that creates TAR files, it only makes sense to examine the TAR file’s contents. It is one thing to blindly create a TAR file, but if you have been a systems administrator for any length of time, you have probably gotten burned by a bad backup, or have been accused of making a bad backup.

To put this situation in perspective and highlight the importance of examining TAR archives, we will share a story about a fictional friend of ours, let’s call it The Case of the Missing TAR Archive. Names, identities, and facts, are fictional; if this story resembles reality, it is completely coincidental.

OUr friend worked at a major television studio as a systems administrator and was responsible for supporting a department led by a real crazy man. This man had a reputation for not telling the truth, acting impulsively, and well, being crazy. If a situation arose where the crazy man was at fault, like he missed a deadline with a client, or didn’t produce a segment according to the specifications he was given, he would gladly just lie and blame it on someone else. Often times, that someone else was our friend, the systems administrator.

Unfortunately, our friend, was responsible for maintaining this lunatic’s backups. His first thought was it was time to look for a new job, but he had worked at this studio for many years, and had many friends, and didn’t want to waste all that on this temporarily bad situation. He needed to make sure he covered all of his bases and so instituted a logging system that categorized the contents of all of the automated TAR archives that were created for the crazy man, as he felt it was only a matter of time before he would get burned when the crazy man missed a deadline, and needed an excuse.

One day our friend, William, gets a call from his boss, “William I need to see you in my office immediately, we have a situation with the backups.” William, immediately walked over to his office, and was told that the crazy man, Alex, had accused William of damaging the archive to his show, and this caused him to miss a deadline with his client. When Alex missed deadlines with his client, it made Alex’s boss Bob, very upset.

William was told by his boss that Alex had told him the backup contained nothing but empty, damaged files, and that he had been depending on that archive to work on his show. William then told his boss, he was certain that he would eventually be accused of messing up an archive, and had secretly written some Python code that inspected the contents of all the TAR archives he had made and created extended information about the attributes of the files before and after they were backed up. It turned out that Alex had never created a show to begin with and that there was an empty folder being archived for months.

When Alex was confronted with this information, he quickly backpeddled and looked for some way to shift attention onto a new issue. Unfortunately for Alex, this was the last straw and a couple of months later, he never showed up to work. He may have either left or been fired, but it didn’t matter, our friend, William had solved, The Case of the Missing TAR Archive.

The moral of this story is that when you are dealing with backups, treat them like nuclear weapons, as backups are fraught with danger in ways you might not even imagine.

Here are several methods to examine the contents of the TAR file we created earlier:

In [1]: import tarfile

In [2]: tar = tarfile.open("temp.tar","r")

In [3]: tar.list()
-rw-r--r-- ngift/wheel          2 2008-04-04 15:17:14 tmp/file00.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 15:15:39 tmp/file1.txt
-rw-r--r-- ngift/wheel          0 2008-04-04 20:50:57 tmp/temp.tar
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:07 tmp/dirA/file0.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:07 tmp/dirA/file00.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:07 tmp/dirA/file1.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:52 tmp/dirB/file0.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:52 tmp/dirB/file00.txt
-rw-r--r-- ngift/wheel          2 2008-04-04 16:19:52 tmp/dirB/file1.txt
-rw-r--r-- ngift/wheel          3 2008-04-04 16:21:50 tmp/dirB/file11.txt

In [4]: tar.name
Out[4]: '/private/tmp/temp.tar'

In [5]: tar.getnames()
Out[5]:
['tmp/file00.txt',
'tmp/file1.txt',
'tmp/temp.tar',
'tmp/dirA/file0.txt',
'tmp/dirA/file00.txt',
'tmp/dirA/file1.txt',
'tmp/dirB/file0.txt',
'tmp/dirB/file00.txt',
'tmp/dirB/file1.txt',
'tmp/dirB/file11.txt']

In [10]: tar.members
Out[10]:
[<TarInfo 'tmp/file00.txt' at 0x109eff0>,
<TarInfo 'tmp/file1.txt' at 0x109ef30>,
<TarInfo 'tmp/temp.tar' at 0x10a4310>,
<TarInfo 'tmp/dirA/file0.txt' at 0x10a4350>,
<TarInfo 'tmp/dirA/file00.txt' at 0x10a43b0>,
<TarInfo 'tmp/dirA/file1.txt' at 0x10a4410>,
<TarInfo 'tmp/dirB/file0.txt' at 0x10a4470>,
<TarInfo 'tmp/dirB/file00.txt' at 0x10a44d0>,
<TarInfo 'tmp/dirB/file1.txt' at 0x10a4530>,
<TarInfo 'tmp/dirB/file11.txt' at 0x10a4590>]

Those examples show how to examine the names of the files in the TAR archive, which could be validated in data verification script. Extracting files is not much more work. If you want to extract a whole TAR archive to the current working directory, you can simply use the following:

In [60]: tar.extractall()

drwxrwxrwx  7 ngift  wheel    238 Apr  4 22:59 tmp/

If you are extremely paranoid, and you should be, then you could also include a step that extracts the contents of the archives and performs random MD5 checksums on files from the archive and compare them against MD5 checksums you made on the file before it was backed up. This could be a very effective way to monitor whether the integrity of the data is what you expect it to be.

No sane archiving solution should just trust that an archive was created properly. At the very least, random spot checking of archives needs to be done automatically. At best, every single archive should be reopened and checked for validity after it has been created.

Chapter 7. SNMP

Introduction

SNMP can change your life as a sysadmin. The rewards of using SNMP are not as instantaneous as writing a few lines of Python to parse a logfile, for example, but when an SNMP infrastructure has been setup, it is amazing to work with.

In this chapter, we will be covering these aspects of SNMP: autodiscovery, polling/monitoring, writing agents, device control, and finally enterprise SNMP integration. Of course, all of these things are going to be done with Python.

If you are unfamiliar with SNMP or need to brush up on SNMP, we highly recommend reading Essential SNMP by Douglas Mauro and Kevin Schmidt (O’Reilly), or at least keeping it handy. A good SNMP reference book is essential to truly understanding SNMP and what it can do. We will go over a few of the basics of SNMP in the next section, but going into much detail is beyond the scope of this book. In fact, there is more than enough material for a complete book on using Python with SNMP.

Brief Introduction to SNMP

SNMP Overview

The 10,000 foot view of SNMP is that it is a protocol for managing devices on an IP network. Typically, this is done via UDP ports 161 and 162, although it is possible, but rare, to use TCP as well. Just about any modern device in a data center supports SNMP; this means it is possible to manage not only switches and routers, but servers, printers, UPSs, storage, and more.

The basic use for SNMP is to send UDP packets to hosts and to wait for a response. This is how monitoring of devices occurs on a very simple level. It is also possible to do other things with the SNMP protocol, though, such as control devices and write agents that respond to conditions.

Some typical things you would do with SNMP are monitor the CPU load, disk usage, and free memory. You may also use it to manage and actually control switches, perhaps even going so far as to reload a switch configuration via SNMP. It is not commonly known that you can monitor software as well, such as web applications and databases. Finally, there is support for Remote Monitoring in the RMON MIB, which supports “flow-based” monitoring; this is different than regular SNMP monitoring, which is “device-based.”

Because we have mentioned the acronym MIB, it is about time to bring this up. SNMP is just a protocol, and it makes no assumptions about the data. On devices that are being monitored, they run an agent, snmpd, that has a list of objects that it keeps track of. The actual list of objects is controlled by MIBs, or management information bases. Every agent implements at least one MIB, and that is MIB-II, which is defined in RFC 1213. One way of thinking of an MIB is as a file that translates names to numbers, just like DNS, although it is slightly more complicated.

Inside this file is where the definitions of these managed objects live. Every object has three attributes: name, type and syntax, and encoding. Of these, name is the one you will need to know the most about. Name is often referred to as an OID, or object identifier. This OID is how you tell the agent what you want to get. The names come in two forms: numeric and “human-readable.” Most often you want to use the human-readable OID name, as the numeric name is very long and difficult to remember. One of the most common OIDs is sysDescr. If you use the command-line tool snmpwalk to determine the value of the sysDescr OID, you can do it by name or number:

[root@rhel][H:4461][J:0]# snmpwalk -v 2c -c public localhost .1.3.6.1.2.1.1.1.0
SNMPv2-MIB::sysDescr.0 = STRING: Linux localhost
2.6.18-8.1.15.el5 #1 SMP Mon Oct 22 08:32:04 EDT 2007 i686

[root@rhel][H:4461][J:0]# snmpwalk -v 2c -c public localhost sysDescr
SNMPv2-MIB::sysDescr.0 = STRING: Linux localhost
2.6.18-8.1.15.el5 #1 SMP Mon Oct 22 08:32:04 EDT 2007 i686

At this point, we have dropped a pile of acryonyms, and an RFC, so fight the urge to get up and walk away or fall asleep. We promise it gets better very soon, and you will be writing code in a few minutes.

SNMP Installation and Configuration

For simplicity’s sake, we will only be dealing with Net-SNMP and the corresponding Python bindings to Net-SNMP. This does not discount some of the other Python-based SNMP libraries out there though, including PySNMP, which both TwistedSNMP and Zenoss utilize. In both Zenoss and TwistedSNMP, PySNMP is used in an asynchronous style. It is a very valid approach, and it is worth looking at as well; we just don’t have room to cover both in this chapter.

In terms of Net-SNMP itself, we will be dealing with two different APIs. Method one is to use the subprocess module to wrap up Net-SNMP command-line tools, and method two is to use the new Python bindings. Each method has advantages and disadvantages depending on what environment they are implemented in.

Finally, we also discuss Zenoss, which is an impressive all-Python, open source, enterprise SNMP monitoring solution. With Zenoss, you can avoid having to write an SNMP management solution from scratch and can instead communicate with it via its public APIs. It is also possible to write plug-ins for Zenoss, contribute patches, and finally extend Zenoss itself.

In order to do anything useful with SNMP, specifically Net-SNMP, you must actually have it installed. Fortunately, most Unix and Linux operating systems already come installed with Net-SNMP, so if you need to monitor a device, usually it just involves adjusting the snmpd.conf file to fit your needs and starting the daemon. If you plan on developing with Net-SNMP with Python bindings, which is what we cover in this chapter, you will need to compile from source to install the Python bindings. If you just plan on wrapping up Net-SNMP command-line tools—such as snmpget, snmpwalk, snmpdf, and others—then you don’t need to do anything if Net-SNMP is already installed.

One option is to download a virtual machine with the source code for this book in it at http://www.oreilly.com/9780596515829. You can also refer to http://www.py4sa.com, the companion site for the book, as it will have the latest information on how to run examples in this section.

We have also configured this virtual machine with Net-SNMP and the Python bindings installed. You can then run all of the examples just by using this virtual machine. If you have beefy enough hardware at your disposal, you can also make a few copies of the virtual machine and simulate some of the other code in this chapter that talks to many machines at the same time.

If you do decide to install the Python bindings, you will need to download the Net-SNMP from sourceforge.net and get a version of Net-SNMP of 5.4.x or higher. The bindings are not built by default, so you should carefully follow the build instructions in the Python/README directory. In a nutshell though, you will first need to compile this version of Net-SNMP and then run the setup.py script in the Python directory. We have found the least painful installation method is on Red Hat, and there is a source RPM available. If you decide to compile, you might want to first try it out on Red Hat to see what a successful build looks like, and then venture out to AIX, Solaris, OS X, HPUX, etc. Finally, if you get stuck, just use the virtual machine to run the examples and figure out how to get it to compile later.

One final note on compiling yourself: make sure you run the Python setup.py build and the python setup.py test. You should find out right away if Net-SNMP works with Python. One tip if you have trouble compiling with Python is to manually run ldconfig like this:

ldconfig -v /usr/local/lib/

In terms of configuration, if you happen to be installing Net-SNMP on a client you want to monitor, you should compile Net-SNMP with the Host Resources MIB. Typically, you can do this as follows:

./configure -with-mib-modules=host

Note that when you run configure, it attempts to run an auto-configure script. You don’t need to do this if you don’t want. Often, it is much easier to just create a custom configuration file yourself. The configuration file on Red-Hat-based systems usually lives in /etc/snmp/snmpd.conf, and it can be as simple as this:

syslocation "O'Reilly"
syscontact bofh@oreilly.com
rocommunity public

Just this simple configuration file is enough for the rest of this chapter, and non-SNMPv3 queries. SNMPv3 is a bit tougher to configure and slightly out of scope for most of this chapter, although we do want to mention that for device control in a production environment, it is highly recommended to use SNMPv3, as v2 and v1 transmit in the clear. For that matter, you should never do SNMP v2 or v1 queries across the Internet, as you may have traffic intercepted. There have been some high-profile break-ins that have occurred as a result of doing just this.

IPython and Net-SNMP

If you haven’t done any SNMP development before, you may have gotten the impression that it is a bit nasty. Well, to be honest, it is. Dealing with SNMP is a bit of a pain, as it involves a very complex protocol, lots of RFCs to read, and a high chance for many things to go wrong. One way to diminish much of the initial pain of getting started with development is to use IPython to explore the SNMP code you will write and to get comfortable with the API.

Example 7-1 is a very brief snippet of live code to run on a local machine.

Example 7-1. Using IPython and Net-SNMP with Python bindings

In [1]: import netsnmp

In [2]: oid = netsnmp.Varbind('sysDescr')

In [3]: result = netsnmp.snmpwalk(oid,
...:                         Version = 2,
...:                         DestHost="localhost",
...:                         Community="public")

Out[4]: ('Linux localhost 2.6.18-8.1.14.el5 #1 SMP Thu Aug 27 12:51:54 EDT 2008 i686',)

Using tab completion when exploring a new library is very refreshing. In this example, we made full use of IPython’s tab completion capabilities and then made a basic SNMP v2 query. As a general note, sysDescr, as we mentioned earlier, is a very important OID query to perform some basic level of identification on a machine. In the output of this example, you will see that it is quite similar, if not identical, to the output of uname -a.

As we will see later in this chapter, parsing the response from a sysDescr query is an important part of initially discovering a data center. Unfortunately, like many parts of SNMP, it is not an exact science. Some equipment may not return any response, some may return something helpful but not complete like “Fibre Switch,” and others will return a vendor identification string. We don’t have space to get into too much detail in solving this problem, but dealing with these differences in responses is where the big boys earn their money.

As you learned in the IPython chapter, you can write out a class or function to a file while inside of IPython by switching to Vim, by typing the following:

ed some_filename.py

Then when you quit Vim, you will get that module’s attributes in your namespace, and you can see them by typing in who. This trick is very helpful for working Net-SNMP, as iterative coding is a natural fit for this problem domain. Let’s go ahead and write this code below out to a file named snmp.py by typing the following:

ed snmp.py

Example 7-2 shows a simple module that allows us to abstract away the boilerplate code associated with creating a session with Net-SNMP.

Example 7-2. Basic Net-SNMP session module

#!/usr/bin/env python
import netsnmp

class Snmp(object):
"""A basic SNMP session"""
def __init__(self,
oid = "sysDescr",
Version = 2,
DestHost = "localhost",
Community = "public"):
self.oid = oid
self.version = Version
self.destHost = DestHost
self.community = Community

def query(self):
"""Creates SNMP query session"""
try:
result = netsnmp.snmpwalk(self.oid,
Version = self.version,
DestHost = self.destHost,
Community = self.community)
except Exception, err:
print err
result = None
return result

When you save this file in IPython and type in who, you will see something like this:

In [2]: who
Snmp netsnmp

Now that we have an object-oriented interface to SNMP, we can begin using it to query our local machine:

In [3]: s = snmp()

In [4]: s.query()
Out[4]: ('Linux localhost 2.6.18-8.1.14.el5 #1 SMP Thu Sep 27 18:58:54 EDT 2007 i686',)

In [5]: result = s.query()

In [6]: len(result)
Out[6]: 1

As you can tell, it is quite easy to get results using our module, but we are basically just running a hardcoded script, so let us change the value of the OID object to walk the entire system subtree:

In [7]: s.oid
Out[7]: 'sysDescr'

In [8]: s.oid = ".1.3.6.1.2.1.1"

In [9]: result = s.query()

In [10]: print result
('Linux localhost 2.6.18-8.1.14.el5 #1 SMP Thu Sep 27 18:58:54 EDT 2007 i686',
'.1.3.6.1.4.1.8072.3.2.10', '121219', 'me@localhost.com', 'localhost', '"My Local Machine"',
'0', '.1.3.6.1.6.3.10.3.1.1', '.1.3.6.1.6.3.11.3.1.1', '.1.3.6.1.6.3.15.2.1.1',
'.1.3.6.1.6.3.1',
'.1.3.6.1.2.1.49', '.1.3.6.1.2.1.4', '.1.3.6.1.2.1.50', '.1.3.6.1.6.3.16.2.2.1',
'The SNMP Management Architecture MIB.',
'The MIB for Message Processing and Dispatching.', 'The management information definitions
for the SNMP User-based Security Model.',
'The MIB module for SNMPv2 entities', 'The MIB module for managing TCP implementations',
'The MIB module for managing IP and ICMP implementations', 'The MIB module for
managing UDP [snip]',
'View-based Access Control Model for SNMP.', '0', '0', '0', '0', '0', '0', '0', '0')

This style of interactive, investigative programming makes dealing with SNMP quite pleasant. At this point, if you feel inclined, you can start investigating various queries with other OIDs, or you can even walk a full MIB tree. Walking a full MIB tree can take quite some time though, as queries will need to occur for the multitude of OIDs; so often, this is not the best practice in a production environment, as it will consume resources on the client machine.

Note

Remember that MIB-II is just a file full of OIDs, and it is included with most systems that support SNMP. Other vendor-specific MIBs are additional files that an agent can refer to and give responses to. You will need to look up vendor-specific documentation to determine what OID in what MIB to query if you want to take this to the next level.

Next, we are going to use an IPython-specific feature that lets you send jobs to the background:

In [11]: bg s.query()
Starting job # 0 in a separate thread.

In [12]: jobs[0].status
Out[12]: 'Completed'

In [16]: jobs[0].result
Out[16]:
('Linux localhost 2.6.18-8.1.14.el5 #1 SMP Thu Sep 27 18:58:54 EDT 2007 i686',
'.1.3.6.1.4.1.8072.3.2.10', '121219', 'me@localhost.com', 'localhost',
'"My Local Machine"',
'0', '.1.3.6.1.6.3.10.3.1.1', '.1.3.6.1.6.3.11.3.1.1', '.1.3.6.1.6.3.15.2.1.1',
'.1.3.6.1.6.3.1',
'.1.3.6.1.2.1.49', '.1.3.6.1.2.1.4', '.1.3.6.1.2.1.50', '.1.3.6.1.6.3.16.2.2.1',
'The SNMP Management Architecture MIB.', 'The MIB for Message Processing and
Dispatching.',
'The management information definitions for the SNMP User-based Security Model.',
'The MIB module for SNMPv2 entities', 'The MIB module for managing TCP implementations',
'The MIB module for managing IP and ICMP implementations', 'The MIB module for
managing UDP implementations',
'View-based Access Control Model for SNMP.', '0', '0', '0', '0', '0', '0', '0', '0')

Before you get too excited, let us tell you that while background threading works like a charm in IPython, it only works with libraries that support asynchronous threading. The Python bindings for Net-SNMP are synchronous. In a nutshell, you cannot write multithreaded code as the underlying C code blocks waiting for a response.

Fortunately, as you found out in the processes and concurrency chapter, it is trivial to use the processing module to fork processes that handle parallel SNMP queries. In the next section, we will address this when we write a sample tool to automatically discover a data center.

Discovering a Data Center

One of the more useful things SNMP is used for is discovery of a data center. In simplistic terms, discovery gathers an inventory of devices on a network and information about those devices. More advanced forms of discovery can be used to make correlations about the data gathered, such as the exact Mac address that a server lives in on a Cisco switch, or what the storage layout is for a Brocade fibre switch.

In this section, we will create a basic discovery script that will gather valid IP addresses, Mac addresses, basic SNMP information, and place that in a record. This can serve as a useful base to implement data center discovery applications at your facility. We will be drawing on information we covered in other chapters to accomplish this.

There are a few different discovery algorithms that we have come across, but we will present this one, as it is one of the more simple. A one-sentence description of the algorithm: send out a bunch of ICMP pings; for each device that responds, send out a basic SNMP query; parse that output; and then do further discovery based on the results. Another algorithm could involve just sending out SNMP queries in a shotgun style and then having another process just collect the responses, but, as we mentioned, we will be focusing on the first algorithm. See Example 7-3.

Note

Just a note about the code below: because the Net-SNMP library is synchronous, we are forking a call to subprocess.call. This gets us around the blocking that occurs. For the ping portion we could have just used subprocess.Popen, but to keep the code consistent, we are using the same pattern for SNMP and ping.

Example 7-3. Basic data center discovery

#!/usr/bin/env python
from processing import Process, Queue, Pool
import time
import subprocess
import sys
from snmp import Snmp

q = Queue()
oq = Queue()
#ips = IP("10.0.1.0/24")
ips = ["192.19.101.250", "192.19.101.251", "192.19.101.252","192.19.101.253",
"192.168.1.1"]
num_workers = 10

class HostRecord(object):
"""Record for Hosts"""
def __init__(self, ip=None, mac=None, snmp_response=None):
self.ip = ip
self.mac = mac
self.snmp_response = snmp_response
def __repr__(self):
return "[Host Record('%s','%s','%s')]" % (self.ip,
self.mac,
self.snmp_response)

def f(i,q,oq):
while True:
time.sleep(.1)
if q.empty():
sys.exit()
print "Process Number: %s Exit" % i
ip = q.get()
print "Process Number: %s" % i
ret = subprocess.call("ping -c 1 %s" % ip,
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)
if ret == 0:
print "%s: is alive" % ip
oq.put(ip)
else:
print "Process Number: %s didn’t find a response for %s " % (i, ip)
pass

def snmp_query(i,out):
while True:
time.sleep(.1)
if out.empty():
sys.exit()
print "Process Number: %s" % i
ipaddr = out.get()
s = Snmp()
h = HostRecord()
h.ip = ipaddr
h.snmp_response = s.query()
print h
return h
try:
q.putmany(ips)

finally:
for i in range(num_workers):
p = Process(target=f, args=[i,q,oq])
p.start()
for i in range(num_workers):
pp = Process(target=snmp_query, args=[i,oq])
pp.start()

print "main process joins on queue"
p.join()
#while not oq.empty():
#    print “Validated", oq.get()

print "Main Program finished"

If we run this script, we get output that looks something like this:

[root@giftcsllc02][H:4849][J:0]> python discover.py
Process Number: 0
192.19.101.250: is alive
Process Number: 1
192.19.101.251: is alive
Process Number: 2
Process Number: 3
Process Number: 4
main process joins on queue
192.19.101.252: is alive
192.19.101.253: is alive
Main Program finished
[Host Record('192.19.101.250','None','('Linux linux.host 2.6.18-8.1.15.el5
#1 SMP Mon Oct 22 08:32:04 EDT 2007 i686',)')]
[Host Record('192.19.101.252','None','('Linux linux.host 2.6.18-8.1.15.el5
#1 SMP Mon Oct 22 08:32:04 EDT 2007 i686',)')]
[Host Record('192.19.101.253','None','('Linux linux.host 2.6.18-8.1.15.el5
#1 SMP Mon Oct 22 08:32:04 EDT 2007 i686',)')]
[Host Record('192.19.101.251','None','('Linux linux.host 2.6.18-8.1.15.el5
#1 SMP Mon Oct 22 08:32:04 EDT 2007 i686',)')]
Process Number: 4 didn't find a response for 192.168.1.1

Looking at the output of this code, we see the beginnings of an interesting algorithm to discover a data center. There are a few things to fix, like adding a Mac address to the Host Record object, and making the code more object-oriented, but that could turn into a whole other book. In fact, that could turn into a whole company. On that note we turn to the next section.

Retrieving Multiple-Values with Net-SNMP

Getting just one value from SNMP is toy code, although it can be very useful to test out responses or to perform an action based on a specific value, like the OS type of a machine. In order to do something more meaningful, we need to get a few values and do something with them.

A very common task is to do an inventory of your data center, or department, to figure out some set of parameters across all of your machines. Here is one hypothetical situation: you are preparing for a major software upgrade, and you have been told all systems will need to have at least 1 GB of RAM. You seem to remember that most of the machines have at least 1 GB of RAM, but there are a few of the thousands of machines you support that do not.

You obviously have some tough decisions to make. Let’s go over some of the options:

Option 1

Physically walk up to every one of your machines and check how much RAM is installed by running a command, or opening the box. This is obviously not a very appealing option.

Option 2

Shell into every box and run a command to determine how much RAM it has. There are quite a few problems with this approach, but at least it could be theoretically scripted via ssh keys. One of the obvious problems is that a cross-platform script would need to be written, as every OS is slightly different. Another problem is that this method depends on knowing where all of the machines live.

Option 3

Write a small script that travels and asks every device on your network how much memory it has via SNMP.

Using option 3 via SNMP, it is easy to generate an inventory report, which shows just the machines that do not have at least 1 GB of RAM. The exact OID name we will need to query is “hrMemorySize.” SNMP is something that can always benefit from being concurrent, but it is best not to optimize until it is absolutely necessary. On that note, let’s dive right into something quick.

We can reuse our code from the earlier example to run a very quick test.

Getting memory value from SNMP:

In [1]: run snmpinput

In [2]: who
netsnmp Snmp

In [3]: s = Snmp()

In [4]: s.DestHost = "10.0.1.2"

In [5]: s.Community = "public"

In [6]: s.oid = "hrMemorySize"

In [7]: result = int(s.query()[0])
hrMemorySize  = None ( None )

In [27]: print result
2026124

As you can see, this is a very straightforward script to write. The result comes back as a tuple, in line [6], so we extracted the index 0 and converted it to an integer. The result is now an integer consisting of KB. One thing to keep in mind is that different machines will calculate RAM in different ways. It is best to account for this by using rough parameters and not hardcoding an absolute value, as you may get results that are different than what you expect. For example you may want to look for a range of value that is slightly below 1 GB of RAM, say 990 MB.

In this case, we can do the math in our heads to estimate that this corresponds to roughly 2 GB of RAM. Having this information, you are now informed by your boss that you need to determine which machines in your data center contain under 2 GBs of RAM, as a new application will need to be installed that requires at least 2 GBs of RAM.

With that bit of information, we can now automate determining memory. What makes the most sense is to query each machine and figure out if it does not contain 2 GBs of RAM and then to put that information into a CSV file so that it can easily be imported into Excel or Open Office Calc.

Next you can write a command-line tool that takes a subnet range as input and an optional OID keyword value but will default to using “hrMemorySize.” We will also want to iterate a range of IP addresses in a subnet.

As always, as a sysadmin writing code, you are faced with some tough decisions. Should you spend a few hours, or a day, writing a really long script that you can reuse for other things, because it is object-oriented, or should you just whip out something quick and dirty? We think in most cases it is safe to say you can do both. If you use IPython, you can log scripts you write and then later turn them into more polished scripts. In general though, it is a good idea to write reusable code, as it becomes like a snowball and soon reaches its own inertia.

Hopefully you now understand the power of SNMP if you didn’t already. Let’s go write our script…

Finding Memory

In this next example, we write a command-line tool to calculate the memory installed on machines via SNMP:

#!/usr/bin/env python
#A command line tool that will grab total memory in a machine

import netsnmp
import optparse
from IPy import IP

class SnmpSession(object):
"""A Basic SNMP Session"""
def __init__(self,
oid="hrMemorySize",
Version=2,
DestHost="localhost",
Community="public"):

self.oid = oid
self.Version = Version
self.DestHost = DestHost
self.Community = Community

def query(self):
"""Creates SNMP query session"""
try:
result = netsnmp.snmpwalk(self.oid,
Version = self.Version,
DestHost = self.DestHost,
Community = self.Community)
except:
#Note this is toy code, but let's us know what exception is raised
import sys
print sys.exc_info()
result = None

return result

class SnmpController(object):
"""Uses optparse to Control SnmpSession"""

def run(self):
results = {}    #A place to hold and collect snmp results
p = optparse.OptionParser(description="A tool that determines
memory installed",
prog="memorator",
version="memorator 0.1.0a",
usage="%prog [subnet range] [options]")
p.add_option('--community', '-c',help='community string',
default='public')
p.add_option('--oid', '-o', help='object identifier',
default='hrMemorySize')
p.add_option('--verbose', '-v', action=’store_true',
help='increase verbosity')
p.add_option('--quiet', '-q', action=’store_true',help=’
suppresses most messages')
p.add_option('--threshold', '-t', action=’store', type="int",
help='a number to filter queries with')

options, arguments = p.parse_args()
if arguments:
for arg in arguments:
try:
ips = IP(arg)   #Note need to convert instance to str
except:
if not options.quiet:
print 'Ignoring %s, not a valid IP address' % arg
continue

for i in ips:
ipAddr = str(i)
if not options.quiet:
print 'Running snmp query for: ', ipAddr

session = SnmpSession(options.oid,
DestHost = ipAddr,
Community = options.community)

if options.oid == "hrMemorySize":
try:
memory = int(session.query()[0])/1024
except:
memory = None
output = memory

else:
#Non-memory related SNMP query results
output = session.query()
if not options.quiet:
print "%s returns %s" % (ipAddr,output)

#Put everything into an IP/result dictionary
#But only if it is a valid response
if output != None:
if options.threshold:   #ensures a specific threshold
if output < options.threshold:
results[ipAddr] = output
#allow printing to standard out
if not options.quiet:
print "%s returns %s" % (ipAddr,output)

else:
results[ipAddr] = output
if not options.quiet:
print output

print "Results from SNMP Query %s for %s:\n" % (options.oid,
arguments), results

else:
p.print_help()  #note if nothing is specified on the
command line, help is printed

def _main():
"""
Runs memorator.
"""
start = SnmpController()
start.run()

if __name__ =='__main__':
try:
import IPy
except:
print "Please install the IPy module to use this tool"
_main()

OK, let’s step through this code a bit and see what it does. We took the whole class from the previous example and placed it into a new module. We next made a controller class that handles option handling via the optparse module. The IPy module, which we refer to over and over again, handles the IP address arguments automatically. We can now place several IP addresses or a subnet range, and our module will look for an SNMP query and return the result as a collected dictionary of IP addresses and SNMP values.

One of the trickier things we did was to create some logic at the end that does not return empty results, and which additionally listens to a threshold number. This means that we set it to return only values under a specific threshold. By using a threshold we can return meaningful results for us and allow for some discrepancies with how different machines handle memory calculations.

Let’s look at an example of the output of this tool in action:

[ngift@ng-lep-lap][H:6518][J:0]> ./memory_tool_netsnmp.py 10.0.1.2 10.0.1.20
Running snmp query for:  10.0.1.2
hrMemorySize  = None ( None )
1978
Running snmp query for:  10.0.1.20
hrMemorySize  = None ( None )
372
Results from SNMP Query hrMemorySize for ['10.0.1.2', '10.0.1.20']:
{'10.0.1.2': 1978, '10.0.1.20': 372}

As you can see, the results come back from machines on the 10.0.1.0/24 subnet. Let’s now use the threshold flag to simulate looking for machines that do not contain at least 2 GBs of RAM. As we mentioned earlier, there are some differences in how machines calculate RAM, so let’s be safe and put in the number 1800, which would correspond roughly to 1800 MBs of RAM. If a machine does not contain at least 1800 MBs or roughly 2GBs of RAM, we will get this in our report.

Here is the output from that query:

[ngift@ng-lep-lap][H:6519][J:0]>
./memory_tool_netsnmp.py --threshold 1800 10.0.1.2 10.0.1.20
Running snmp query for:  10.0.1.2
hrMemorySize  = None ( None )
Running snmp query for:  10.0.1.20
hrMemorySize  = None ( None )
10.0.1.20 returns 372
Results from SNMP Query hrMemorySize for ['10.0.1.2', '10.0.1.20']:
{'10.0.1.20': 372}

Although our script does its job, there are a couple of things we can do to optimize the tool. If you need to query thousands of machines, then this tool might take a day to run or more. This might be OK, but if you need the results quickly, you will need to add concurrency and fork each query using a third-party library. The other improvement we could make is to generate a CSV report automatically from our dictionary.

Before we get to automating those tasks, let me show you one additional benefit that you may not have noticed. The code was written in a way to allow any OID to be queried, not just one specific to memory calculation. This comes in very handy because we now have both a tool that calculates memory and a general-purpose tool to perform SNMP queries.

Let’s take a look at an example of what we mean:

[ngift@ng-lep-lap][H:6522][J:0]> ./memory_tool_netsnmp.py -o sysDescr 10.0.1.2 10.0.1.20
Running snmp query for:  10.0.1.2
sysDescr  = None ( None )
10.0.1.2 returns ('Linux cent 2.6.18-8.1.14.el5 #1 SMP
Thu Sep 27 19:05:32 EDT 2007 x86_64',)
('Linux cent 2.6.18-8.1.14.el5 #1 SMP Thu Sep 27 19:05:32 EDT 2007 x86_64',)
Running snmp query for:  10.0.1.20
sysDescr  = None ( None )
10.0.1.20 returns ('Linux localhost.localdomain 2.6.18-8.1.14.el5 #1 SMP
Thu Sep 27 19:05:32 EDT 2007 x86_64',)
('Linux localhost.localdomain 2.6.18-8.1.14.el5 #1 SMP
Thu Sep 27 19:05:32 EDT 2007 x86_64',)
Results from SNMP Query sysDescr for ['10.0.1.2', '10.0.1.20']:
{'10.0.1.2': ('Linux cent 2.6.18-8.1.14.el5 #1 SMP
Thu Sep 27 19:05:32 EDT 2007 x86_64',), '10.0.1.20':
('Linux localhost.localdomain 2.6.18-8.1.14.el5 #1 SMP
Thu Sep 27 19:05:32 EDT 2007 x86_64',)}

It is good to keep this fact in mind when writing what could be a one-off tool. Why not spend an extra 30 minutes and make it generic? As a result, you may have a tool that you find yourself using over and over again, and that 30 minutes becomes a drop in the bucket compared to how much time you saved in the future.

Creating Hybrid SNMP Tools

Since we have shown a few examples of separate tools, it’s good to note that these techniques can be combined to create some very sophisticated tools. Let’s start by creating a whole slew of one-off tools, and then later we can make sure of these in bigger scripts.

There is a useful tool called snmpstatus that grabs a few different snmp queries and combines them into a “status”:

import subprocess

class Snmpdf(object):
"""A snmpstatus command-line tool"""
def __init__(self,
Version="-v2c",
DestHost="localhost",
Community="public",
verbose=True):

self.Version = Version
self.DestHost = DestHost
self.Community = Community
self.verbose = verbose

def query(self):
"""Creates snmpstatus query session"""
Version = self.Version
DestHost = self.DestHost
Community = self.Community
verbose = self.verbose

try:
snmpstatus = "snmpstatus %s -c %s %s" % (Version, Community, DestHost)
if verbose:
print "Running: %s" % snmpstatus
p = subprocess.Popen(snmpstatus,
shell=True,
stdout=subprocess.PIPE)

out = p.stdout.read()
return out

except:
import sys
print >> sys.stderr, "error running %s" % snmpstatus

def _main():
snmpstatus = Snmpdf()
result = snmpstatus.query()
print result
if __name__ == "__main__":
_main()

We hope you are paying attention to the fact that this script has very few differences from the snmpdf command, with the exception of things being named differently. This is a perfect example of when it becomes a good idea to create another level of abstraction and then resuse common components. If we created a module to handle all of the boilerplate code, our new script would be only a few lines long. Keep this in mind; we will revist this later.

Another tool, related to SNMP, is ARP, which uses the ARP protocol. By using ARP protocol, it is possible to obtain Mac addresses of devices based on their IP address if you are physically located on the same network. Let’s write one of those tools too. This one-off tool will come in handy a little later.

ARP is so easy to wrap up into a script that it is better to just show an example by using IPython interactively. Go ahead and fire up IPython, and try this out:

import re
import subprocess

#some variables
ARP = "arp"
IP = "10.0.1.1"
CMD = "%s %s " % (ARP, IP)
macPattern = re.compile(":")

def getMac():
p = subprocess.Popen(CMD, shell=True, stdout=subprocess.PIPE)
out = p.stdout.read()
results = out.split()
for chunk in results:
if re.search(macPattern, chunk):
return chunk

if __name__ == "__main__":
macAddr = getMac()
print macAddr

This snippet of code is not a reusable tool yet, but you could easily take this idea and use it as part of a general data center discovery library.

Extending Net-SNMP

As we have discussed earlier, Net-SNMP is installed as an agent on most *nix machines. There is a default set of information that an agent can return, but it is also possible to extend an agent on a machine. It is reasonably straightforward to write an agent that collects just about anything and then returns the results via the SNMP protocol.

The EXAMPLE.conf file is one of the best sources for information on extending Net-SNMP, and it is included with Net-SNMP. Doing a man on snmpd.conf is also useful for more verbose information that documents the API. Both of these would be good sources of information to reference if you are interested in further study on extending native agents.

For a Python programmer, extending Net-SNMP is one of the most exciting aspects of working with SNMP, as it allows a developer to write code to monitor whatever they see fit, and to additionally have the agent internally respond to conditions that you have assigned to it.

Net-SNMP offers quite a few ways to extend its agent, but to get started we are going to write a Hello World program that we will query from snmp. The first step is to create a very simple snmpd.conf file that executes our Hello World program in Python. Example 7-4 shows what that looks like on a Red Hat machine.

Example 7-4. SNMP configuration file with Hello World

syslocation "O'Reilly"
syscontact bofh@oreilly.com
rocommunity public
exec helloworld /usr/bin/python -c "print 'hello world from Python'"

Next we need to tell snmpd to re-read the configuration file. We can do that three different ways. On Red Hat you can use:

service snmpd reload

or you can also do:

ps -ef | grep snmpd
root     12345    1  0 Apr14 ?
00:00:30 /usr/sbin/snmpd -Lsd -Lf /dev/null -p /var/run/snmpd.pid -a

Then you can send it:

kill -HUP 12345

Finally, the snmpset command can assign an integer (1) to UCD-SNMP-MIB::versionUpdateConfig.0, which will tell snmpd to reread the configuration file.

Now that we have modified the snmpd.conf file and told snmpd to reread the configuration file, we can go ahead and query our machine by using either the snmpwalk command-line tool or the Net-SNMP binding with IPython. Here is what it looks like from the snmpwalk command:

[root@giftcsllc02][H:4904][J:0]> snmpwalk -v 1 -c public localhost .1.3.6.1.4.1.2021.8
UCD-SNMP-MIB::extIndex.1 = INTEGER: 1
UCD-SNMP-MIB::extNames.1 = STRING: helloworld
UCD-SNMP-MIB::extCommand.1 = STRING: /usr/bin/python
-c "print 'hello world from Python'"
UCD-SNMP-MIB::extResult.1 = INTEGER: 0
UCD-SNMP-MIB::extOutput.1 = STRING: hello world from Python
UCD-SNMP-MIB::extErrFix.1 = INTEGER: noError(0)
UCD-SNMP-MIB::extErrFixCmd.1 = STRING:

This query bears some explanation, as the observant reader may wonder where we got it. 1.3.6.1.4.1.2021.8 from. This OID is the ucdavis.extTable. When you create an extension to snmpd.conf, it will assign it to this OID. Things get slightly more complicated if you would like to query a custom OID that you create. The proper way to do this is to fill out a request with iana.org and to get an enterprise number. You can then use that number to create custom queries to an agent. The main reason for this is to keep a uniform namespace that avoids collisions with other future vendor numbers you may run into.

Getting output from one-liners isn’t really Python’s strength, and it is kind of silly. Here is an example of a script that parses the total number of Firefox hits in an Apache log and then returns the number for a custom enterprise number. Let’s start backward this time and see what it looks like when we query it:

snmpwalk -v 2c -c public localhost .1.3.6.1.4.1.2021.28664.100
UCD-SNMP-MIB::ucdavis.28664.100.1.1 = INTEGER: 1
UCD-SNMP-MIB::ucdavis.28664.100.2.1 = STRING: "FirefoxHits"
UCD-SNMP-MIB::ucdavis.28664.100.3.1 = STRING:
"/usr/bin/python /opt/local/snmp_scripts/agent_ext_logs.py"
UCD-SNMP-MIB::ucdavis.28664.100.100.1 = INTEGER: 0
UCD-SNMP-MIB::ucdavis.28664.100.101.1 = STRING:
"Total number of Firefox Browser Hits: 15702"
UCD-SNMP-MIB::ucdavis.28664.100.102.1 = INTEGER: 0
UCD-SNMP-MIB::ucdavis.28664.100.103.1 = ""

If you look at the value of 100.101.1, you will see the output of a script that uses a generator pipeline to parse an Apache log and look for all Firefox hits in the log. It then sums them and returns the output via SNMP. Example 7-5 is the script that gets run when we query this OID.

Example 7-5. Generator pipeline to look for total firefox hits in Apache logfile

import re

"""Returns Hit Count for Firefox"""

def grep(lines,pattern="Firefox"):
pat = re.compile(pattern)
for line in lines:
if pat.search(line): yield line

def increment(lines):
num = 0
for line in lines:
num += 1
return num

wwwlog = open("/home/noahgift/logs/noahgift.com-combined-log")
column = (line.rsplit(None,1)[1] for line in wwwlog)
match  = grep(column)
count = increment(match)
print "Total Number of Firefox Hits: %s" % count

In order for our query to work in the first place, we needed to tell snmpd.conf about this script, and here is what that section looks like:

syslocation "O'Reilly"
syscontact bofh@oreilly.com
rocommunity public
exec helloworld /usr/bin/python -c "print 'hello world from Python'"
exec .1.3.6.1.4.1.2021.28664.100 FirefoxHits /usr/bin/python
/opt/local/snmp_scripts/agent_ext_logs.py

The magic portion is the last line, in which .1.3.6.1.4.1.2021 is the ucdavis enterprise number, 28664 our enterprise number, and 100 is some contrived value that we decided we wanted to use. It is really important to follow best practices and use your our enterprise number if you plan on extending SNMP. The main reason is that you will avoid causing havoc if you decide to use a range already occupied by someone else and then make changes via snmpset.

We would like to close with the fact that this is one of the more exciting topics in the book, and SNMP is still a very untouched playground. There are many things that customizing Net-SNMP can be useful for, and if you are careful to use SNMP v3, you can do some incredible things that are most easily accomplished through the SNMP protocol; and that ssh or sockets would be the most natural choice.

SNMP Device Control

One of the more interesting things SNMP can do is control a device through the SNMP protocol. Obviously, this creates a significant advantage over using something like Pyexpect (http://sourceforge.net/projects/pexpect/) to control a router, as it is much more straightforward.

For brevity’s sake, we will only cover SNMP v1 in the example, but if you are communicating with a device over an insecure network, it should be done via SNMP v3. For this section, it would be good to reference Essential SNMP and Cisco IOS Cookbook by Kevin Dooley and Ian J. Brown (O’Reilly) if you have a Safari account or have bought those books. They include some excellent information about both talking to Cisco devices via SNMP and basic configuration.

Because reloading a Cisco configuration via SNMP is plain cool, it seems like a perfect choice to talk about device control. For this example it is necessary to have a running TFTP server from which the router will pull the IOS file, and the router must be configured to allow read/write access for SNMP. Example 7-6 is what the Python code looks like.

Example 7-6. Upload new switch configuration Cisco router

import netsnmp

vars = netsnmp.Varbind(netsnmp.VarList(netsnmp.Varbind(".1.2.6.1.4.1.9.2.10.6.0", "1"),
(netsnmp.Varbind("cisco.example.com.1.3.6.1.4.1.9.2.10.12.172.25.1.1",
"iso-config.bin")

result = netsnmp.snmpset(vars,
Version = 1,
DestHost='cisco.example.com',
Community='readWrite')

In this example, we used Net-SNMP’s VarList to assign the instruction to first erase the flash for the switch and second load a new IOS image file. This could be the basis for a script that upgrades the IOS of every switch at once in a data center. As with all code in this book, you should test this out in a nonproduction environment before just seeing what happens.

One final thing to point out is that SNMP is often not thought of in terms of device control, but it is a powerful way to programmatically control devices in a data center, as it serves as a uniform specification for device control that has been under development since 1988. The future probably holds a very interesting story for SNMP v3.

Enterprise SNMP Integration with Zenoss

Zenoss is a fascinating new option for enterprise SNMP management systems. Not only is Zenoss a completely open source application, it is also written in pure Python. Zenoss is a new breed of enterprise application that is both incredibly powerful and extendable via an XML-RPC or ReST API. For more information on ReST, take a look at RESTful Web Services by Leonard Richardson and Sam Ruby (O’Reilly).

Finally, if you want to help develop Zenoss, you can contribute patches.

Zenoss API

For the latest information on the Zenoss API, please visit http://www.zenoss.com/community/docs/howtos/send-events/.

Using Zendmd

Not only does Zenoss come with a robust SNMP monitoring and discovery system, it also includes a high-level API called zendmd. You can open up a customized shell and run commands directly against Zenoss.

Using zendmd:

>>> d = find('build.zenoss.loc')
>>> d.os.interfaces.objectIds()
['eth0', 'eth1', 'lo', 'sit0', 'vmnet1', 'vmnet8']
>>> for d in dmd.Devices.getSubDevices():
>>>    print d.id, d.getManageIp()

Device API

You can also communicate directly with Zenoss via an XML-RPC API and add or remove devices. Below are two examples.

Using ReST:

[zenos@zenoss $]
wget 'http://admin:zenoss@MYHOST:8080/zport/dmd
/ZenEventManager/manage_addEvent?device=MYDEVICE&component=MYCOMPONENT&summary=↴
MYSUMMARY&severity=4&eclass=EVENTCLASS&eventClassKey=EVENTCLASSKEY

Using XML-RPC:

>>> from xmlrpclib import ServerProxy
>>> serv = ServerProxy('http://admin:zenoss@MYHOST:8080/zport/dmd/ZenEventManager')
>>> evt = {'device':'mydevice', 'component':'eth0',
'summary':'eth0 is down','severity':4, 'eventClass':'/Net'}
>>> serv.sendEvent(evt)

Chapter 8. OS Soup

Introduction

Being a sysadmin often means that you get thrown to the wolves. Rules, a predictable schedule, or even choice of an operating system is often out of your control. To be even a marginally effective sysadmin nowadays, you need to know it all, and we mean all the operating systems. From Linux, to Solaris, to OS X, to FreeBSD, it needs to be in your toolbelt. Although only time will tell, it does seem as if the proprietary operating systems, such as AIX and HP-UX, won’t last forever, but they still are necessary to know for many people.

Fortunately, Python comes to the rescue yet again—we hope you are noticing a trend here—by offering a mature standard library that has just about anything a multi-OS systems administrator needs. Python’s massive standard library has a module that deals with just about anything a sysadmin could want to do, from tarring up a directory, to comparing files and directories, to parsing config files. Python’s maturity, coupled with its elegance and readability, is why it is the 800 pound gorilla of systems administration.

Many complex systems administration facilities, such as animation pipelines and data centers, are switching away from Perl to Python because it offers much more readable and elegant code. While Ruby is an interesting language that shares many of the positive features of Python, when one compares the standard library and maturity of the language, Ruby lacks in comparison to Python for a systems administration language.

Since this chapter is going to be a mixture of many different operating systems, we won’t have time to explore any of them in great depth, but we will explore them enough to demonstrate how Python can act in as both a generic, cross-platform scripting language and a unique weapon for each operating system. Finally, there is a new “operating system” on the horizon, and it comes in the form of a data center. Some people refer to this new platform as Cloud Computing, and we will talk about Amazon’s and Google’s offerings.

Enough of the idle banter, something smells delicious in the kitchen…is that OS soup?

Cross-Platform Unix Programming in Python

While there are some significant differences between *nix operating systems, there is much more in common that not. One way to bring the different versions of *nix back together is to write cross-platform tools and libraries that bridge the divide between the operating system differences. One of the most basic ways to accomplish this is to write conditional statements that check for the operating systems, platform, and version in code that you write.

Python takes the “batteries included” philosophy quite seriously, and includes a tool for just about any problem you could think of. For the problem of determing what platform your code is running on, there is the platform module. Let’s look at the essentials of using the platform module.

An easy way to get comfortable with the platform module is to create a tool the prints out all available information about a system. See Example 8-1.

Example 8-1. Using the platform module to print a system report

#!/usr/bin/env python
import platform

profile = [
platform.architecture(),
platform.dist(),
platform.libc_ver(),
platform.mac_ver(),
platform.machine(),
platform.node(),
platform.platform(),
platform.processor(),
platform.python_build(),
platform.python_compiler(),
platform.python_version(),
platform.system(),
platform.uname(),
platform.version(),
]

for item in profile:
print item

Here is the output of that script on OS X Leopard 10.5.2:

[ngift@Macintosh-6][H:10879][J:0]% python cross_platform.py
('32bit', '')
('', '', '')
('', '')
('10.5.2', ('', '', ''), 'i386')
i386
Macintosh-6.local
Darwin-9.2.0-i386-32bit
i386
('r251:54863', 'Jan 17 2008 19:35:17')
GCC 4.0.1 (Apple Inc. build 5465)
2.5.1
Darwin
('Darwin', 'Macintosh-6.local', '9.2.0', 'Darwin Kernel Version 9.2.0:
Tue Feb  5 16:13:22 PST 2008; root:xnu-1228.3.13~1/RELEASE_I386', 'i386', 'i386')
Darwin Kernel Version 9.2.0: Tue Feb  5 16:13:22 PST 2008;
root:xnu-1228.3.13~1/RELEASE_I386

This gives us some idea of the kind of information we can gather. The next step on the road to writing cross-platform code is to create a fingerprint module that will “fingerprint” which platform and version we are running on. In this example, we will fingerprint the following operating systems: Mac OS X, Ubuntu, Red Hat/Cent OS, FreeBSD, and SunOS. See Example 8-2.

Example 8-2. Fingerprinting an operating system type

#!/usr/bin/env python
import platform

"""
Fingerprints the following Operating Systems:

* Mac OS X
* Ubuntu
* Red Hat/Cent OS
* FreeBSD
* SunOS

"""
class OpSysType(object):
"""Determins OS Type using platform module"""

def __getattr__(self, attr):
if attr == "osx":
return "osx"
elif attr == "rhel":
return "redhat"
elif attr == "ubu":
return "ubuntu"
elif attr == "fbsd":
return "FreeBSD"
elif attr == "sun":
return "SunOS"
elif attr == "unknown_linux":
return "unknown_linux"
elif attr == "unknown":
return "unknown"
else:
raise AttributeError, attr

def linuxType(self):
"""Uses various methods to determine Linux Type"""

if platform.dist()[0] == self.rhel:
return self.rhel
elif platform.uname()[1] == self.ubu:
return self.ubu
else:
return self.unknown_linux

def queryOS(self):
if platform.system() == "Darwin":
return self.osx
elif platform.system() == "Linux":
return self.linuxType()
elif platform.system() == self.sun:
return self.sun
elif platform.system() == self.fbsd:
return self.fbsd

def fingerprint():
type = OpSysType()
print type.queryOS()

if __name__ == "__main__":
fingerprint()

Let’s take a look at this output when we run it on the various platforms.

Red Hat:

[root@localhost]/# python fingerprint.py
redhat

Ubuntu:

root@ubuntu:/# python fingerprint.py
ubuntu

Solaris 10 or SunOS:

bash-3.00# python fingerprint.py
SunOS

FreeBSD

# python fingerprint.py
FreeBSD

While the output of the command is not tremendously interesting, it does do a very powerful thing for us. This simple module allows us to write cross-platform code, as we can, perhaps, query a dictionary for these operating system types, and if they match, run the appropriate platform-specific code. One of the ways the benefits of cross-platform APIs are most tangible is writing scripts that manage a network via ssh keys. Code can run simultaneously on many platforms yet provide consistent results.

Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

One way to manage a diverse infrastructure of *nix machines is to use a combination of ssh keys, a commonly shared NFS-mounted src directory, and cross-platform Python code. Breaking this process into steps will make it clearer.

Step 1: create a public ssh key on the system from which you will manage machines. Note that this can vary by platform. Please consult your operating system documentation or do a man on ssh for details. See Example 8-3.

Note

One thing to point out in the example below is that for demonstration we are creating ssh keys for the root user, but perhaps a better security strategy would be to create a user account that has sudo privileges to run only this script.

Example 8-3. Creating a public ssh key

[ngift@Macintosh-6][H:11026][J:0]% ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
6c:2f:6e:f6:b7:b8:4d:17:05:99:67:26:1c:b9:74:11 root@localhost.localdomain
[ngift@Macintosh-6][H:11026][J:0]%

Step 2: SCP the public key to the host machines and create an authorized_keys file. See Example 8-4.

Example 8-4. Distributing ssh key

[ngift@Macintosh-6][H:11026][J:0]% scp id_leop_lap.pub root@10.0.1.51:~/.ssh/
root@10.0.1.51’s password:
id_leop_lap.pub
100%  403     0.4KB/s   00:00
[ngift@Macintosh-6][H:11027][J:0]% ssh root@10.0.1.51
root@10.0.1.51’s password:
Last login: Sun Mar  2 06:26:10 2008
[root@localhost]~# cd .ssh
[root@localhost]~/.ssh# ll
total 8
-rw-r--r-- 1 root root  403 Mar  2 06:32 id_leop_lap.pub
-rw-r--r-- 1 root root 2044 Feb 14 05:33 known_hosts
[root@localhost]~/.ssh# cat id_leop_lap.pub > authorized_keys
[root@localhost]~/.ssh# exit

Connection to 10.0.1.51 closed.
[ngift@Macintosh-6][H:11028][J:0]% ssh root@10.0.1.51
Last login: Sun Mar  2 06:32:22 2008 from 10.0.1.3
[root@localhost]~#

Step 3: mount a common NFS src directory that contains the modules you need clients to run. Often, the easiest way to accomplish this is to use autofs and then make a symbolic link. Alternately, this could be done via a version control system, in which a command is issued via ssh to tell the remote hosts to update their local svn repository full of code. Next, the script would run the newest module. For example, on a Red Hat-based system, you might do something like this:

ln -s /net/nas/python/src /src

Step 4: write a dispatcher to run code against a network of machines. This is a fairly simple task now that we have ssh keys and a common NFS-mounted src directory, or version control-monitored src directory. As usual, let’s start with the simplest possible example of a ssh-based dispatch system. If you have never done this before, you will slap yourself thinking how easy it is to do incredibly powerful things. In Example 8-5, we run a simple uname command.

Example 8-5. Simple ssh-based dispatcher

#!/usr/bin/env python
import subprocess

"""
A ssh based command dispatch system

"""
machines = ["10.0.1.40",
"10.0.1.50",
"10.0.1.51",
"10.0.1.60",
"10.0.1.80"]

cmd = "uname"
for machine in machines:
subprocess.call("ssh root@%s %s" % (machine, cmd), shell=True)

Running that script on those five IP addresses, which are a mixture of CentOS 5, FreeBSD 7, Ubuntu 7.1, and Solaris 10, gives the following:

[ngift@Macintosh-6][H:11088][J:0]% python dispatch.py
Linux
Linux
Linux
SunOS
FreeBSD

Since we wrote a more accurate operating system fingerprint script, let’s use that to get xx a more accurate description of the host machines to which we’re dispatching commands in order to temporarily create src directory on the remote machines and copy our code to each machine. Of course, since we have a dispatch script, it is becoming painfully obvious we need a robust CLI to our tool, as we have to change the script each time we want to do anything different such as the following:

cmd = "mkdir /src"
or:
cmd = "python /src/fingerprint.py"
or even:
subprocess.call("scp fingerprint.py root@%s:/src/" % machine, shell=True)

We will change that right after we get our fingerprint.py script to run, but let’s look at the new cmd first:

#!/usr/bin/env python
import subprocess

"""
A ssh based command dispatch system

"""
machines = ["10.0.1.40",
"10.0.1.50",
"10.0.1.51",
"10.0.1.60",
"10.0.1.80"]

cmd = "python /src/fingerprint.py"
for machine in machines:
subprocess.call("ssh root@%s %s" % (machine, cmd), shell=True)

Now, let’s look at the new output:

[ngift@Macintosh-6][H:11107][J:0]# python dispatch.py
redhat
ubuntu
redhat
SunOS
FreeBSD

This is much better thanks to our fingerprint.py module. Of course, our few lines of dispatch code need a major overhaul to be considered useful, as we have to change things by editing the script. We need a better tool, so let’s make one.

Creating a Cross-Platform, Systems Management Tool

Using ssh keys with a simple ssh-based dispatch system was marginally useful, but hardly extensible or reusable. Let’s make a list of problems with our previous tool, and then a list of requirements to fix those problems. Problems: the list of machines is hardcoded into our script; the command we dispatch is hardcoded into our script; we can only run one command at a time; we have to run the same list of commands to all machines, we cannot pick and choose; and our dispatch code blocks waiting for each command to respond. Requirements: we need a command-line tool that reads in a config file with IP addresses and commands to run; we need a CLI interface with options to send a command(s) to machine(s); and we need to run dispatch in a separate thread pool so the processes do not block.

It seems like we can get away with creating a very basic configuration file syntax to parse, with a section for machines, and a section for commands. See Example 8-6.

Example 8-6. Dispatch config file

[MACHINES]
CENTOS: 10.0.1.40
UBUNTU: 10.0.1.50
REDHAT: 10.0.1.51
SUN: 10.0.1.60
FREEBSD: 10.0.1.80
[COMMANDS]
FINGERPRINT : python /src/fingerprint.py

Next, we need to write a function that reads the config file and splits the MACHINES and COMMANDS up so we can iterate over them one at a time. See Example 8-7.

Note

One thing to note is that our commands will be imported from the config file randomly. In many cases, this is a showstopper, and it would be better to just write a Python file and use that as a configuration file.

Example 8-7. Advanced ssh dispatcher

#!/usr/bin/env python
import subprocess
import ConfigParser

"""
A ssh based command dispatch system

"""

def readConfig(file="config.ini"):
"""Extract IP addresses and CMDS from config file and returns tuple"""
ips = []
cmds = []
Config = ConfigParser.ConfigParser()
Config.read(file)
machines = Config.items("MACHINES")
commands = Config.items("COMMANDS")
for ip in machines:
ips.append(ip[1])
for cmd in commands:
cmds.append(cmd[1])
return ips, cmds

ips, cmds = readConfig()

#For every ip address, run all commands
for ip in ips:
for cmd in cmds:
subprocess.call("ssh root@%s %s" % (ip, cmd), shell=True)

This trivial piece of code is fun to use. We can arbitrarily assign a list of commands and machines and run them at once. If we look at the output of the command, we can see if it looks the same:

[ngift@Macintosh-6][H:11285][J:0]# python advanced_dispatch1.py
redhat
redhat
ubuntu
SunOS
FreeBSD

Even though we have a fairly sophisticated tool, we still have not met our original requirements specification of running dispatched commands in a separate thread pool. Fortunately, we can use some of the tricks from the processes chapter to create a thread pool for our dispatcher quite easily. Example 8-8 shows what adding threading can do.

Example 8-8. Multithreaded command dispatch tool

#!/usr/bin/env python
import subprocess
import ConfigParser
from threading import Thread
from Queue import Queue
import time
"""
A threaded ssh-based command dispatch system

"""
start = time.time()
queue = Queue()

def readConfig(file="config.ini"):
"""Extract IP addresses and CMDS from config file and returns tuple"""
ips = []
cmds = []
Config = ConfigParser.ConfigParser()
Config.read(file)
machines = Config.items("MACHINES")
commands = Config.items("COMMANDS")
for ip in machines:
ips.append(ip[1])
for cmd in commands:
cmds.append(cmd[1])
return ips, cmds

def launcher(i,q, cmd):
"""Spawns command in a thread to an ip"""
while True:
#grabs ip, cmd from queue
ip = q.get()
print "Thread %s: Running %s to %s" % (i, cmd, ip)
subprocess.call("ssh root@%s %s" % (ip, cmd), shell=True)
q.task_done()

#grab ips and cmds from config
ips, cmds = readConfig()

#Determine Number of threads to use, but max out at 25
if len(ips) < 25:
num_threads = len(ips)
else:
num_threads = 25

#Start thread pool
for i in range(num_threads):
for cmd in cmds:
worker = Thread(target=launcher, args=(i, queue,cmd))
worker.setDaemon(True)
worker.start()

print "Main Thread Waiting"
for ip in ips:
queue.put(ip)
queue.join()
end = time.time()
print "Dispatch Completed in %s seconds" % end - start

If we look at the output of our new threaded dispatch engine, we can see that the commands were dispatched and returned in about 1.2 seconds. To really see the speed difference, we should add a timer to our original dispatcher and compare the results:

[ngift@Macintosh-6][H:11296][J:0]# python threaded_dispatch.py
Main Thread Waiting
Thread 1: Running python /src/fingerprint.py to 10.0.1.51
Thread 2: Running python /src/fingerprint.py to 10.0.1.40
Thread 0: Running python /src/fingerprint.py to 10.0.1.50
Thread 4: Running python /src/fingerprint.py to 10.0.1.60
Thread 3: Running python /src/fingerprint.py to 10.0.1.80
redhat
redhat
ubuntu
SunOS
FreeBSD
Dispatch Completed in 1 seconds

By adding some simple timing code to our original dispatcher, we get this new output:

[ngift@Macintosh-6][H:11305][J:0]# python advanced_dispatch1.py
redhat
redhat
ubuntu
SunOS
FreeBSD
Dispatch Completed in 3 seconds

From this sample test, we can tell our threaded version is roughly three times quicker. If we were using our dispatch tool to monitor a network full of machines, say 500 machines, and not 5, it would make a substantial difference in performance. So far, our cross-platform systems management tool is proceeding nicely, so let’s step it up another notch and use it to create a cross-platform build network.

Note

We should note that another, perhaps even better, solution would be to implement this using the parallel version of IPython. See http://ipython.scipy.org/moin/Parallel_Computing.

Creating a Cross-Platform Build Network

Since we know how to distribute jobs in parallel to a list full of machines, identify what operating system they are running, and finally, create a uniform manifest with EPM that can create a vendor-specific package, doesn’t it make sense to put all of this together? We can use these three techniques to quite easily build a cross-platform build network.

With the advent of virtual machines technology, it is quite easy to create a virtual machine for any nonproprietary *nix operating system, such as Debian/Ubuntu, Red Hat/CentOS, FreeBSD, and Solaris. Now, when you create a tool you need to share to the world, or just the people at your company, you can quite easily create a “build farm,” perhaps even running on your laptop, in which you run a script, and then instantly create a vendor package for it.

So how would that work? The most automated way to accomplish this would be to create a common NFS-mounted package build tree, and give all of your build servers access to this mount point. Then, use the tools we created earlier to have the build servers spawn package builds into the NFS-mounted directory. Because EPM allows you to create a simple manifest or “list” file, and because we have created a “fingerprint” script, all the hard work is done. OK, let’s write that code to do just that.

Here is an example of what a build script could look like:

#!/usr/bin/env python
from fingerprint import fingerprint
from subprocess import call

os = fingerprint()

#Gets epm keyword correct
epm_keyword = {"ubuntu":"dpkg", "redhat":"rpm", "SunOS":"pkg", "osx":"osx"}

try:
epm_keyword[os]
except Exception, err:
print err

subprocess.call("epm -f %s helloEPM hello_epm.list" % platform_cmd, shell=True)

Now, with that out of the way, we can edit our config.ini file and change it to run our new script.

[MACHINES]
CENTOS: 10.0.1.40
UBUNTU: 10.0.1.50
REDHAT: 10.0.1.51
SUN: 10.0.1.60
FREEBSD: 10.0.1.80
[COMMANDS]
FINGERPRINT = python /src/create_package.py

Now, we just run our threaded version distribution tool, and eureka, we have packages built for CentOS, Ubuntu, Red Hat, FreeBSD, and Solaris in seconds. This example isn’t what we could consider production code, as there needs to be error handling in place, but it is a great example of what Python can whip up in a matter of a few minutes or a few hours.

PyInotify

If you have the privilege of working with GNU/Linux platforms, then you will appreciate PyInotify. According to the documentation, it is “a Python module for watching filesystem changes.” The official project page is here: http://pyinotify.sourceforge.net.

Example 8-9 shows how it could work.

Example 8-9. Event-monitoring Pyinotify script

import os
import sys
from pyinotify import WatchManager, Notifier, ProcessEvent, EventsCodes

class PClose(ProcessEvent):
"""
Processes on close event
"""

def __init__(self, path):
self.path = path
self.file = file

def process_IN_CLOSE(self, event):
"""
process 'IN_CLOSE_*' events
can be passed an action function
"""
path = self.path
if event.name:
self.file = "%s" % os.path.join(event.path, event.name)
else:
self.file = "%s" % event.path
print "%s Closed" % self.file
print "Performing pretend action on %s...." % self.file
import time
time.sleep(2)
print "%s has been processed" % self.file

class Controller(object):

def __init__(self, path='/tmp'):
self.path = path

def run(self):
self.pclose = PClose(self.path)
PC = self.pclose
# only watch these events
mask = EventsCodes.IN_CLOSE_WRITE | EventsCodes.IN_CLOSE_NOWRITE

# watch manager instance
wm = WatchManager()
notifier = Notifier(wm, PC)

print 'monitoring of %s started' % self.path

added_flag = False
# read and process events
while True:
try:
if not added_flag:
# on first iteration, add a watch on path:
# watch path for events handled by mask.
wm.add_watch(self.path, mask)
added_flag = True
notifier.process_events()
if notifier.check_events():
notifier.read_events()
except KeyboardInterrupt:
# ...until c^c signal
print 'stop monitoring...'
# stop monitoring
notifier.stop()
break
except Exception, err:
# otherwise keep on watching
print err

def main():
monitor = Controller()
monitor.run()

if __name__ == '__main__':
main()

If we run this script, it will “pretend” to do things when something is placed in the /tmp directory. This should give you some idea of how to actually do something useful, such as adding a callback that performs an action. Some of the code in the data section could be useful for doing something that finds duplicates and deletes them automatically, or performs a TAR archive if they match a fnmatch expression you defined. All in all, it is fun and useful that the Python module works on Linux.

OS X

OS X is a weird beast to say the least. On one hand, it has, arguably, the world’s finest user interface in Cocoa; on the other hand, as of Leopard, it has a completely POSIX-compliant Unix operating system. OS X accomplished what every Unix operating system vendor tried to do and failed: it brought Unix to the mainstream. With Leopard, OS X included Python 2.5.1, Twisted, and many other Python goodies.

OS X also follows a somewhat strange paradigm of offering a server and regular version of its operating system. For all the things Apple has done right, it might need to rethink that dinosaur-era thinking, but we can get into the one-OS, one-price discussion later. The server version of the operating system offers some better command-line tools for administration, and a few Apple-specific goodies, such as the ability to NetBoot machines, run LDAP Directory Servers, and more.

Scripting DSCL or Directory Services Utility

DSCL stands for Directory Services Command Line, and it is a convenient hook into OS X’s directory services API. DSCL will let you read, create and delete records, so Python is a natural fit. Example 8-10 shows using IPython to script DSCL to read Open Directory attributes and their values.

Note

Note in the example we read only attributes, but it easy enough to modify them as well using the same technique.

Example 8-10. Getting user record interactively with DSCL and IPython

In [42]: import subprocess

In [41]: p = subprocess.Popen("dscl . read /Users/ngift", shell=True,stdout=subprocess.PIPE)

In [42]: out = p.stdout.readlines()

In [43]: for line in out:
line.strip().split()

Out[46]: ['NFSHomeDirectory:', '/Users/ngift']
Out[46]: ['Password:', '********']
Out[46]: ['Picture:']
Out[46]: ['/Library/User', 'Pictures/Flowers/Sunflower.tif']
Out[46]: ['PrimaryGroupID:', '20']
Out[46]: ['RealName:', 'ngift']
Out[46]: ['RecordName:', 'ngift']
Out[46]: ['RecordType:', 'dsRecTypeStandard:Users']
Out[46]: ['UniqueID:', '501']
Out[46]: ['UserShell:', '/bin/zsh']

It is good to point out that Apple has centralized both local and LDAP/Active Directory account management to use the dscl command. The dscl utility is a wonderful breath of fresh air when dealing with it in comparison to other LDAP management tools, even if you take Python out of the equation. Although we don’t have the space to go into the details, it is quite easy to script the dscl utility with Python to programatically manage accounts either on a local database or a LDAP database such as Open Directory, and the previous code should give you an idea of where to start if you choose to do this.

OS X Scripting APIs

Often, with OS X it is a requirement for a sysadmin to know a bit about high-level scripting that interacts with the actual UI itself. With OS X Leopard, Python, and Ruby, we’re given first-class access to the Scripting Bridge. Refer to this documentation for more information: http://developer.apple.com/documentation/Cocoa/Conceptual/RubyPythonCocoa/Introduction/Introduction.html.

One of the options for accessing the OSA, or Open Scripting Architecture, is py-appscript, which has a project page here: http://sourceforge.net/projects/appscript.

Using py-appscript is quite fun and powerful, as it gives Python the ability to interact with the very rich OSA architecture. Before we dig into it, though, let’s build a simple osascript command-line tool that shows you how the scripting API works. With Leopard, it is now possible to write osascript command-line tools and execute them like Bash or Python scripts. Let’s build this script below, call it bofh.osa, and then make it executable. See Example 8-11.

Example 8-11. Hello, Bastard Operator From Hell osascript

#!/usr/bin/osascript
say "Hello, Bastard Operator From Hell" using "Zarvox"

If we run this from the command line, an alien-sounding voice says hello to us. This was a bit silly, but hey, this is OS X; you are supposed to do things like this.

Now, let’s dig into using appscript to access this same API, in Python, but let’s do this with IPython interactively. Here is an interactive version of an example included with the source code of appscript that prints out all of the running processes in alphabetical order:

In [4]: from appscript import app

In [5]: sysevents = app('System Events')

In [6]: processnames = sysevents.application_processes.name.get()

In [7]: processnames.sort(lambda x, y: cmp(x.lower(), y.lower()))

In [8]: print '\n'.join(processnames)
Activity Monitor
AirPort Base Station Agent
AppleSpell
Camino
DashboardClient
DashboardClient
Dock
Finder
Folder Actions Dispatcher
GrowlHelperApp
GrowlMenu
iCal
iTunesHelper
JavaApplicationStub
loginwindow
mdworker
PandoraBoy
Python
quicklookd
Safari
Spotlight
System Events
SystemUIServer
Terminal
TextEdit
TextMate

If you happen to need to perform work-flow automation tasks with OS X-specific applications, appscript can be a godsend, as it can also do things in Python that were commmonly done via Applescript. Noah wrote an article that goes into some of this: http://www.macdevcenter.com/pub/a/mac/2007/05/08/using-python-and-applescript-to-get-the-most-out-of-your-mac.html.

Some of the things that a sysadmin might do are script Final Cut Pro and create batch operations that interact with, say, Adobe After Effects. One final point of advice is that a very quick-and-dirty way to create GUIs in Python on OS X can be done through Applescript Studio and calls via “do shell script” to Python. A little-known fact is that the original versions of Carbon Copy Cloner were written in Applescript Studio. If you have some time, it is worth exploring.

Automatically Re-Imaging Machines

Yet another revolutionary tool OS X has developed that is ahead of its time is the ASR command-line tool. This tool is a key component in a very popular freeware cloning utility called Carbon Copy Cloner, and it has played a role in automating many environments. Noah used the asr utility in tandom with Netboot to automatically re-image machines; in fact, he fully automated at one place he worked. A user would just need to reboot his machine and hold down the “N” key for a netboot, and it was “game over,” or the machine would fix itself.

Please don’t tell anyone, though, as they still think he works there. Here is a hardcoded and simplistic version of an automated startup script that could be run on a netboot image to automatically re-image a machine, or alternately, it could be run from a second partition on a hard drive. In terms of setup, the /Users directory and any other important directory should be symbolically linked to another partition or should live on the network, which is even better. See Example 8-12.

Example 8-12. Automatically re-image a partition on OS X and show progress with WXPython progress widget

#!/usr/bin/env pythonw
#automatically reimages partition

import subprocess
import os
import sys
import time
from wx import PySimpleApp, ProgressDialog, PD_APP_MODAL, PD_ELAPSED_TIME

#commands to rebuild main partition using asr utility
asr = '/usr/sbin/asr -source '

#path variables
os_path = '/Volumes/main’
ipath = '/net/server/image.dmg '
dpath = '-target /Volumes/main -erase -noprompt -noverify &'
reimage_cmd = "%s%s%s" % (asr,ipath, dpath)

#Reboot Variables
reboot = ‘reboot’
bless = '/usr/sbin/bless -folder /Volumes/main/System/Library/CoreServices -setOF'

#wxpython portion
application = PySimpleApp()
dialog = ProgressDialog ('Progress', 'Attempting Rebuild of Main Partition',
maximum = 100, style = PD_APP_MODAL | PD_ELAPSED_TIME)

def boot2main():
"""Blesses new partition and reboots"""
subprocess.call(bless, shell=True)
subprocess.call(reboot, shell=True)

def rebuild():
"""Rebuilds Partition"""
try:
time.sleep(5)   #Gives dialog time to run
subprocess.call(reimage_cmd)
except OSError:
print "CMD: %s [ERROR:  invalid path]" % reimage_cmd
sys.exit(1)
time.sleep(30)
while True:
if os.path.exists(os_path):
x = 0
wxSleep(1)
dialog.Update ( x + 1, "Rebuild is complete...\n rebooting to main partition\n
...in 5 seconds..")
wxSleep(5)
print "repaired volume.." + os_path
boot2main()        #calls reboot/bless function
break
else:
x = 0
wxSleep(1)
dialog.Update ( x + 1, 'Reimaging.... ')

def main():
if os.path.exists(os_path):
rebuild()
else:
print "Could not find valid path...FAILED.."
sys.exit(1)
if __name__ == "__main__":
main()

To review the code, the script attempts to re-image a partition and pops up a WXPython progress bar. If the path is set correctly, and there are no errors, it then proceeds to re-image the hard drive with the ASR command and a self-updating progress bar, “blesses” the partition that was re-imaged to become the boot volume again, and then tells the machine to reboot.

This script could quite easily become the basis for an enterprise software distribution and management system, as it could be told to distribute different images based on a fingerprint of the hardware, or even by looking at the “old” name of the hard drive. Next, software packages could be distributed programatically using OS X’s package management system, or using the open source tool radmind. One interesting scenario in which Noah has deployed OS X was to first automatically re-image a fresh installation of OS X with a base operating system, and then to finish of the rest of the installation with radmind.

If you are doing any serious OS X systems administration, it would be worth taking a look at radmind. Radmind is a type of tripwire system that detects changes in a filesystem and is able to restore machines based on this changeset. You can refer to http://rsug.itd.umich.edu/software/radmind/ if you would like more information. Although radmind is not written in Python, it can be scripted in Python quite easily.

Managing Plist Files from Python

In Chapter 3, we parsed an XML stream generated from the system_profiler with ElementTree, but Python on OS X comes bundled with plistlib, which allows you to parse and create Plist files. The name of the module itself is plistlib. We won’t have time to get into a use case for it, but it is worth exploring on your own.

Red Hat Linux Systems Administration

Red Hat is doing a whole slew of things with Python as a company and as an operating system. Some of the most interesting new uses of Python at Red Hat are coming from the Emerging Technologies group: http://et.redhat.com/page/Main_Page. Here is a list of some of the projects using Python:

- Libvert, the virtualization API virtual machine manager

- A Python + PyGTK management application built with libvirt VirtInst

- A Python library for simplifying provisioning of guest VMs with libvirt

- Cobbler, which sets up fully automated network boot servers for PXE and virtualization

- Virt-Factory: web-based virtualization management with an application focus

- FUNC (Fedora Unified Network Controller)

Ubuntu Administration

Of all of the mainstream Linux distributions, you could say that Ubuntu is perhaps the one most enamored with Python. Part of this could be that Mark Shuttleworth, the founder, is a long-time Python hacker, going back to the early ’90s. One good source for Python packages on Ubuntu is Launchpad: https://launchpad.net/.

Solaris Systems Administration

From the late ’90s to the early 2000s Solaris was a preferred, “Big Iron” distribution of Unix. Linux’s metioric rise in the early 2000s rapidly cut into Solaris’ and Sun was in some real trouble. However, recently, a lot of sysadmins, developers, and enterprises are talking about Sun again.

Some of the interesting developments in Sun’s future are a 6-month release cycle, just like Ubuntu, with a 18-month support window. It is also copying the single CD approach of Ubuntu as well and ditching the big DVD distribution. Finally, it is mixing some of the ideas from Red Hat and Fedora by having a community-developed version of Solaris mixed. You can know download a live CD or order one here: http://www.opensolaris.com.

What does all this mean for a sysadmin who uses Python? Sun is suddently exciting, and it has a slew of interesting technologies from ZFS, to Containers, to LDOMs which are equivalent to VMware virtual machines in some respects. There is even a correlation to this book. Python works just fine on Solaris, and it is even used quite heavily in its developing package management system.

Virtualization

On August 14, 2007, VMware went public in an IPO that raised billions of dollars and solidified “virtualization” as the next big thing for data centers and systems administrators everywhere. Predicting the future is always a dicey bet, but the words “data center operating system,” are being tossed around by large companies, and everyone from Microsoft to Red Hat to Oracle are jumping on the virtualization bandwagon. It is safe to say that virtualization is going to completely change the data center and the job of systems administration. Virtualization is a no-brainer example of the often overused phrase, “distruptive technology.”

Virtualization is a double-edged sword for systems administrators, as on one hand, it creates the ability to easily test configurations and applications, but on the other hand, it dramatically increases the complexity of administration. No longer does one machine hold one operating system, one machine could hold a hold small business, or a large chunk of a data center. All of the efficiency has to come at some cost, and it does, right out of the hide of the average systems administrator.

You might be at home reading this right now thinking: what could this possibly have to do with Python? Well, quite a bit. Noah’s recent employer Racemi has written a comprehensive data center management application in Python that deals with virtualization. Python can and does interact with virtualization in a very fundamental way, from controlling virtual machines, to moving physical machines to virtual machines via Python APIs. Python is right at home in this new virtualized world, and it is a safe bet it will play a big role in whatever future the data center operating system has.

VMware

VMware as, we mentioned earlier, is the current powerhouse in virtualization. Having full control programatically over a virtual machine is obviously the Holy Grail. Luckily, there are several APIs to look at: Perl, XML-RPC, Python, and C. As of this writing, some of the Python implementations are somewhat limited, but that could change. The new direction for VMware appears to be in terms of the XML-RPC API.

VMware has a few different products with a few different APIs. Some of the products you may want to consider scripting are VMware Site Recovery Manager, VMware ESX Server, VMware Server, and VMware Fusion.

We won’t have room to cover scripting these technologies, as they fall outside the scope of this book, but it would pay to closely monitor these products and examine what role Python will play.

Cloud Computing

Just when the buzz was settling from virtualization, suddenly cloud computing is raising the buzz yet again. Simply put, “cloud computing” is about using resources that respond on demand to workload requirements. The two big players in cloud computing are Amazon and Google. Google just literally dropped the “C” bomb just a few weeks before this book went to the publisher. Google offered an interesting twist in it that only currently supports Python. This being a book on Python programming, we are sure this doesn’t disappoint you too much. For some reason, this whole ordeal with Google offering only Python reminds us of an American Express commercial.

In this section, we go into some of the available APIs that you may need to deal with for both Amazon and Google App Engine. Finally, we talk about how this may impact systems administration.

Amazon Web Services with Boto

An exciting option for dealing with Amazon’s cloud computing infrastructure is Boto. With Boto, you can do the following things: Simple Storage Service, Simple Queue Service, Elastic Compute Cloud, Mechanical Turk, SimpleDB. Because this is a very new yet powerful API, we recommend that you just take a look at the project home page yourself: http://code.google.com/p/boto/. This will give you the latest information better than we can give you in dead tree format.

Here is a brief example though of how it works with SimpleDB:

Initial connection:

In [1]: import boto
In [2]: sdb = boto.connect_sdb()

Create a new domain:

In [3]: domain = sdb.create_domain('my_domain')

Adding a new item:

In [4]: item = domain.new_item('item')

This is the feel for how the API works currently, but you should take a look at the tests in svn repository to get a real idea of how things work: http://code.google.com/p/boto/source/browse. On a side note, looking at tests is one of the best ways to understand how a new library works.

Google App Engine

Google App Engine was released as a beta service, and it was massively buzzworthy from the day it was announced. It lets you run your application on Google’s infrastructure for free. App Engine applications have a strictly Python API for now, but that could change at some point. One of the other interesting things about App Engine is that it also integrates with Google’s other services.

Celebrity Profile: Google App Engine Team: Kevin Gibbs

Kevin Gibbs is the technical lead for Google App Engine. Kevin joined Google in 2004. Prior to his work on Google App Engine, Kevin worked for a number of years in Google’s systems infrastructure group, where he worked on the cluster management systems that underlie Google’s products and services. Kevin is also the creator of Google Suggest, a product which provides interactive search suggestions as you type. Prior to joining Google, Kevin worked with the Advanced Internet Technology group at IBM, where he focused on developer tools.

One of the ways this affects a systems administrator is that it is increasingly becoming feasible to host major portions of what used to live in your data center into another data center. Knowing how to interact with Google App Engine could be the killer new skill for sysadmins, so it makes sense to investigate it a bit.

We interviewed several people from the App Engine Team and talked to them about how this would affect a systems administrator. They mentioned the following tasks:

1. Bulk Data Uploader: http://code.google.com/appengine/articles/bulkload.html.
Sysadmins often deal with moving large chunks of data around, and this is a tool for doing that in the context of an app on Google App Engine.

2. Logging: http://code.google.com/appengine/articles/logging.html.

3. Mail API: send_mail_to_admins() function: http://code.google.com/appengine/docs/mail/functions.html.
In a sysadmin context, this could be useful for monitoring. For important exceptions or key actions, you could automatically send an email to the app’s admins.

4. Cron jobs for regular tasks.
This is something that is not directly a part of Google App Engine, but you could use cron on your own servers to send requests to your app at regular intervals. For example, you could have a cron job that hit http://yourapp.com/emailsummary every hour, which triggered an email to be sent to admins with a summary of important events over the last hour.

5. Version management: http://code.google.com/appengine/docs/configuringanapp.html#Required_Elements.
One of the required fields you set for your app is the “version.” Each time you upload an app with the same version ID, it replaces it with the new code. If you change the version ID, you can have multiple versions of your app running in production and use the admin console to select which version receives life traffic.

Building a sample Google App Engine application

To get started with building a Google App Engine application, you will need to first download the SDK for Google app engine here: http://code.google.com/appengine/downloads.html. You also might do well to go through the excellent tutorial for Google App Engine: http://code.google.com/appengine/docs/gettingstarted/.

In this section, we offer a reverse tutorial on Google App Engine, as there is already an excellent tutorial. If you go to http://greedycoin.appspot.com/, you can test out a running version of what we are going to cover, along with the latest version of the source code. The application takes change as an input, stores it in the database, and then returns proper change. It also has the ability to log in via Google’s authentication API and perform a recent actions query. See Example 8-13.

Example 8-13. Greedy coin web application

#!/usr/bin/env python2.5
#Noah Gift

import decimal
import wsgiref.handlers
import os

from google.appengine.api import users
from google.appengine.ext import webapp
from google.appengine.ext import db
from google.appengine.ext.webapp import template

class ChangeModel(db.Model):
user = db.UserProperty()
input = db.IntegerProperty()
date = db.DateTimeProperty(auto_now_add=True)

class MainPage(webapp.RequestHandler):
"""Main Page View"""

def get(self):
user = users.get_current_user()

if users.get_current_user():
url = users.create_logout_url(self.request.uri)
url_linktext = ‘Logout’
else:
url = users.create_login_url(self.request.uri)
url_linktext = ‘Login’

template_values = {
‘url’: url,
‘url_linktext’: url_linktext,
}
path = os.path.join(os.path.dirname(__file__), ‘index.html')
self.response.out.write(template.render(path, template_values))

class Recent(webapp.RequestHandler):
"""Query Last 10 Requests"""

def get(self):

#collection
collection = []
#grab last 10 records from datastore
query = ChangeModel.all().order('-date')
records = query.fetch(limit=10)

#formats decimal correctly
for change in records:
collection.append(decimal.Decimal(change.input)/100)

template_values = {
'inputs': collection,
'records': records,
}

path = os.path.join(os.path.dirname(__file__), 'query.html')
self.response.out.write(template.render(path,template_values))

class Result(webapp.RequestHandler):
"""Returns Page with Results"""
def __init__(self):
self.coins = [1,5,10,25]
self.coin_lookup = {25: "quarters", 10: "dimes", 5: "nickels", 1: "pennies"}

def get(self):
#Just grab the latest post
collection = {}

#select the latest input from the datastore
change = db.GqlQuery("SELECT * FROM ChangeModel ORDER BY date DESC LIMIT 1")
for c in change:
change_input = c.input

#coin change logic
coin = self.coins.pop()
num, rem  = divmod(change_input, coin)
if num:
collection[self.coin_lookup[coin]] = num
while rem > 0:
coin = self.coins.pop()
num, rem = divmod(rem, coin)
if num:
collection[self.coin_lookup[coin]] = num

template_values = {
'collection': collection,
'input': decimal.Decimal(change_input)/100,
}

#render template
path = os.path.join(os.path.dirname(__file__), 'result.html')
self.response.out.write(template.render(path, template_values))

class Change(webapp.RequestHandler):

def post(self):
"""Printing Method For Recursive Results and While Results"""
model = ChangeModel()
try:
change_input = decimal.Decimal(self.request.get('content'))
model.input = int(change_input*100)
model.put()
self.redirect('/result')
except decimal.InvalidOperation:
path = os.path.join(os.path.dirname(__file__), 'submit_error.html')
self.response.out.write(template.render(path,None))

def main():
application = webapp.WSGIApplication([('/', MainPage),
('/submit_form', Change),
('/result', Result),
('/recent', Recent)],
debug=True)
wsgiref.handlers.CGIHandler().run(application)

if __name__ == "__main__":
main()

As a reverse tutorial, let’s start by looking at the version running at http://greedycoin.appspot.com/, or your development version at http://localhost:8080/. There is a pumpkin-colored theme that has two floating boxes; on the left is a form that lets you input change, and on the right there is a navigation box. These pretty, or ugly, colors and layout are just a combination of Django templating and CSS. The Django templates can be found in the main directory, and the CSS we used is found in stylesheets. This really has little to do with Google App Engine, so we will just refer you to the Django templating reference material for more: http://www.djangoproject.com/documentation/templates/.

Now that we have covered this, let’s actually get into some Google App Engine specifics. If you notice the “Login” link in the right navigation box, it is made possible by the clever user authentication API. Here is what that actual code looks like:

class MainPage(webapp.RequestHandler):
"""Main Page View"""

def get(self):
user = users.get_current_user()

if users.get_current_user():
url = users.create_logout_url(self.request.uri)
url_linktext = 'Logout'
else:
url = users.create_login_url(self.request.uri)
url_linktext = 'Login'

template_values = {
'url': url,
'url_linktext': url_linktext,
}
path = os.path.join(os.path.dirname(__file__), 'index.html')
self.response.out.write(template.render(path, template_values))

There is a class that inherits from webapp.RequestHandler, and if you define a get method, you can make a page that checks to see if a user is logged in or not. If you notice the few lines at the bottom, you will see that the user information gets tossed into the template system and then gets rendered to the Django template file index.html. What is incredibly powerful is that it is trivial to leverage the Google User Accounts database to create authorization for pages. If you look at the previous code, it is as simple as saying:

user = users.get_current_user()

if users.get_current_user():

At this point, we would suggest fiddling around with this code and trying to add code that only shows up for authenticated users. You don’t even need to understand how things work; you could just use the existing conditional statements to do something.

Now that we have a vague understanding of authentication, let’s get into the powerful stuff. The datastore API lets you store persistent data and then retrieve it throughout your application. In order to do this, you need to import the datastore, as shown in the previous code, and then also define a model like this:

class ChangeModel(db.Model):
user = db.UserProperty()
input = db.IntegerProperty()
date = db.DateTimeProperty(auto_now_add=True)

With that simple class, we can now create and use persistent data. Here is a class in which we use Python API to the datastore to retrieve the last 10 changes made to the database, and then display them:

class Recent(webapp.RequestHandler):
"""Query Last 10 Requests"""

def get(self):

#collection
collection = []
#grab last 10 records from datastore
query = ChangeModel.all().order('-date')
records = query.fetch(limit=10)

#formats decimal correctly
for change in records:
collection.append(decimal.Decimal(change.input)/100)

template_values = {
'inputs': collection,
'records': records,
}

path = os.path.join(os.path.dirname(__file__), 'query.html')
self.response.out.write(template.render(path,template_values))

The two most important lines are:

query = ChangeModel.all().order('-date')
records = query.fetch(limit=10)

These pull the results out of the datastore and then “fetch” 10 records in a query. At this point, a simple thing to do for fun would be to experiment with this code and to try to fetch more records, or to sort them in a different way. This should give you some immediate and fun feedback.

Finally, if we look closely at the code below, we might be able to guess that each of the URLs corresponds to a class we defined in our change.py file. At this point, we would recommend trying to tweak the names of URLs by changing the parts of the application that depend on a URL; this will give you a good idea of how things get routed around.

def main():
application = webapp.WSGIApplication([('/', MainPage),
('/submit_form', Change),
('/result', Result),
('/recent', Recent)],
debug=True)
wsgiref.handlers.CGIHandler().run(application)

This is the end of this reverse tutorial on Google App Engine, but it should give you some ideas on how you could implement a more sysadmin-like tool on your own. If you are interested in writing more applications, you should also take a look a Guido’s source code for his Google App Engine application: http://code.google.com/p/rietveld/source/browse.

Using Zenoss to Manage Windows Servers from Linux

If you have the unfortunate task of managing one or more Windows servers, the task just became a little less unpleasant. Zenoss is an amazing tool that will help us out here. We talk about Zenoss in the Chapter 7, SNMP. In addition to being an industry-leading SNMP tool, Zenoss also provides the tools to talk to a Windows server via WMI—from Linux! We still get the giggles when thinking about the practical implications of this as well as the technology used to make it possible. From a discussion that we had with the good folks at Zenoss, they push WMI messages down to Samba (or possibly CIFS now) on a Linux box and send them over to your Windows server. And possibly the most interesting part of this (at least for readers of this book, anyway) is that you can script this WMI connection with Python.

Note

A discussion of the syntax and features of WMI is beyond the scope of this book.

Currently, the Zenoss documentation is pretty light on the WMI-from-Linux-using-Python functionality. However, the examples that we are about to review should provide a good foundation for you to build on. First off, let’s look at a non-Python tool for talking WMI to a Windows server from Linux. wmic. wmic is a simple command-line utility that takes username, password, server address, and WMI query as command-line parameters, connects to the appropriate server with the given credentials, passes the query to the server, and displays the result to standard output. The syntax for using the utility is something like this:

wmic -U username%password //SERVER_IP_ADDRESS_OR_HOSTNAME "some WMI query"

And here is an example of connecting to a server with an IP address of 192.168.1.3 as Administrator and asking for its event logs:

wmic -U Administrator%password //192.168.1.3 "SELECT * FROM Win32_NTLogEvent"

And here is part of the result of running that command:

CLASS: Win32_NTLogEvent
Category|CategoryString|ComputerName|Data|EventCode|EventIdentifier|
EventType|InsertionStrings|Logfile|Message|RecordNumber|SourceName|
TimeGenerated|TimeWritten|Type|User
...
|3|DCOM|20080320034341.000000+000|20080320034341.000000+000|Information|(null)
0|(null)|MACHINENAME|NULL|6005|2147489653|3|(,,,,14,0,0 )|System|The Event log
service was started.
|2|EventLog|20080320034341.000000+000|20080320034341.000000+000|Information|(null)
0|(null)|MACHINENAME|NULL|6009|2147489657|3|(5.02.,3790,Service Pack
2,Uniprocessor Free)|System|Microsoft (R) Windows (R) 5.02. 3790 Service Pack 2
Uniprocessor Free.
|1|EventLog|20080320034341.000000+000|20080320034341.000000+000|Information|(null)

In order to write a similar Python script, we first have to set up our environment. For the following examples, we used the Zenoss v2.1.3 VMware appliance. In this appliance, some of the Zenoss code is located in the home directory of the zenoss user. The biggest part of that is to add the directory where the wmiclient.py module lives to your PYTHONPATH. We prepended the directory to our already existing PYTHONPATH like this:

export PYTHONPATH=~/Products/ZenWin:$PYTHONPATH

Once we have access to the needed libraries in Python, we can execute a script something like the following:

#!/usr/bin/env python

from wmiclient import WMI

if __name__ == '__main__':
w = WMI('winserver', '192.168.1.3', 'Administrator', passwd='foo')
w.connect()
q = w.query('SELECT * FROM Win32_NTLogEvent')
for l in q:
print "l.timewritten::", l.timewritten
print "l.message::", l.message

Rather than printing out all fields as the wmic example did, this script prints only out the timestamp and the body of the log message. This script connects to the server 192.168.1.3 as Administrator with the password foo. Then, it executes the WMI query 'SELECT * FROM Win32_NTLogEvent'. It then iterates over the results of the query and prints the timestamp and the log message body. It really couldn’t get much easier than that.

Here is some of the output from running this script:

l.timewritten:: 20080320034359.000000+000
l.message:: While validating that \Device\Serial1 was really a serial port, a
fifo was detected. The fifo will be used.

l.timewritten:: 20080320034359.000000+000
l.message:: While validating that \Device\Serial0 was really a serial port, a
fifo was detected. The fifo will be used.

l.timewritten:: 20080320034341.000000+000
l.message:: The COM sub system is suppressing duplicate event log entries for a
duration of 86400 seconds. The suppression timeout can be controlled by a
REG_DWORD value named SuppressDuplicateDuration under the following registry
key: HKLM\Software\Microsoft\Ole\EventLog.

l.timewritten:: 20080320034341.000000+000
l.message:: The Event log service was started.

l.timewritten:: 20080320034341.000000+000
l.message:: Microsoft (R) Windows (R) 5.02. 3790 Service Pack 2 Uniprocessor
Free.

But how did we know to use the timewritten and message attributes for these records? It took just a bit of hacking to find that information. Here is a script that we ran to help us find which attributes we needed to use:

#!/usr/bin/env python

from wmiclient import WMI

if __name__ == '__main__':
w = WMI('winserver', '192.168.1.3', 'Administrator', passwd='foo')
w.connect()
q = w.query('SELECT * FROM Win32_NTLogEvent')
for l in q:
print "result set fields::->", l.Properties_.set.keys()
break

You may notice that this script looks quite similar to the other WMI script. The two differences between this script and the other WMI script are rather than printing out the timestamp and the log message body, this script prints out l.Properties_.set.keys(), and this script breaks after the first result. The set object that we call keys() on is actually a dictionary. (Which makes sense, because keys() is a dictionary method.) Each resulting record from the WMI query should have a set of attributes that correspond to these keys. So, here is the output from running the script that we just discussed:

result set fields::-> ['category', 'computername', 'categorystring',
'eventidentifier', 'timewritten', 'recordnumber', 'eventtype', 'eventcode',
'timegenerated', 'sourcename', 'insertionstrings', 'user', 'type', 'message',
'logfile', 'data']

The two attributes that we chose to pull from in the first WMI script, 'message' and 'timewritten', are both in this list of keys.

While we aren’t huge fans of working with Windows, we recognize that sometimes the job dictates the technology that we use. This tool from Zenoss can make that task a lot less painful. Plus, it’s just cool to be able to run a WMI query from Linux. If you have to do much work with Windows, then Zenoss could easily find a prominent place in your toolbox.

Chapter 9. Package Management

Introduction

Package management is a one of the most critical factors in the success of a software development project. Package management can be thought of as the shipping company in an e-commerce business, such as Amazon. If there were no shipping companies, Amazon would not exist. Likewise, if there is not a functional, simple, robust package management system for an operating system or language, then it will be limited to some degree.

When we mention “package management,” your first thoughts are probably drawn toward .rpm files and yum, or .deb files and apt or some other operating system level package management system. We’ll get into that in this chapter, but the primary focus is on packaging and managing Python code and your Python environment. Python has always had ways to make Python code universally accessible to the entire system. Recently, though, there have been some projects which have improved the flexibility and usability of packaging, managing, and deploying Python code.

Some of these recent projects include setuptools, Buildout, and virtualenv. Buildout, setuptools, and virtualenv are often about development, development libraries, and dealing with development environments. But at their heart, they are mostly about using Python to deploy Python code in operating system-agnostic ways. (Note that we did say “mostly” here.)

Another deployment scenario involves creating operating system-specific packages and deploying them to an end user’s machine. Sometimes, these are two completely different problems, although there is some degree of overlap. We will be discussing an open source tool called EPM that generates native platform packages for AIX, Debian/Ubuntu, FreeBSD, HP-UX, IRIX, Mac OS X, NetBSD, OpenBSD, Red Hat, Slackware, Solaris, and Tru64 Unix.

Package mangement isn’t good just for software developers. It is critical for system administrators as well. In fact, a system administrator is often the person with whom the buck stops for package management. Understanding the latest techniques in package management for Python and for other operating systems is one way to make yourself an invaluable resource. Hopefully, this chapter will help you in that regard. A very valuable reference for the topics we cover in this chapter can also be found here: http://wiki.python.org/moin/buildout/pycon2008_tutorial.

Setuptools and Python Eggs

According the official documentation, “setuptools is a collection of enhancements to the Python distutils (for Python 2.3.5 on most platforms, although 64-bit platforms require a minimum of Python 2.4) that allow you to more easily build and distribute packages, especially ones that have dependencies on other packages.”

Until the creation of setuptools, distutils was the primary way of creating installable Python packages. setuptools is a library that enhances distutils. “Eggs” refers to the final bundle of Python packages and modules, much like an .rpm or .deb file. They are typically distributed in a zipped format and installed in either the zipped format or are unzipped so that you can navigate the package contents. Eggs is a feature of the setuptools library that works with easy_install. According to the official documentation, “Easy Install is a python module (easy_install) bundled with setuptools that let’s you automatically download, build, install and manage Python packages.” While it is a module, it is most often thought of and interacted with as a command-line tool. In this section, we will cover and explain setuptools, easy_install, and eggs, and clear up any confusion about what each provides.

We’ll outline what we feel are the most useful features of setuptools and easy_install in this chapter. However, to get the full set of documentation on them, you can visit http://peak.telecommunity.com/DevCenter/setuptools and http://peak.telecommunity.com/DevCenter/EasyInstall, respectively.

Complex tools that do amazing things are often hard to understand. Parts of setuptools are difficult to grasp as a direct result of the amazing things it can do. With this section acting as a quickstart guide, and then later referring to the manual, you should be able to get a handle on using setuptools, easy_install, and Python eggs as a user and developer.

Using easy_install

The basics of understanding and using easy_install are very easy to grasp. The majority of people reading this book have very likely used rpm, yum, apt-get, fink, or a similar package management tool at some point. The phrase “Easy Install,” often refers to the use of a command-line tool named easy_install to do similar things as yum on Red Hat-based systems, and apt-get on Debian-based systems, but for Python packages.

The easy_install tool can be installed by running a “bootstrap” script named ez_setup.py with the version of Python you wish easy_install to work with. ez_setup.py grabs the latest version of setuptools and then automatically installs easy_install as a script to the default “scripts” location, which on *nixes is typically the same directory that your python binary lives in. Let’s take a look at how “easy” that is. See Example 9-1.

Example 9-1. Bootstrapping easy_install

$ curl http://peak.telecommunity.com/dist/ez_setup.py
> ez_setup.py
% Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
Dload  Upload   Total   Spent    Left  Speed
100  9419  100  9419    0     0    606      0  0:00:15  0:00:15 --:--:-- 83353
$ ls
ez_setup.py
$ sudo python2.5 ez_setup.py
Password:
Searching for setuptools
Reading http://pypi.python.org/simple/setuptools/
Best match: setuptools 0.6c8
Processing setuptools-0.6c8-py2.5.egg
setuptools 0.6c8 is already the active version in easy-install.pth
Installing easy_install script to /usr/local/bin
Installing easy_install-2.5 script to /usr/local/bin

Using /Library/Python/2.5/site-packages/setuptools-0.6c8-py2.5.egg
Processing dependencies for setuptools
Finished processing dependencies for setuptools
$

In this situation, easy_install was placed into /usr/local/bin under two different names.

$ ls -l /usr/local/bin/easy_install*
-rwxr-xr-x  1 root  wheel  364 Mar  9 18:14 /usr/local/bin/easy_install
-rwxr-xr-x  1 root  wheel  372 Mar  9 18:14 /usr/local/bin/easy_install-2.5

This has been a convention that Python itself has used for quite a while: when installing an executable, install one with a version number denoting the version of Python and one without the version number. This means that the one that doesn’t have the version number will be used by default when a user doesn’t explicitly reference the versioned script. This also means that the last-installed version will become the default. It is convenient, though, that the older version still sticks around.

Here are the contents of the newly installed /usr/local/bin/easy_install:

#!/System/Library/Frameworks/Python.framework/Versions/2.5/Resources/Python.app/
Contents/MacOS/Python
# EASY-INSTALL-ENTRY-SCRIPT: 'setuptools==0.6c8','console_scripts','easy_install'
__requires__ = 'setuptools==0.6c8'
import sys
from pkg_resources import load_entry_point

sys.exit(
load_entry_point('setuptools==0.6c8', 'console_scripts', 'easy_install')()
)

The main point here is that when you install setuptools, it installs a script for you named easy_install that you can use to install and manage Python code. A secondary point here that we were making by showing the contents of the easy_install script is that this is the type of script that is automatically created for you when you use “entry points” when defining packages. Don’t worry right now about the contents of this script or entry points or how to create scripts like this. We’ll get to all of that later in this chapter.

Now that we have easy_install, we can install any package that is located in the central repository for uploaded Python Modules, commonly referred to as PyPI (Python Package Index), or the “Cheeseshop”: http://pypi.python.org/pypi.

To install IPython, the shell we use exclusively in examples throughout the book, we can just issue this command:

sudo easy_install ipython

Notice that easy_install required sudo privileges in this setup, as it installed packages to the global Python site-pacakges directory. It also placed scripts in the default scripts directory of the operating system, which is the same directory that the python executable lives in. Basically, easy_installing a package requires permissions to write files to the site-packages directory and the script directory for you Python installation. If this is a problem, you should refer to the section of this chapter where we discuss using virtualenv and setuptools. Alternatively, you could even compile and install Python in a directory that you own, such as your home directory.

Before we get into advanced use of the easy_install tool, here’s a quick summary for basic use of easy_install:

1. Download the ez_setup.py bootstrap script.

2. Run ez_setup.py with the version of Python you wish to install packages with.

3. Explicitly run easy_install with the version of python that installed it if you have several versions of Python running on your system.

Celebrity Profile: Easy Install: Phillip J. Eby

Phillip J. Eby has been responsible for numerous Python Enhancement Proposals, the WSGI standard, setuptools, and more. He was featured in the book Dreaming in Code (Three Rivers Press). You can read his programming blog at: http://dirtsimple.org/programming/.

easy_install Advanced Features

For most casual users, using easy_install and passing it only one command-line argument without any additional options will fit all of their needs. (By the way, giving easy_install only one argument, a package name, will simply download and install that package, as in the previous example with IPython.) There are cases, though, where it is nice to have more power under the hood to do various things other than just download eggs from the Python Package Index. Fortunately, easy_install has quite a few tricks up its sleeve and is flexible enough to do a whole assortment of advanced miscellanea.

Search for Packages on a Web Page

As we saw earlier, easy_install can automatically search the central repository for packages and automatically install them. It can also install packages in just about any way you can think of. Following is an example of how to search a web page and install or upgrade package by name and version:

$ easy_install -f http://code.google.com/p/liten/ liten
Searching for liten
Reading http://code.google.com/p/liten/
Best match: liten 0.1.3
Downloading http://liten.googlecode.com/files/liten-0.1.3-py2.4.egg
[snip]

In this situation, there is a Python2.4 and a Python2.5 egg at http://code.google.com/p/liten/. easy_install -f specifies a location to look for eggs. It found both eggs and then installed the Python2.4 egg, as it was the best match. Obviously, this is quite powerful, as easy_install not only found the egg link to begin with, but also found the correct egg version.

Install Source Distribution from URL

Now, we’ll automatically install a source distribution from a URL:

% easy_install http://superb-west.dl.sourceforge.net/sourceforge
/sqlalchemy/SQLAlchemy-0.4.3.tar.gz

Downloading http://superb-west.dl.sourceforge.net/sourceforge
/sqlalchemy/SQLAlchemy-0.4.3.tar.gz
Processing SQLAlchemy-0.4.3.tar.gz
Running SQLAlchemy-0.4.3/setup.py -q bdist_egg --dist-dir
/var/folders/LZ/LZFo5h8JEW4Jzr+ydkXfI++++TI/-Tmp-/
easy_install-Gw2Xq3/SQLAlchemy-0.4.3/egg-dist-tmp-Mf4jir
zip_safe flag not set; analyzing archive contents...
sqlalchemy.util: module MAY be using inspect.stack
sqlalchemy.databases.mysql: module MAY be using inspect.stack
Adding SQLAlchemy 0.4.3 to easy-install.pth file

Installed /Users/ngift/src/py24ENV/lib/python2.4/site-packages/SQLAlchemy-0.4.3-py2.4.egg
Processing dependencies for SQLAlchemy==0.4.3
Finished processing dependencies for SQLAlchemy==0.4.3

We passed the URL of a gzipped tarball to easy_install. It was able to figure out that it should install this source distribution without being explicitly told to do so. This is a neat trick, but the source must include a setup.py file at the root level for it to work. For example, at the time of this writing, if someone nested their package several levels deep into empty folders, then this will fail.

Install Egg Located on Local or Network Filesystem

Here is an example of how to install an egg located on a filesystem or NFS-mounted storage:

easy_install /net/src/eggs/convertWindowsToMacOperatingSystem-py2.5.egg

You can also install eggs from an NFS-mounted directory or a local partition. This can be a very efficient to distribute packages in a *nix environment, especially across a number of machines you’d like to keep in sync with one another regarding the versions of code they are running. Some of the other scripts in this book could help with creating a polling daemon. Each client could run such a daemon to check for updates to the centralized repository of eggs. If there is a new version, then it could automatically update itself.

Upgrading Packages

Another way of using easy_install is by getting it to upgrade packages. In the next few examples, we’ll walk through installing and then upgrading the CherryPy package.

First, we’ll install version 2.2.1 of CherryPy:

$ easy_install cherrypy==2.2.1
Searching for cherrypy==2.2.1
Reading http://pypi.python.org/simple/cherrypy/
....
Best match: CherryPy 2.2.1
Downloading http://download.cherrypy.org/cherrypy/2.2.1/CherryPy-2.2.1.tar.gz
....
Processing dependencies for cherrypy==2.2.1
Finished processing dependencies for cherrypy==2.2.1

Now, we’ll show you what happens when you try to easy_install something that has already been installed:

$ easy_install cherrypy
Searching for cherrypy
Best match: CherryPy 2.2.1
Processing CherryPy-2.2.1-py2.5.egg
CherryPy 2.2.1 is already the active version in easy-install.pth

Using /Users/jmjones/python/cherrypy/lib/python2.5/site-packages/CherryPy-2.2.1-py2.5.egg
Processing dependencies for cherrypy
Finished processing dependencies for cherrypy

After you’ve installed some version of a package, you can upgrade to a newer version of the same package by explicitly declaring which version to download and install:

$ easy_install cherrypy==2.3.0 Searching for
cherrypy==2.3.0
Reading http://pypi.python.org/simple/cherrypy/
....
Best match: CherryPy 2.3.0
Downloading http://download.cherrypy.org/cherrypy/2.3.0/CherryPy-2.3.0.zip
....
Processing dependencies for cherrypy==2.3.0
Finished processing dependencies for cherrypy==2.3.0

Notice that we didn’t use the --upgrade flag in this particular example. You only really ever need to use --upgrade if you already have some version of a package installed and want to update it to the latest version of that package.

Next, we upgrade to CherryPy 3.0.0 using the --upgrade flag. Here, --upgrade was purely unnecessary:

$ easy_install --upgrade cherrypy==3.0.0
Searching for cherrypy==3.0.0
Reading http://pypi.python.org/simple/cherrypy/
....
Best match: CherryPy 3.0.0
Downloading http://download.cherrypy.org/cherrypy/3.0.0/CherryPy-3.0.0.zip
....
Processing dependencies for cherrypy==3.0.0
Finished processing dependencies for cherrypy==3.0.0

Giving the --upgrade flag without specifying a version upgrades the package to the latest version. Notice that this is different from specifying easy_install cherrypy. With easy_install cherrypy, there already existed some version of the CherryPy package, so no action was taken. In the following example, CherryPy will be upgraded to the most current version:

$ easy_install --upgrade cherrypy
Searching for cherrypy
Reading http://pypi.python.org/simple/cherrypy/
....
Best match: CherryPy 3.1.0beta3
Downloading http://download.cherrypy.org/cherrypy/3.1.0beta3/CherryPy-3.1.0beta3.zip
....
Processing dependencies for cherrypy
Finished processing dependencies for cherrypy

Now, CherryPy is at 3.1.0b3. If we specify to upgrade to something greater than 3.0.0, no action will be taken, since it is already there:

$ easy_install --upgrade cherrypy>3.0.0
$

Install an Unpacked Source Distribution in Current Working Directory

Although this looks trivial, it can be useful. Rather than going through the python setup.py install routine, you can just type the following (it’s a few less characters to type, so it’s definitely a tip for the lazy):

easy_install

Extract Source Distribution to Specified Directory

You can use the following example to find either a source distribution or checkout URL for a package and then either extract it or check it out to a specified directory:

easy_install --editable --build-directory ~/sandbox liten

This is handy, as it allows easy_install to take a source distribution and put it in the directory you specify. Since installing a package with easy_install doesn’t always install everything (such as documentation or code examples), this is a good way to look at everything included in the source distribution. easy_install will only pull down the package source. If you need to install the package, you will need to run easy_install again.

Change Active Version of Package

This example assumes that you have liten version 0.1.3 and some other version of liten installed. It also assumes that the other version is the “active version.” This is how you would reactivate 0.1.3:

easy_install liten=0.1.3

This will work whether you need to downgrade to an older package or if you need to get back to a more current version of a package.

Changing Standalone .py File into egg

Here is how you convert a regular standalone Python package into an egg (note the -f flag):

easy_install -f "http://svn.colorstudy.com/virtualenv/
trunk/virtualenv.py#egg=virtualenv-1.0" virtualenv

This is useful when you want to package a single .py file as an egg. Sometimes, using this method is your best choice if you want to use a previously unpackaged standalone Python filesystem-wide. Your other alternative is to set your PYTHONPATH whenever you want to use that standalone module. In this example, we are packaging the virtualenv.py script from that project’s trunk and putting our own version and name label on it. In the URL string, the #egg=virtualenv-1.0 simply specifies the package name and version number we are choosing to give this script. The argument that we give after the URL string is the package name we are looking for. It makes sense to use the consistent names between the URL string and the standalone package name argument, because we are telling easy_install to install a package with the same name as what we just created. While it makes sense to keep these two in sync, you shouldn’t feel constrained to keep the package name in sync with the name of the module. For example:

easy_install -f "http://svn.colorstudy.com/virtualenv/
trunk/virtualenv.py#egg=foofoo-1.0" foofoo

This does exactly the same thing as the previous example, except that it creates a package named foofoo rather than virtualenv. What you choose to name these types of packages is entirely up to you.

Authenticating to a Password Protected Site

There may be cases where you need to install an egg from a website that requires authentication before allowing you to pull down any files. In that case, you can use this syntax for a URL to specify a username and password:

easy_install -f http://uid:passwd@example.com/packages

You may have a secret skunkworks project you are developing at work that you don’t want your coworkers to find out about. (Isn’t everyone doing this?) One way to distribute your packages to coworkers “behind the scenes,” is to create a simple .htaccess file and then tell easy_install to do an authenticated update.

Using Configuration Files

easy_install has yet another trick for power users. You can specify default options using config files that are formatted using .ini syntax. For systems administrators, this is a godsend of a feature, as it allows a declarative configuration of clients who use easy_install. easy_install will look for config files in the following places, in this order: current_working_directory/setup.cfg, ~/.pydistutils.cfg, and distutils.cfg in the distutils package directory.

So, what can you put in this configuration file? Two of the most common items to set are a default intranet site(s) for package downloads, and a custom install directory for the packages. Here is what a sample easy_install configuration file could look like:

[easy_install]

#Where to look for packages
find_links = http://code.example.com/downloads

#Restrict searches to these domains
allow_hosts = *.example.com

#Where to install packages. Note, this directory has to be on the PYTHONPATH
install_dir = /src/lib/python

This configuration file, which we could call ~/.pydistutils.cfg, defines a specific URL to search for packages, allows only searches for packages to come from example.com (and subdomains), and finally places packages into a custom python package directory.

Easy Install Advanced Features Summary

This was not meant to be a replacement for the comprehensive official documentation for easy_install, but it was meant to highlight some of the key features of the tool for power users. Because easy_install is still in active development, it would be a good idea to frequently check http://peak.telecommunity.com/DevCenter/EasyInstall for updates to the documentation. There is also a mailing list called the distutils-sig (sig stands for special interest group) that discusses all things Python distribution-related. Sign up at http://mail.python.org/mailman/listinfo/distutils-sig, and you can report bugs and get help for easy_install there, too.

Finally, by doing a simple easy_install --help, you will find even more options that we did not discuss. Chances are very good that something you want to do has already been included as a feature in easy_install.

Creating Eggs

We mentioned earlier that an egg is a bundle of Python modules, but we didn’t give a much better definition at the time than that. Here is a definition of “egg” from the setuptools website:

Python Eggs are the preferred binary distribution format for EasyInstall, because they are cross-platform (for “pure” packages), directly importable, and contain project metadata including scripts and information about the project’s dependencies. They can be simply downloaded and added to sys.path directly, or they can be placed in a directory on sys.path and then automatically discovered by the egg runtime system.

And we certainly didn’t give any reason why a system administrator would be interested in creating eggs. If all that you do is write one-off scripts, eggs won’t help you much. But if you start to recognize patterns and common tasks you find your self reinventing frequently, eggs could save you a lot of trouble. If you create a little library of common tasks that you use, you could bundle them as an egg. And if you do that, you’ve not only saved yourself time in writing code by reusing it, but you’ve also made it easy to install on multiple machines.

Creating Python eggs is an incredibly simple process. It really involves just four steps:

1. Install setuptools.

2. Create the files you want to be in your egg.

3. Create a setup.py file.

4. Run.

python setup.py bdist_egg

We already have setuptools installed, so we’ll go ahead and create the files we want in our egg:

$ cd /tmp
$ mkdir egg-example
$ cd egg-example
$ touch hello-egg.py

In this case, it will only contain an empty Python module named hello-egg.py.

Next, create the simplest possible setup.py file:

from setuptools import setup, find_packages
setup(
name = "HelloWorld",
version = "0.1",
packages = find_packages(),
)

Now, we can create the egg:

$ python setup.py bdist_egg
running bdist_egg
running egg_info
creating HelloWorld.egg-info
writing HelloWorld.egg-info/PKG-INFO
writing top-level names to HelloWorld.egg-info/top_level.txt
writing dependency_links to HelloWorld.egg-info/dependency_links.txt
writing manifest file 'HelloWorld.egg-info/SOURCES.txt'
reading manifest file 'HelloWorld.egg-info/SOURCES.txt'
writing manifest file 'HelloWorld.egg-info/SOURCES.txt'
installing library code to build/bdist.macosx-10.5-i386/egg
running install_lib
warning: install_lib: 'build/lib' does not exist -- no Python modules to install
creating build
creating build/bdist.macosx-10.5-i386
creating build/bdist.macosx-10.5-i386/egg
creating build/bdist.macosx-10.5-i386/egg/EGG-INFO
copying HelloWorld.egg-info/PKG-INFO -> build/bdist.macosx-10.5-i386/egg/EGG-INFO
copying HelloWorld.egg-info/SOURCES.txt -> build/bdist.macosx-10.5-i386/egg/EGG-INFO
copying HelloWorld.egg-info/dependency_links.txt -> build/bdist.macosx-10.5-i386/egg/EGG-INFO
copying HelloWorld.egg-info/top_level.txt -> build/bdist.macosx-10.5-i386/egg/EGG-INFO
zip_safe flag not set; analyzing archive contents...
creating dist
creating 'dist/HelloWorld-0.1-py2.5.egg' and adding 'build/bdist.macosx-10.5-i386/egg' to it
removing 'build/bdist.macosx-10.5-i386/egg' (and everything under it)
$ ll
total 8
drwxr-xr-x  6 ngift  wheel  204 Mar 10 06:53 HelloWorld.egg-info
drwxr-xr-x  3 ngift  wheel  102 Mar 10 06:53 build
drwxr-xr-x  3 ngift  wheel  102 Mar 10 06:53 dist
-rw-r--r--  1 ngift  wheel    0 Mar 10 06:50 hello-egg.py
-rw-r--r--  1 ngift  wheel  131 Mar 10 06:52 setup.py

Install the egg:

$ sudo easy_install HelloWorld-0.1-py2.5.egg
sudo easy_install HelloWorld-0.1-py2.5.egg
Password:
Processing HelloWorld-0.1-py2.5.egg
Removing /Library/Python/2.5/site-packages/HelloWorld-0.1-py2.5.egg
Copying HelloWorld-0.1-py2.5.egg to /Library/Python/2.5/site-packages
Adding HelloWorld 0.1 to easy-install.pth file

Installed /Library/Python/2.5/site-packages/HelloWorld-0.1-py2.5.egg
Processing dependencies for HelloWorld==0.1
Finished processing dependencies for HelloWorld==0.1

As you can see, creating an egg is extremly simple. Because this egg was really a blank file, though, we’ll create a Python script and go into building an egg in a little more detail.

Here is a very simple Python script that shows the files in a directory that are symlinks, where their corresponding real file is, and whether the real file exists or not:

#!/usr/bin/env python

import os
import sys

def get_dir_tuple(filename, directory):
abspath = os.path.join(directory, filename)
realpath = os.path.realpath(abspath)
exists = os.path.exists(abspath)
return (filename, realpath, exists)

def get_links(directory):
file_list = [get_dir_tuple(f, directory) for f in os.listdir(directory)
if os.path.islink(os.path.join(directory, f))]
return file_list

def main():
if not len(sys.argv) == 2:
print 'USAGE: %s directory' % sys.argv[0]
sys.exit(1)
directory = sys.argv[1]
print get_links(directory)

if __name__ == '__main__':
main()

Next, we’ll create a setup.py that uses setuptools. This is another minimal setup.py file as in our previous example:

from setuptools import setup, find_packages
setup(
name = "symlinkator",
version = "0.1",
packages = find_packages(),
entry_points = {
'console_scripts': [
'linkator = symlinkator.symlinkator:main',
],
},
)

This declares that name of the package is “symlinkator”, that it is at version 0.1, and that setuptools will try to find any appropriate Python files to include. Just ignore the entry_points section for the moment.

Now, we’ll build the egg by running python setup.py bdist_egg:

$ python setup.py bdist_egg
running bdist_egg
running egg_info
creating symlinkator.egg-info
writing symlinkator.egg-info/PKG-INFO
writing top-level names to symlinkator.egg-info/top_level.txt
writing dependency_links to symlinkator.egg-info/dependency_links.txt
writing manifest file 'symlinkator.egg-info/SOURCES.txt'
writing manifest file 'symlinkator.egg-info/SOURCES.txt'
installing library code to build/bdist.linux-x86_64/egg
running install_lib
warning: install_lib: 'build/lib' does not exist -- no Python modules to install
creating build
creating build/bdist.linux-x86_64
creating build/bdist.linux-x86_64/egg
creating build/bdist.linux-x86_64/egg/EGG-INFO
copying symlinkator.egg-info/PKG-INFO -> build/bdist.linux-x86_64/egg/EGG-INFO
copying symlinkator.egg-info/SOURCES.txt -> build/bdist.linux-x86_64/egg/EGG-INFO
copying symlinkator.egg-info/dependency_links.txt -> build/bdist.linux-x86_64/egg/EGG-INFO
copying symlinkator.egg-info/top_level.txt -> build/bdist.linux-x86_64/egg/EGG-INFO
zip_safe flag not set; analyzing archive contents...
creating dist
creating 'dist/symlinkator-0.1-py2.5.egg' and adding 'build/bdist.linux-x86_64/egg' to it
removing 'build/bdist.linux-x86_64/egg' (and everything under it)

Verify the egg contents. Let’s go into the dist directory that was created and verify there is an egg located in there:

$ ls -l dist
total 4
-rw-r--r-- 1 jmjones jmjones 825 2008-05-03 15:34 symlinkator-0.1-py2.5.egg

Now, we’ll install the egg:

$ easy_install dist/symlinkator-0.1-py2.5.egg
Processing symlinkator-0.1-py2.5.egg
....
Processing dependencies for symlinkator==0.1
Finished processing dependencies for symlinkator==0.1

Next, let’s fire up IPython, import, and use our module:

In [1]: from symlinkator.symlinkator import get_links

In [2]: get_links('/home/jmjones/logs/')
Out[2]: [('fetchmail.log.old', '/home/jmjones/logs/fetchmail.log.3', False),
('fetchmail.log', '/home/jmjones/logs/fetchmail.log.0', True)]

Just in case you’re interested, here is the directory that we ran the get_links() function on:

$ ls -l ~/logs/
total 0
lrwxrwxrwx 1 jmjones jmjones 15 2008-05-03 15:11 fetchmail.log -> fetchmail.log.0
-rw-r--r-- 1 jmjones jmjones  0 2008-05-03 15:09 fetchmail.log.0
-rw-r--r-- 1 jmjones jmjones  0 2008-05-03 15:09 fetchmail.log.1
lrwxrwxrwx 1 jmjones jmjones 15 2008-05-03 15:11 fetchmail.log.old -> fetchmail.log.3

Entry Points and Console Scripts

From the setuptools documentation page:

Entry points are used to support dynamic discovery of services or plugins provided by a project. See Dynamic Discovery of Services and Plugins for details and examples of the format of this argument. In addition, this keyword is used to support Automatic Script Creation.

The only kinds of entry points that we’ll cover in this book are the console script variety. setuptools will automatically create a console script for you given just a couple of pieces of information that you place in your setup.py. Here is the relevant section from the setup.py in the previous example:

entry_points = {
'console_scripts': [
'linkator = symlinkator.symlinkator:main',
],
},

In this example, we specified that we wanted to have a script named "linkator" and that when the script was executed, we wanted it to call the main() function in the symlinkator.symlinkator module. When we installed the egg, this linkator script was placed in the same directory with our python binary:

#!/home/jmjones/local/python/scratch/bin/python
# EASY-INSTALL-ENTRY-SCRIPT: 'symlinkator==0.1','console_scripts','linkator'
__requires__ = 'symlinkator==0.1'
import sys
from pkg_resources import load_entry_point

sys.exit(
load_entry_point('symlinkator==0.1', 'console_scripts', 'linkator')()
)

Everything that you see was generated by setuptools. It’s really not important to understand everything that’s in this script. Actually, it’s probably not important at all to understand anything in this script. The important thing to know is that when you define a console_scripts entry point in your setup.py, setuptools will create a script that calls your code into the place that you designated. And here is what happens when we call this script in a comparable manner to calling it in a previous example:

$ linkator ~/logs/
[('fetchmail.log.old', '/home/jmjones/logs/fetchmail.log.3', False),
('fetchmail.log', '/home/jmjones/logs/fetchmail.log.0', True)]

There are some complex aspects to understand about entry points, but on a very high level, it is only important to know that you can use an entry point to “install” your script as a command-line tool in the user’s path. In order to do this, you only need to follow the syntax listed above and define a function that runs your command-line tool.

Registering a Package with the Python Package Index

If you write a cool tool or useful module, naturally, you want to share it with other people. This is one of the most enjoyable parts of open source software development. Thankfully, it is a relatively simple process to upload a package to the Python Package Index.

The process is only slightly different from creating an egg. Two things to pay attention to are to remember to include a ReST, reStructuredText, formatted description in the long_description, and to provide a download_url value. We talked about ReST formatting in Chapter 4.

Although we discussed ReST formatting earlier, we should emphasize here that it is a good idea to format your documentation as ReST because it will be converted to HTML when it is uploaded to the cheeseshop. You can use the tool Aaron Hillegass created, ReSTless, to preview the formatted text to insure it is properly formatted while you preview it. One caveat to look out for is to make sure that you properly format your ReST. If you do not have properly formatted ReST, the text will display as plain text, and not HTML, when you upload your documentation.

See Example 9-2 for a look at a working setup.py for a command-line tool and library that Noah created.

Example 9-2. Sample setup.py for upload to Python Package Index

#!/usr/bin/env python

# liten 0.1.4.2 -- deduplication command-line tool
#
# Author: Noah Gift
try:
from setuptools import setup, find_packages
except ImportError:
from ez_setup import use_setuptools
use_setuptools()
from setuptools import setup, find_packages
import os,sys

version = '0.1.4.2'
f = open(os.path.join(os.path.dirname(__file__), 'docs', 'index.txt'))
long_description = f.read().strip()
f.close()

setup(

name='liten',
version='0.1.4.2',
description='a de-duplication command line tool',
long_description=long_description,
classifiers=[
'Development Status :: 4 - Beta',
'Intended Audience :: Developers',
'License :: OSI Approved :: MIT License',
],
author='Noah Gift',
author_email='noah.gift@gmail.com',
url='http://pypi.python.org/pypi/liten',
download_url="http://code.google.com/p/liten/downloads/list",
license='MIT',
py_modules=['virtualenv'],
zip_safe=False,
py_modules=['liten'],
entry_points="""
[console_scripts]
liten = liten:main
""",
)

Using this setup.py file, we can now “automatically” register a package with the Python Package Index by issuing this command:

$ python setup.py register
running register
running egg_info
writing liten.egg-info/PKG-INFO
writing top-level names to liten.egg-info/top_level.txt
writing dependency_links to liten.egg-info/dependency_links.txt
writing entry points to liten.egg-info/entry_points.txt
writing manifest file 'liten.egg-info/SOURCES.txt'
Using PyPI login from /Users/ngift/.pypirc
Server response (200): OK

This setup.py adds some additional fields compared to the symlinkator example. Some of the additional fields include description, long_description, classifiers, author, and download_url. The entry point, as we discussed earlier, allows the tool to be run from the command line and installed into the default scripts directory.

The download_url is critical because it tells easy_install where to search for your package. You can include a link to a page and easy_install is smart enough to find the package or egg, but you can also explicitly create the link to an egg you created.

The long_description reuses documentation that exists in a /doc relative directory that was created with an index.txt file in it. The index.txt file is formatted as ReST, and then the setup.py script reads that information in, and puts it into the field as it is registered with the Python Package Index.

Where Can I Learn More About …

The following are important resources:

Easy install

http://peak.telecommunity.com/DevCenter/EasyInstall

Python eggs

http://peak.telecommunity.com/DevCenter/PythonEggs

The setuptools module

http://peak.telecommunity.com/DevCenter/setuptools

The package resources module

http://peak.telecommunity.com/DevCenter/PkgResources

Architectural overview of pkg_resources and Python eggs in general

http://mail.python.org/pipermail/distutils-sig/2005-June/004652.html

And don’t forget the Python mailing list at http://mail.python.org/pipermail/distutils-sig/.

Distutils

As of the time of this writing, setuptools is the preferred way of creating packages and distributing them for many people, and it seems possible that parts of the setuptools library will make it into the standard library. That being said, it is still important to know how the distutils package works, what setuptools enhances, and what it doesn’t.

When distutils has been used to create a package for distribution, the typical way to install the package will be to run:

python setup.py
install

Regarding building packages for distribution, we will be covering four topics:

- How to write a setup script, which is a setup.py file

- Basic configuration options in the setup.py file

- How to build a source distribution

- Creating binaries such as rpms, Solaris, pkgtool, and HP-UX swinstall

The best way to demonstrate how distutils works is to just jump in feet first.

Step 1: create some code. Let’s use this simple script as an example to distribute:

#!/usr/bin/env python
#A simple python script we will package
#Distutils Example. Version 0.1

class DistutilsClass(object):
"""This class prints out a statement about itself."""
def __init__(self):
print "Hello, I am a distutils distributed script." \
"All I do is print this message."

if __name__ == '__main__':
DistutilsClass()

Step 2: make a setup.py in the same directory as your script.

#Installer for distutils example script

from distutils.core import setup

setup(name="distutils_example",
version="0.1",
description="A Completely Useless Script That Prints",
author="Joe Blow",
author_email = "joe.blow@pyatl.org",
url = "http://www.pyatl.org")

Notice that we’re passing setup() several keyword arguments that can later identify this package by this metadata. Please note this is a very simple example, as there are many more options, such as dealing with multiple dependencies, etc. We won’t get into more advanced configurations, but we do recommend reading more about them in the official Python online documentation.

Step 3: create a distribution.

Now that we have a very basic setup.py script, we can create a source distribution package very easily by running this command in the same directory as your script, README and setup.py file:

python setup.py sdist

You will get the following output:

running sdist
warning: sdist: manifest template 'MANIFEST.in' does not exist
(using default file list)
writing manifest file 'MANIFEST'
creating distutils_example-0.1
making hard links in distutils_example-0.1...
hard linking README.txt distutils_example-0.1
hard linking setup.py distutils_example-0.1
creating dist
tar -cf dist/distutils_example-0.1.tar distutils_example-0.1
gzip -f9 dist/distutils_example-0.1.tar
removing 'distutils_example-0.1' (and everything under it)

As you can tell from the output, now all someone has to do is unpack and install using:

python setup.py install

If you would like to build binaries, here are a few examples. Note that they rely on the underlying operating system to do the heavy lifting, so you cannot build an rpm on, say, OS X. With the plethora of virtualization products around, though, this shouldn’t be a problem for you. Just keep a few virtual machines laying around that you can activate when you need to do builds.

To build an rpm:

python setup.py bdist_rpm

To build a Solaris pkgtool:

python setup.py bdist_pkgtool

To build a HP-UX swinstall:

python setup.py bdist_sdux

Finally, when you distribute the package you make, you may want to customize the installation directory when you get around to installing your package. Normally, the build and installation processes happen all at once, but you may want to select a customized build direction like the following:

python setup.py build --build-base=/mnt/python_src/ascript.py

When you actually run the install command, it copies everything in the build directory to an installation directory. By default, the installation directory is the site-packages directory in the Python environment in which you execute the command, but you can also specify a custom installation directory, such as an NFS mount point, as shown in the previous example.

Buildout

Buildout is a tool created by Jim Fulton of Zope Corporation to manage “building out” new applications. These applications can be Python programs or other programs, such as Apache. One of the main goals of Buildout is to allow buildouts to become repeatable across platforms. One of the author’s first experiences using Buildout was to deploy a Plone 3.x site. Since then, he realized this was just the tip of the iceberg.

Buildout is one of the more buzz-worthy new package management tools that Python has to offer, as it allows complex applications that have complex dependencies to bootstrap themselves if they have a bootstrap.py and a config file. In the coming sections, we will separate our discussion into two pieces: using Buildout and developing with Buildout. We would also recommend you read the Buildout manual at http://pypi.python.org/pypi/zc.buildout, as it is an invaluable resource for the latest information about Buildout. In fact, this documentation is about as comprehensive as it gets for Buildout, and is a must-read for any Buildout user.

Celebrity Profile: Buildout: Jim Fulton

Jim Fulton is the creator and one of the maintainers of the Zope Object Database. Jim is also one of the creators of the Zope Object Publishing Environment and the CTO at Zope Corporation.

Using Buildout

Although many people that deal with Zope technologies are aware of Buildout, it has been a secret for the rest of the Python world. Buildout is the recommended mechanism by which Plone is deployed. If you are not familiar with Plone, it is an enterprise-grade content management system with a tremendous development community behind it. Plone used to be extremely complex to install until the invention of Buildout. Now Buildout makes Plone installation trivial.

What many people do not know is that you can use Buildout to manage a Python environment as well. Buildout is a very clever piece of software because it requires only two things:

- The latest copy of bootstrap.py. You can always download it here: http://svn.zope.org/*checkout*/zc.buildout/trunk/bootstrap/bootstrap.py.

- A buildout.cfg file, with the names of the “recipes” or “eggs” to install.

The best way to demonstrate Buildout is to use it to install something. Noah has written a de-duplication command-line tool called liten that is available from the central Python repository, PyPI. We are going to use Buildout to “bootstrap” a Python environment to run this tool.

Step 1: download the bootstrap.py script.

mkdir -p ~/src/buildout_demo
curl http://svn.zope.org/*checkout*/zc.buildout/trunk/
bootstrap/bootstrap.py > ~/src/buildout_demo/bootstrap.py

Step 2: define a simple buildout.cfg. As we stated earlier, Buildout requires a buildout.cfg file to be present. If we tried to run the bootstrap.py script without the buildout.cfg file, we would get the output below:

$ python bootstrap.py
While:
Initializing.
Error: Couldn't open /Users/ngift/src/buildout_demo/buildout.cfg

For example, we will create the configuration file shown in Example 9-3.

Example 9-3. Example Buildout configuration file

[buildout]
parts = mypython
[mypython]
recipe = zc.recipe.egg
interpreter = mypython
eggs = liten

If we save that file as buildout.cfg and then run the bootstrap.py script again, we will get the output shown in Example 9-4.

Example 9-4. Poking the buildout environment with a stick

$ python bootstrap.py
Creating directory '/Users/ngift/src/buildout_demo/bin'.
Creating directory '/Users/ngift/src/buildout_demo/parts'.
Creating directory '/Users/ngift/src/buildout_demo/eggs'.
Creating directory '/Users/ngift/src/buildout_demo/develop-eggs'.
Generated script '/Users/ngift/src/buildout_demo/bin/buildout'.

If we poke around these newly created directories, we will find executables, including a custom Python interpreter inside of the bin directory:

$ ls -l bin
total 24
-rwxr-xr-x  1 ngift  staff  362 Mar  4 22:17 buildout
-rwxr-xr-x  1 ngift  staff  651 Mar  4 22:23 mypython

Now that we finally have a Buildout tool installed, we can run it and our egg we defined earlier will work. See Example 9-5.

Example 9-5. Running Buildout and testing installation

$ bin/buildout
Getting distribution for 'zc.recipe.egg'.
Got zc.recipe.egg 1.0.0.
Installing mypython.
Getting distribution for 'liten'.
Got liten 0.1.3.
Generated script '/Users/ngift/src/buildout_demo/bin/liten'.
Generated interpreter '/Users/ngift/src/buildout_demo/bin/mypython'.
$ bin/mypython

>>>
$ ls -l bin
total 24
-rwxr-xr-x  1 ngift  staff  362 Mar  4 22:17 buildout
-rwxr-xr-x  1 ngift  staff  258 Mar  4 22:23 liten
-rwxr-xr-x  1 ngift  staff  651 Mar  4 22:23 mypython
$ bin/mypython

>>> import liten

Finally, because the “liten” was created with an entry point, which we discussed earlier in this chapter, the egg is able to automatically install a console script in addition to the module inside of the local Buildout bin directory. If we take a look at that, we will see the following output:

$ bin/liten
Usage: liten [starting directory] [options]

A command-line tool for detecting duplicates using md5 checksums.

Options:
--version             show program's version number and exit
-h, --help            show this help message and exit
-c, --config          Path to read in config file
-s SIZE, --size=SIZE  File Size Example:  10bytes, 10KB, 10MB,10GB,10TB, or
plain number defaults to MB (1 = 1MB)
-q, --quiet           Suppresses all STDOUT.
-r REPORT, --report=REPORT
Path to store duplication report. Default CWD
-t, --test            Runs doctest.
$ pwd
/Users/ngift/src/buildout_demo

That is a very powerful and simple example of how Buildout can be used to create an isolated environment and automatically deploy the correct dependencies for a project or environment. To really show the power of Buildout, though, we should look at another aspect of Buildout. Buildout has complete “control” of the directory in which it is run, and everytime that Buildout runs, it reads the buildout.cfg file to look for instructions. This means that if we remove the egg we listed, it will effectively remove the command-line tool and the library. See Example 9-6

Example 9-6. Stripped-down Buildout configuration file

[buildout]
parts =

Now, here is a rerunning of Buildout with the egg and interpreter removed. Note that Buildout has quite a few command-line options, and in this case, we are selecting -N, which will only modify changed files. Normally, Buildout will rebuild everything from scratch each time it is rerun.

$ bin/buildout -N
Uninstalling mypython.

When we look inside of the bin directory, the interpreter and the command-line tool are gone. The only item left is the actual Buildout command-line tool:

$ ls -l bin/
total 8
-rwxr-xr-x  1 ngift  staff  362 Mar  4 22:17 buildout

If we look inside of the eggs directory, though, the egg is installed but not activated. But we couldn’t run it, as it doesn’t have an interpreter:

$ ls -l eggs
total 640
drwxr-xr-x  7 ngift  staff     238 Mar  4 22:54 liten-0.1.3-py2.5.egg
-rw-r--r--  1 ngift  staff  324858 Feb 16 23:47 setuptools-0.6c8-py2.5.egg
drwxr-xr-x  5 ngift  staff     170 Mar  4 22:17 zc.buildout-1.0.0-py2.5.egg
drwxr-xr-x  4 ngift  staff     136 Mar  4 22:23 zc.recipe.egg-1.0.0-py2.5.egg

Developing with Buildout

Now that we have gone through a simple example of creating and destroying a Buildout-controlled environment, we can now go a step further and create a Buildout-controlled development environment.

One of the most common scenarios where Buildout is used is quite simple. A developer may work on an individual package that lives in version control. The developer then checks out her project into a top-level src directory. Inside of her src directory, she would then run Buildout as described earlier, with an example configuration file such as this:

[buildout]
develop = .
parts = test

[python]
recipe = zc.recipe.egg
interpreter = python
eggs = ${config:mypkgs}

[scripts]
recipe = zc.recipe.egg:scripts
eggs = ${config:mypkgs}

[test]
recipe = zc.recipe.testrunner
eggs = ${config:mypkgs}

virtualenv

“virtualenv is a tool to create isolated Python environments,” according to the documentation on the Python Package Index page. The basic problem that virtualenv solves is to eliminate problems with conflicting packages. Often, one tool will require one version of a package, and another tool will require a different version of a package. This can create a dangerous scenario in which a production web application could be broken because someone “accidentally” modifies the global site-packages directory to run a different tool by upgrading a package.

Alternately, a developer may not have write access to a global site-packages directory, and can use virtualenv to keep a separate virtualenv that is isolated from the system Python. virtualenv is a great way to eliminate problems before they start, as it allows for the creation of new sandbox that can be, optionally, completely isolated from the global site-packages directory.

virtualenv can also “bootstrap” a virtual environment by allowing a developer to pre-populate a virtual environment with a custom environment. This is very similar to what Buildout does, although Buildout uses a declarative config file. We should note that Buildout and virtualenv both extensively use setuptools, of which Phillip J. Eby is the current maintainer.

Celebrity Profile: Virtualenv: Ian Bicking

Ian Bicking is responsible for so many Python packages it is often hard to keep track. He has written Webob, which is part of Google App Engine, Paste, virtualenv, SQLObject, and much more. You can read his famous blog here: http://blog.ianbicking.org/.

So, how do you use virtualenv? The most straightforward approach is to use easy_install to install virtualenv:

sudo easy_install virtualenv

If you plan on using virtualenv with only one version of Python, this approach works quite well. If you have several versions of Python installed on your machine, such as Python 2.4, Python 2.5, Python 2.6, and perhaps Python 3000, and they share the same main bin directory, such as /usr/bin, then an alternate approach could work best, as only one virtualenv script can be installed at a time in the same scripts directory.

One way to create several virtualenv scripts that work with multiple versions of Python is to just download the latest version of virtualenv and create an alias to each Python version. Here are the steps to do that:

1. curl http://svn.colorstudy.com/virtualenv/trunk/virtualenv.py > virtualenv.py

2. sudo cp virtualenv.py /usr/local/bin/virtualenv.py

3. Create two aliases in your Bash or zsh:

alias virtualenv-py24="/usr/bin/python2.4 /usr/local/bin/virtualenv.py"
alias virtualenv-py25="/usr/bin/python2.5 /usr/local/bin/virtualenv.py"
alias virtualenv-py26="/usr/bin/python2.6 /usr/local/bin/virtualenv.py"

With a multi-script environment behind us, we can go ahead and create several virtualenv containers for each version of Python we need to deal with. Here is an example of what that looks like.

Creating a Python2.4 virtual environment:

$ virtualenv-py24 /tmp/sandbox/py24ENV
New python executable in /tmp/sandbox/py24ENV/bin/python
Installing setuptools.................done.
$ /tmp/sandbox/py24ENV/bin/python
Python 2.4.4 (#1, Dec 24 2007, 15:02:49)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
$ ls /tmp/sandbox/py24ENV/
bin/  lib/
$ ls /tmp/sandbox/py24ENV/bin/
activate           easy_install*      easy_install-2.4*  python*            python2.4@

Creating a Python2.5 virtual environment:

$ virtualenv-py25 /tmp/sandbox/py25ENV
New python executable in /tmp/sandbox/py25ENV/bin/python
Installing setuptools..........................done.
$ /tmp/sandbox/py25ENV/bin/python
Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
$ ls /tmp/sandbox/py25ENV/
bin/  lib/
$ ls /tmp/sandbox/py25ENV/bin/
activate           easy_install*      easy_install-2.5*  python*            python2.5@

If we look at the output of the commands, we can observe that virtualenv creates a relative bin directory and a relative lib directory. Inside the bin directory is a python interpretor that uses the lib directory as its own local site-packages directory. Another great feature is the prepopulated easy_install script that allows an easy_install of packages into the virtual environment.

Finally, it is important to take note that there are two ways to work with the virtual environment you create. You can always explicitly call the full path to a virtual environment:

$ /src/virtualenv-py24/bin/python2.4

Alternately, you can use the activate script located in the bin directory of your virtualenv to set your environment to use that “sandbox” without typing in a full path. This is an optional tool you can use, but it is not necessary, as you can always type in the full path to your virtualenv. Doug Hellmann, one of the reviewers for the book, created a clever hack you can find here: http://www.doughellmann.com/projects/virtualenvwrapper/. It uses activate with a Bash wrapper menu that let’s you select which sandbox to work on at a time.

Creating a Custom Bootstrapped Virtual Environment

The release of virtualenv 1.0, which is current as of the writing of this book, includes support to create bootstrap scripts for virtualenv environments. One method of doing that is to call virutalenv.create_bootstrap_script(text). What this does is create a bootstrap script, which is like virtualenv, but with additional features to extend option parsing, adjust_options, and use after_install hooks.

Let’s go over how easy it is to create a custom bootstrap script that will install virtualenv and a custom set of eggs into a new environment. Going back to the liten package as an example, we can use virtualenv to create a brand new virtual environment and prepopulate it with liten. Example 9-7 shows exactly how to create a custom bootstrap script that installs liten.

Example 9-7. Bootstrap creator example

import virtualenv, textwrap
output = virtualenv.create_bootstrap_script(textwrap.dedent("""
import os, subprocess
def after_install(options, home_dir):
etc = join(home_dir, 'etc')
if not os.path.exists(etc):
os.makedirs(etc)
subprocess.call([join(home_dir, 'bin', 'easy_install'),
'liten'])
"""))
f = open('liten-bootstrap.py', 'w').write(output)

This example was adapted from the virtualenv documentation, and the last two lines are the important lines to pay attention to:

subprocess.call([join(home_dir, 'bin', 'easy_install'),
'liten'])
"""))
f = open('liten-bootstrap.py', 'w').write(output)

In a nutshell, this tells our after_install function to write a new file in the current working directory called liten-bootstrap.py and then include a custom easy_install of the module liten. It is important to note that this snippet of code will create a bootstrap.py, and then this bootstrap.py file will need to be run. After running this script, we will have a liten-bootstrap.py file that can be distributed to a developer or end user.

If we run liten-bootstrap.py without any options, we get the following output:

$ python liten-bootstrap.py
You must provide a DEST_DIR
Usage: liten-bootstrap.py [OPTIONS] DEST_DIR

Options:
--version           show program's version number and exit
-h, --help          show this help message and exit
-v, --verbose       Increase verbosity
-q, --quiet         Decrease verbosity
--clear             Clear out the non-root install and start from scratch
--no-site-packages  Don't give access to the global site-packages dir to the
virtual environment

When we actually run this tool with a destination directory, we get this output:

$ python liten-bootstrap.py  --no-site-packages /tmp/liten-ENV
New python executable in /tmp/liten-ENV/bin/python
Installing setuptools..........................done.
Searching for liten
Best match: liten 0.1.3
Processing liten-0.1.3-py2.5.egg
Adding liten 0.1.3 to easy-install.pth file
Installing liten script to /tmp/liten-ENV/bin

Using /Library/Python/2.5/site-packages/liten-0.1.3-py2.5.egg
Processing dependencies for liten
Finished processing dependencies for liten

Our clever bootstrap script automatically creates an environment with our module. So, if we run the full path to the virtualenv on our the liten tool, we get the following:

$ /tmp/liten-ENV/bin/liten
Usage: liten [starting directory] [options]

A command-line tool for detecting duplicates using md5 checksums.

Options:
--version             show program's version number and exit
-h, --help            show this help message and exit
-c, --config          Path to read in config file
-s SIZE, --size=SIZE  File Size Example:  10bytes, 10KB, 10MB,10GB,10TB, or
plain number defaults to MB (1 = 1MB)
-q, --quiet           Suppresses all STDOUT.
-r REPORT, --report=REPORT
Path to store duplication report. Default CWD
-t, --test            Runs doctest.

This is a great trick to know about, as it allows a completely isolated and bootstrapped virtual environment.

We hope it is clear from this section on virtualenv that one of its core strengths is how simple it is to use and understand. More than anything, virtualenv respects the sacred rule of KISS, and that alone is reason enough to consider using it to help manage isolated development environments. Be sure to visit the virtualenv mailing list at http://groups.google.com/group/python-virtualenv/ if you have more questions about it.

EPM Package Manager

Because EPM creates native packages for each operating system, it will need to be installed on each “build” system. Due to the incredible advances in virtualization in the past few years, it is trivial to get a few build virtual machines set up. I created a small cluster of virtual machines running in the equivalent of Red Hat run level init 3, with a minimal allocation of RAM, to test out the code examples in this book.

A coworker and contributor to EPM first introduced me to what EPM can do. I was looking for a tool that would allow me to create operating system-specific software packages for a tool I had developed, and he mentioned EPM. After reading through some of the online documentation at http://www.epmhome.org/epm-book.html, I was pleasantly suprised at how painless the process was. In this section, we are going to walk through the steps involved to create a software package ready for installation on multiple platforms: Ubuntu, OS X, Red Hat, Solaris, and FreeBSD. These steps can easily be applied to other systems that EPM supports, such as AIX or HP-UX.

Before we jump into the tutorial, here is a little background on EPM. According to the official documentation for EPM, it was designed from the beginning to build a binary software distribution using a common software specification format. Because of this design goal, the same distribution files work for all operating systems and all distribution formats.

EPM Package Manager Requirements and Installation

EPM requires only a Bourne type shell, a C compiler, the make program and gzip. These utilities are easily obtained on almost every *nix system, if they are not already installed. After downloading the source for EPM, it is necessary to run the following:

./configure
make
make install

Creating a Hello World Command-Line Tool to Distribute

To get started with building packages for almost every *nix operating system made, we need something to actually distribute. In the spirit of tradition, we are going to create a simple command-line tool called hello_epm.py. See Example 9-8.

Example 9-8. Hello EPM command-line tool

#!/usr/bin/env python

import optparse

def main():
p = optparse.OptionParser()
p.add_option('--os', '-o', default="*NIX")
options, arguments = p.parse_args()
print 'Hello EPM, I like to make packages on %s' % options.os

if __name__ == '__main__':
main()

If we run this tool, we get the following output:

$ python hello_epm.py
Hello EPM, I like to make packages on *NIX

$ python hello_epm.py --os RedHat
Hello EPM, I like to make packages on RedHat

Creating Platform-Specific Packages with EPM

The “basics,” are so simple that you may wonder why you never used EPM to package cross-platform software before. EPM reads a “list” file(s) that describe your software package. Comments begin with a # character, directives begin with a % character, variables start with a $ character, and finally, file, directory, init script, and symlink lines start with a letter.

It is possible to create a generic cross-platform install script as well as platform-specific packages. We will focus on creating vendor package files. The next step to creating platform-specific packages is to create a manifest or “list” that describes our package. Example 9-9 is a template we used to create packages for our hello_epm command-line tool. It is general enough that you could get away with changing it slightly to create your own tools.

Example 9-9. “List” template for EPM

#EPM List File Can Be Used To Create Package For Any Of These Vendor Platforms
#epm -f format foo bar.list ENTER
#The format option can be one of the following keywords:

#aix - AIX software packages.
#bsd - FreeBSD, NetBSD, or OpenBSD software packages.
#depot or swinstall - HP-UX software packages.
#dpkg - Debian software packages.
#inst or tardist - IRIX software packages.
#native - "Native" software packages (RPM, INST, DEPOT, PKG, etc.) for the platform.
#osx - MacOS X software packages.
#pkg - Solaris software packages.
#portable - Portable software packages (default).
#rpm - Red Hat software packages.
#setld - Tru64 (setld) software packages.
#slackware - Slackware software packages.

# Product Information Section

%product hello_epm
%copyright 2008 Py4SA
%vendor O’Reilly
%license COPYING
%readme README
%description Command Line Hello World Tool
%version 0.1

# Autoconfiguration Variables

$prefix=/usr
$exec_prefix=/usr
$bindir=${exec_prefix}/bin
$datadir=/usr/share
$docdir=${datadir}/doc/
$libdir=/usr/lib
$mandir=/usr/share/man
$srcdir=.

# Executables

%system all
f 0555 root sys ${bindir}/hello_epm hello_epm.py

# Documentation

%subpackage documentation
f 0444 root sys ${docdir}/README $srcdir/README
f 0444 root sys ${docdir}/COPYING $srcdir/COPYING
f 0444 root sys ${docdir}/hello_epm.html $srcdir/doc/hello_epm.html

# Man pages

%subpackage man
%description Man pages for hello_epm
f 0444 root sys ${mandir}/man1/hello_epm.1 $srcdir/doc/hello_epm.man

If we examine this file, which we will call hello_epm.list, you will notice that we define the $srcdir variable as the current working directory. In order to create packages on any platform, we now just need to create the following in our current working directory: a README file, a COPYING file, a doc/hello_epm.html file, and a doc/hello_epm.man file, and our script hello_epm.py has to be in this same directory.

If we wanted to “cheat” for our hello_epm.py tool, and just place blank files in our packaging directory, we could do this:

$ pwd
/tmp/release/hello_epm
$ touch README
$ touch COPYING
$ mkdir doc
$ touch doc/hello_epm.html
$ touch doc/hello_epm.man

Looking inside of our directory, we have this layout:

$ ls -lR
total 16
-rw-r--r--  1 ngift  wheel     0 Mar 10 04:45 COPYING
-rw-r--r--  1 ngift  wheel     0 Mar 10 04:45 README
drwxr-xr-x  4 ngift  wheel   136 Mar 10 04:45 doc
-rw-r--r--  1 ngift  wheel  1495 Mar 10 04:44 hello_epm.list
-rw-r--r--@ 1 ngift  wheel   278 Mar 10 04:10 hello_epm.py

./doc:
total 0
-rw-r--r--  1 ngift  wheel  0 Mar 10 04:45 hello_epm.html
-rw-r--r--  1 ngift  wheel  0 Mar 10 04:45 hello_epm.man

Making the Package

Now, we have a directory with a “list” file that contains generic directives that will work on any platform EPM supports. All that is left is to run the epm -f command appended with what platform we are on and the name of our list file. Example 9-10 shows what it looks like on OS X.

Example 9-10. Creating a native OS X installer with EPM

$ epm -f osx hello_epm hello_epm.list
epm: Product names should only contain letters and numbers!
^C
$ epm -f osx helloEPM hello_epm.list
$ ll
total 16
-rw-r--r--  1 ngift  wheel     0 Mar 10 04:45 COPYING
-rw-r--r--  1 ngift  wheel     0 Mar 10 04:45 README
drwxr-xr-x  4 ngift  wheel   136 Mar 10 04:45 doc
-rw-r--r--  1 ngift  wheel  1495 Mar 10 04:44 hello_epm.list
-rw-r--r--@ 1 ngift  wheel   278 Mar 10 04:10 hello_epm.py
drwxrwxrwx  6 ngift  staff   204 Mar 10 04:52 macosx-10.5-intel

Notice the warning when the package name had an underscore in it. As a result, we renamed the package without an underscore and ran it again. It then creates a macosx-10.5-intel directory that contains the following.

$ ls -la macosx-10.5-intel
total 56
drwxrwxrwx  4 ngift  staff    136 Mar 10 04:54 .
drwxr-xr-x  8 ngift  wheel    272 Mar 10 04:54 ..
-rw-r--r--@ 1 ngift  staff  23329 Mar 10 04:54 helloEPM-0.1-macosx-10.5-intel.dmg
drwxr-xr-x  3 ngift  wheel    102 Mar 10 04:54 helloEPM.mpkg

This is convenient, as it makes both a .dmg image archive that is native to OS X and contains our installer and the native OS X installer.

If we run the installer, we will notice that OS X will install our blank man pages and documentation and show our blank license file. Finally, it places our tool exactly where we told it to and creates the custom name we gave it the following:

$ which hello_epm
/usr/bin/hello_epm
$ hello_epm
Hello EPM, I like to make packages on *NIX
$ hello_epm -h
Usage: hello_epm [options]

Options:
-h, --help      show this help message and exit
-o OS, --os=OS
$

EPM Summary: It Really Is That Easy

If we scp -r the /tmp/release/hello_epm to a Red Hat, Ubuntu, or Solaris machine, we can run the exact same command, except with the platform-specific name, and it will “just work.” In Chapter 8, we examined how to create a build farm using this technique so that you can instantly create cross-platform packages by running a script. Please note that all of this source code is available for download along with the example package created. You should be able to slightly modify it and create your own cross-platform packages in minutes.

There are quite a few additional advanced features that EPM has to offer, but going into those is beyond the scope of this book. If you are curious about creating packages that handle dependencies, run pre- and post-install scripts, etc., then you owe it to yourself to read EPM’s official documentation, which covers all of these scenarios and more.

Chapter 10. Processes and Concurrency

Introduction

Dealing with processes as a Unix/Linux systems administrator is a fact of life. You need to know about startup scripts, run levels, daemons, cron jobs, long-running processes, concurrency, and a host of other issues. Fortunately, Python makes dealing with processes quite easy. Since Python 2.4, Subprocess has been the one-stop shop module that allows you to spawn new processes and talk to standard input, standard output, and standard error. While talking to a processes is one aspect of dealing with processes, it is also import to understand how to deploy and manage long-running processes as well.

Subprocess

With Python 2.4 came subprocess, which takes the place of several older modules: os.system, os.spawn, os.popen, and popen2 commands. Subprocess is a revolutionary change for systems administrators and developers who need to deal with processes and “shelling out.” It is now a one-stop shop for many things dealing with processes and it may eventually include the ability to manage a flock of processes.

Subprocess might just be the single most important module for a systems administrator, as it is the unified API to “shelling out.” Subprocess is responsible for the following things in Python: spawning new processes connecting to standard input, connecting to standard output, connecting to error streams, and listening to return codes.

To whet your appetite, let’s use the KISS principle (Keep It Simple Stupid), and do the absolute simplest possible thing we can with Subprocess and make a trivial system call. See Example 10-1.

Example 10-1. Simplest possible use of Subprocess

In [4]: subprocess.call('df -k', shell=True)
Filesystem    1024-blocks     Used Available Capacity  Mounted on
/dev/disk0s2     97349872 80043824  17050048    83%    /
devfs                 106      106         0   100%    /dev
fdesc                   1        1         0   100%    /dev
map -hosts              0        0         0   100%    /net
map auto_home           0        0         0   100%    /home
Out[4]: 0

Using that same simple syntax it is possible to include shell variables as well. Example 10-2 is an example of finding out the summary of the space used in our home directory.

Example 10-2. Summary of disk usage

In [7]: subprocess.call('du -hs $HOME', shell=True)
28G    /Users/ngift
Out[7]: 0

One interesting trick to point out with Subprocess is the ability to suppress standard out. Many times, someone is just interested in running a system call, but is not concerned about the stdout. In these cases, it is often desirable to suprocess the stdout of subprocess.call. Fortunately, there is a very easy way to do this. See Example 10-3.

Example 10-3. Suppressing stdout of subprocess.call

In [3]: import subprocess

In [4]: ret = subprocess.call("ping -c 1 10.0.1.1",
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)

There are a few things to point out about these two examples and subprocess.call in general. You typically use subprocess.call when you do not care about the ouptut of the shell command and you just want it to run. If you need to capture the output of a command, then you will want to use subprocess.Popen. There is another sizable difference between subprocess.call and subprocess.Popen. Subprocess.call will block waiting for a response, while subprocess.Popen will not.

Using Return Codes with Subprocess

One interesting thing to note about subprocess.call is that you can use return codes to determine the success of your command. If you have experience with programming in C or Bash, you will be quite at home with return codes. The phrases “exit code” and “return code” are often used interchangeably to describe the status code of a system process.

Every process will have a return code when it exits, and the status of the return code can be used to determine what actions a program should take. Generally, if a program exits with a code of anything but zero, it is an error. The obvious use of a return code for a developer is to determine that if a process it needs to use return with an exit code of anything but zero, then it was a failure. The not-so-obvious use of return codes has many interesting possibilities. There are special return codes for a program not being found, a program not being executable, and a program being terminated by Ctrl-C. We will explore the use of these return codes in Python programs in this section.

Let’s look at a list of common return codes with special meaning:

0

Success

1

General errors

2

Misuse of shell built-ins

126

Command invoked cannot execute

127

Command not found

128

Invalid argument to exit

Fatal error signal “n”

130

Script terminated by Ctrl-C 255 Exit status out of range

The most useful scenario where this may come into play is with the use of return codes 0 and 1, which generally signifies success or failure of a command you just ran. Let’s take a look at some common examples of this with subprocess.call. See Example 10-4.

Example 10-4. Failure return code with subprocess.call

In [16]: subprocess.call("ls /foo", shell=True)
ls: /foo: No such file or directory
Out[16]: 1

Because that directory did not exist, we received a return code of 1 for failure. We can also capture the return code and use it to write conditional statements. See Example 10-5.

Example 10-5. Conditional statements based on return code true/false with subprocess.call

In [25]: ret = subprocess.call("ls /foo", shell=True)
ls: /foo: No such file or directory

In [26]: if ret == 0:
....:     print "success"
....: else:
....:     print "failure"
....:
....:
failure

Here is an example of a “command not found” return code, which is 127. This might be a useful way to write a tool that attempted to run several similar shell commands based on what was available. You might first try to run rsync, but if you get a return code of 127, then you would move on to scp -r. See Example 10-6.

Example 10-6. Conditional statements based on return code 127 with subprocess.call

In [28]: subprocess.call("rsync /foo /bar", shell=True)
/bin/sh: rsync: command not found
Out[28]: 127

Let’s take the previous example and make it less abstract. Often, when writing cross-platform code that needs to run on a variety of *nix boxes, you may find yourself in a situation in which you need to accomplish something that requires a different system program depending on which OS the program is run. HP-UX, AIX, Solars, FreeBSD, and Red Hat could each have a slightly different utility that does what you want. A program could listen to the return code of the first program it attemps to call via subprocess and if return code 127 is given, then the next command could be tried, etc.

Unfortunately, exit codes can vary from OS to OS, so if you are writing a cross-platform script, you may want to only rely a zero or nonzero exit code. To give you an example, this is an exit code on Solaris 10 for the exact same command we ran earlier on Red Hat Enterprise Linux 5:

ash-3.00# python
Python 2.4.4 (#1, Jan  9 2007, 23:31:33) [C] on sunos5
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess
>>> subprocess.call("rsync", shell=True)
/bin/sh: rsync: not found
1

We could still use a specific exit code, but we might first want to determine what the operating system is. After we have determined the operating system, then we could check for the platform-specific command’s existence. If you find yourself writing this type of code, then it is a good idea to become intimately familiar with the platform module. The process module is talked about in detail in Chapter 8, so you can refer to that chapter for more information.

Let’s look at Example 10-7 to see how to use the platform module interactively in IPython to determine what to pass to subprocess.call.

Example 10-7. Using platform and Subprocess module to determine command execution on Solaris 10

In [1]: import platform
In [2]: import subprocess
In [3]: platform?

Namespace:      Interactive
File:           /usr/lib/python2.4/platform.py
Docstring:
This module tries to retrieve as much platform-identifying data as
possible. It makes this information available via function APIs.

If called from the command line, it prints the platform
information concatenated as single string to stdout. The output
format is useable as part of a filename.

In [4]: if platform.system() == 'SunOS':
....:     print "yes"
....:
yes

In [5]: if platform.release() == '5.10':
....:     print "yes"
....:
yes

In [6]: if platform.system() == 'SunOS':
...:     ret = subprocess.call('cp /tmp/foo.txt /tmp/bar.txt', shell=True)
...:     if ret == 0:
...:         print "Success, the copy was made on %s %s " % (platform.system(),
platform.release())
...:
Success, the copy was made on SunOS 5.10

As you can see, using the platform module with subprocess.call can be an effective weapon in writing cross-platform code. Please refer to Chapter 8 for detailed information on using the platform module to write cross-platform *nix code. See Example 10-8.

Example 10-8. Capturing standard out with Subprocess

In [1]: import subprocess

In [2]: p = subprocess.Popen("df -h", shell=True, stdout=subprocess.PIPE)

In [3]: out = p.stdout.readlines()

In [4]: for line in out:
...:     print line.strip()
...:
...:
Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   78Gi   15Gi    85%    /
devfs          107Ki  107Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home

Note that readlines() returns a list with newline characters. We had to use line.strip() to remove the newlines. Subprocess also has the ability to communicate with stdin and stdout to create pipes. Here is a simple example of communicating to the standard input of a process. One interesting thing we can do with Python that would be horrendous in Bash is to create a piping factory. With a trivial few lines of code, we have arbitrary commands that get created and printed depending on the number of arguments. See Example 10-9.

Example 10-9. Subprocess piping factory

def multi(*args):
for cmd in args:
p = subprocess.Popen(cmd, shell=True, stdout = subprocess.PIPE)
out = p.stdout.read()
print out

Here is an example of this simple function in action:

In [28]: multi("df -h", "ls -l /tmp", "tail /var/log/system.log")
Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   80Gi   13Gi    87%    /
devfs          107Ki  107Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home

lrwxr-xr-x@ 1 root  admin  11 Nov 24 23:37 /tmp -> private/tmp

Feb 21 07:18:50 dhcp126 /usr/sbin/ocspd[65145]: starting
Feb 21 07:19:09 dhcp126 login[65151]: USER_PROCESS: 65151 ttys000
Feb 21 07:41:05 dhcp126 login[65197]: USER_PROCESS: 65197 ttys001
Feb 21 07:44:24 dhcp126 login[65229]: USER_PROCESS: 65229 ttys002

Due to the power of python and *args, we can arbitrarily run commands using our function as a factory. Each command gets popped off a list starting at the beginning due to the args.pop(0) syntax. If we used args.pop(), it would have popped the arguments in reverse order. Since this may be confusing, we can also write the same command factory function using a simple iteration for loop:

def multi(*args):
for cmd in args:
p = subprocess.Popen(cmd, shell=True, stdout = subprocess.PIPE)
out = p.stdout.read()
print out

Sysadmins quite frequently need to run a sequence of commands, so creating a module that simplifies this process could make quite a bit of sense. Let’s take a look at how we could do that with a simple example of inheritance. See Example 10-10.

Example 10-10. Creating a module around Subprocess

#!/usr/bin/env python
from subprocess import call
import time
import sys

"""Subtube is module that simplifies and automates some aspects of subprocess"""

class BaseArgs(object):
"""Base Argument Class that handles keyword argument parsing"""

def __init__(self, *args, **kwargs):
self.args = args
self.kwargs = kwargs
if self.kwargs.has_key("delay"):
self.delay = self.kwargs["delay"]
else:
self.delay = 0
if self.kwargs.has_key("verbose"):
self.verbose = self.kwargs["verbose"]
else:
self.verbose = False

def run (self):
"""You must implement a run method"""
raise NotImplementedError

class Runner(BaseArgs):
"""Simplifies subprocess call and runs call over a sequence of commands

Runner takes N positional arguments, and optionally:

[optional keyword parameters]
delay=1, for time delay in seconds
verbose=True for verbose output

Usage:

cmd = Runner("ls -l", "df -h", verbose=True, delay=3)
cmd.run()

"""
def run(self):
for cmd in self.args:
if self.verbose:
print "Running %s with delay=%s" % (cmd, self.delay)
time.sleep(self.delay)
call(cmd, shell=True)

Let’s take a look at how we would actually use our newly created module:

In [8]: from subtube import Runner
In [9]: r = Runner("df -h", "du -h /tmp")

In [10]: r.run()
Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   80Gi   13Gi    87%    /
devfs          108Ki  108Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
4.0K    /tmp

In [11]: r = Runner("df -h", "du -h /tmp", verbose=True)

In [12]: r.run()
Running df -h with delay=0
Filesystem      Size   Used  Avail Capacity  Mounted on
/dev/disk0s2    93Gi   80Gi   13Gi    87%    /
devfs          108Ki  108Ki    0Bi   100%    /dev
fdesc          1.0Ki  1.0Ki    0Bi   100%    /dev
map -hosts       0Bi    0Bi    0Bi   100%    /net
map auto_home    0Bi    0Bi    0Bi   100%    /home
Running du -h /tmp with delay=0
4.0K    /tmp

If we had ssh keys set up on all of our systems, we could easily code something like this:

machines = ['homer', 'marge','lisa', 'bart']
for machine in machines:
r = Runner("ssh " + machine + "df -h", "ssh " + machine + "du -h /tmp")
r.run()

This is a crude example of a remote command runner, but the idea is a good one, because the Red Hat Emerging Technology group has a project that facilitates wholesale scripting of large clusters of machines in Python. According to the Func website, “Here’s an interesting and contrived example—rebooting all systems that are running httpd. It’s contrived, yes, but it’s also very simple, thanks to Func.” We got into more detailed use of Func in Chapter 8, and we covered a home-brew “dispatching” system that works on any *nix platform.

results = fc.Client("*").service.status("httpd")
for (host, returns) in results.iteritems():
if returns == 0:
fc.Client(host).reboot.reboot()

Because subprocess is a unified API for “shelling out,” we can quite easily write to stdin. In Example 10-11, we will tell the word count utility to listen to standard in, and then we will write a string of characters for word count to process.

Example 10-11. Communicating to standard in with Subprocess

In [35]: p = subprocess.Popen("wc -c", shell=True, stdin=subprocess.PIPE)
In [36]: p.communicate("charactersinword")
16

The equivalent Bash is the following:

> echo charactersinword | wc -c

Let’s emulate Bash this time and redirect a file to the standard input. First, we need to write something to a file, so let’s do that with the new Python 2.6 syntax. Remember that if you are using Python 2.5, you must you the future import idiom:

In [5]: from __future__ import with_statement

In [6]: with open('temp.txt', 'w') as file:
...:     file.write('charactersinword')

We can reopen the file the classic way and read the file in as a string assigned to f:

In [7]: file = open('temp.txt')

In [8]: f = file.read()

Then we “redirect” the file output to our waiting process:

In [9]: p = subprocess.Popen("wc -c", shell=True, stdin=subprocess.PIPE)

In [10]: p.communicate(f)

In [11]: p.communicate(f)
16

In Bash, this would be equivalent to the following sequence of commands:

% echo charactersinword > temp.txt
% wc -c < temp.txt
16

Next, let’s take a look at actually piping several commands together as we would do in a typical shell scenario. Let’s take a look at a series of commands piped together in Bash and then the same series of commands piped together in Python. A realistic example that we encounter quite often is when dealing with logfiles. In Example 10-12, we are looking for the successful logins to the screensaver on a Macintosh laptop.

Example 10-12. Chaining commands with Subprocess

In Bash here is a simple chain:

[ngift@Macintosh-6][H:10014][J:0]> cat /etc/passwd | grep 0:0 | cut -d ':' -f 7
/bin/sh

Here is the same chain in Python:

In [7]: p1 = subprocess.Popen("cat /etc/passwd", shell=True, stdout=subprocess.PIPE)
In [8]: p2 = subprocess.Popen("grep 0:0", shell=True, stdin=p1.stdout, stdout=subprocess.PIPE)
In [9]: p3 = subprocess.Popen("cut -d ': ' -f 7", shell=True, stdin=p2.stdout,
stdout=subprocess.PIPE)

In [10]: print p3.stdout.read()
/bin/sh

Just because we can do something using subprocess piping, it doesn’t mean we have to. In the previous example, we grabbed the shell of the root user by piping a series of commands. Python has a built-in module that does this for us, so it is important to know that sometimes you don’t even need to use Subprocess; Python might have a built-in module that does the work for you. Many things you might want to do in the shell, such as tar or zip, Python can also do. It is always a good idea to see if Python has a built-in equivalent if you find yourself doing a very complex shell pipeline using Subprocess. See Example 10-13.

Example 10-13. Using pwd, the password database module instead of Subprocess

In [1]: import pwd

In [2]: pwd.getpwnam('root')
Out[2]: ('root', '********', 0, 0, 'System Administrator', '/var/root', '/bin/sh')

In [3]: shell = pwd.getpwnam('root')[-1]

In [4]: shell
Out[4]: '/bin/sh'

Subprocess can also handle sending input and receiving output at the same time, and also listening to standard error. Let’s take a look at an example of that.

Note that inside of IPython we use the “ed upper.py” feature to automatically switch to Vim when we want to write a snippet of code that may block such as the one in Example 10-14.

Example 10-14. Sending input and receiving output and standard error

import subprocess

p = subprocess.Popen("tr a-z A-Z", shell=True,stdin=subprocess.PIPE,
stdout=subprocess.PIPE)
output, error = p.communicate("translatetoupper")
print output

So when we exit Vim inside of IPython, it automatically runs this snippet of code and we get the following:

done. Executing edited code...
TRANSLATETOUPPER

Using Supervisor to Manage Processes

As a sysadmin, you often need to manage and deal with processes. When the web developers find out that their sysadmin is a Python expert, they are going to be very excited because many Python web frameworks do not offer an elegant way to temporarily manage long-running processes. Supervisor can help these situations by managing how a long process is controlled and ensuring it starts back up in the case of a reboot of the system.

Supervisor does quite a bit more than just help web applications get deployed; it has much more general applications. Supervisor can act as cross-platform controller to manage and interact with processes. It can start, stop, and restart other programs on a *nix system. It can also restart crashed processes, which can come in quite handy. The coauthor of Supervisor, Chris McDonough, tells us that it can also help manage “bad” processes, too. This could include processes that consume too much memory or hog the CPU, for example. Supervisor does remote control via XML-RPC XML-RPC Interface Extensions Event Notification System.

Most *nix systems administrators will mainly be concerned with “supervisord,” which is the daemon program that runs designed programs as child processes, and “supervisorctl,” which is a client program that can view the logs and control the processes from a unified session. There is a web interface as well, but well, this is a book on *nix, so let’s move right along.

As of this writing, the latest version of Supervisor is 3.0.x. The latest version of the manual can always be found at http://supervisord.org/manual/current/. Installing Supervisor is a piece of cake, thanks to the fact that you can easy_install it. Assuming you have used virtualenv to create a virtual Python installation directory, you can use the following command to easy_install supervisor:

bin/easy_install supervisor

This will install Supervisor into your bin directory. If you did an easy_install to your system Python, then it will be installed in something like /usr/local/bin, or your system scripts directory.

The next step to getting a very basic Supervisor daemon running is to create a very simple script that prints, sleeps for 10 seconds, and then dies. This is the exact opposite of a long-running process, but it shows one of the more powerful aspects of Supervisor, the ability to auto-restart and daemonize a program. Now, we can simply echo out a supervisord.conf file somewhere by using a special supervisor command called echo_supervisord_conf. In this example, we will just echo this out to /etc/supervisord.conf. It is good to note that the Supervisor config file can live anywhere, as the supervisord daemon can be run with an option to specify the location of a config file.

echo_supervisord_conf > /etc/supervisord.conf

With those few basic steps out of the way, we are ready to create a very simple example of a process that will die after a few seconds. We will use the upervisor autostart feature to keep this process alive. See Example 10-15.

Example 10-15. Simple example of Supervisor restarting a dying process

#!/usr/bin/env python
import time
print "Daemon runs for 3 seconds, then dies"
time.sleep(3)
print "Daemons dies"

As we mentioned earlier, in order to actually run a child program inside of supervisord, we need to edit the configuration file, and add our application. Let’s go ahead and add a couple lines to /etc/supervisord.conf:

[program:daemon]
command=/root/daemon.py              ; the program (relative uses PATH, can take args)
autorestart=true              ; retstart at unexpected quit (default: true)

Now, we can start supervisord and then use the supervisorectl to watch and start the process:

[root@localhost]~# supervisord
[root@localhost]~# supervisorctl
daemon                           RUNNING    pid 32243, uptime 0:00:02
supervisor>

At this point, we can run the help command to see what options are available for supervisorctl:

supervisor> help

Documented commands (type help topic):
========================================
EOF    exit  maintail  quit    restart   start   stop  version
clear  help  open      reload  shutdown  status  tail

Next, let’s start our process which we called daemon in the config file and then tail it to watch it while it dies, then reawakens magically, in an almost Frankenstein-like way…mwahahaha. It’s alive, then dead, and then alive again.

supervisor> stop daemon
daemon: stopped
supervisor> start daemon
daemon: started

And for the final part in our play, we can interactively tail the stdout of this program:

supervisor> tail -f daemon
== Press Ctrl-C to exit ==
for 3 seconds, then die
I just died
I will run for 3 seconds, then die

Using Screen to Manage Processes

An alternate approach to manage long-running processes is to use the GNU screen application. As a sysadmin, if you do not use screen, it is worth knowing even if you will not be managing Python programs with it. One of the core features of screen that makes it so useful is its ability to allow you to detach from a long-running process and come back to it. This is so useful, we would consider it an essential Unix skill to know.

Let’s take a look at a typical scenario in which we want to detach from a long-running web application such as trac. There are a few ways to configure trac, but one of the most simple is to just detach from the standalone trac process with screen.

All that is necessary to run a process is screen the append screen to the front of the long-running process, Ctrl-A, and then Ctrl-D to detach. To reattach to that process, you just need to type in screen and then press Ctrl-A again.

In Example 10-16, we tell tracd to run within screen. Once the process starts, we can then simply detach using Ctrl-A, then Ctrl-D, if we ever want to reattach.

Example 10-16. Running Python processes in screen

screen python2.4 /usr/bin/tracd --hostname=trac.example.com --port 8888
-r --single-env --auth=*
,/home/noahgift/trac-instance/conf/password,tracadminaccount /home/example/trac-instance/

If I ever need to reattach I can run:

[root@cent ~]# screen -r
There are several suitable screens on:
4797.pts-0.cent    (Detached)
24145.pts-0.cent    (Detached)
Type “screen [-d] -r [pid.]tty.host” to resume one of them.

This approach might not be the best to use in a production environment, but while doing development work, or for personal use, it certaintly has its advantages.

Threads in Python

Threads could be described as a necessary evil to some people, although many people dislike threads, to solve many problems that require dealing with multiple things at once. Threads are different than processes because they all run inside of the same process and share state and memory. That is both the thread’s greatest advantage and disadvantage. The advantage is that you can create a data structure that all threads can access without creating an IPC, or interprocess communication mechanism.

There are also hidden complexities in dealing with threads. Often, a trivial program of just a few dozens lines of code can become extremely complex with the introduction of threads. Threads are difficult to debug without adding extensive tracing and even then it is complex, as the output of the tracing can become confusing and overwhelming. While one of the authors was writing an SNMP discovery system that discovered data centers, the sheer magnitude of threads that needed to be spawned was very difficult to handle.

There are strategies to deal with threads, however, and often implementing a robust tracing library is one of them. That said, they can become a very handy tool in solving a complex problem.

For systems administrators, knowing some of the basics of programming with threads may be useful. Here are some of the ways that threads are useful for everyday sysadmin tasks: autodiscovering a network, fetching multiple web pages at the same time, stress-testing a server, and performing network-related tasks.

In keeping with our KISS theme, let’s use one of the most basic threading examples possible. It is good to note that threading a module requires an understanding of object-oriented programming. This can be a bit of a challenge, and if you have not had much, or any, exposure to object-oriented programming (OOP), then this example may be somewhat confusing. We would recommend picking up a copy of Mark Lutz’s Learning Python (O’Reilly) to understand some of the basics of OOP, although you can also refer to our Introduction and practice some of the techniques there. Ultimately, practicing OOP programming is the best way to learn it.

Because this book is about pragmatic Python, let’s get right into a threading example using the simplest possible threading example we could think of. In this simple threading script, we inherit from threading.Thread, set a global count variable, and then override the run method for threading. Finally, we launch five threads that explicitly print their number.

In many ways, this example is overly simplistic and has a bad design because we are using a global count so that multiple threads can share state. Often, it is much better to use queues with threads, as they take care of the complexity of dealing with shared state for you. See Example 10-17.

Example 10-17. KISS example of threading

#subtly bad design because of shared state
import threading
import time
count = 1
class KissThread(threading.Thread):
def run(self):
global count
print “Thread # %s:  Pretending to do stuff” % count
count += 1
time.sleep(2)
print “done with stuff”
for t in range(5):
KissThread().start()

[ngift@Macintosh-6][H:10464][J:0]> python thread1.py
Thread # 1:  Pretending to do stuff
Thread # 2:  Pretending to do stuff
Thread # 3:  Pretending to do stuff
Thread # 4:  Pretending to do stuff
Thread # 5:  Pretending to do stuff
done with stuff
done with stuff
done with stuff
done with stuff
done with stuff

#common.py
import subprocess
import time

IP_LIST = [ 'google.com',
'yahoo.com',
'yelp.com',
'amazon.com',
'freebase.com',
'clearink.com',
'ironport.com' ]

cmd_stub = 'ping -c 5 %s'

def do_ping(addr):
print time.asctime(), "DOING PING FOR", addr
cmd = cmd_stub % (addr,)
return subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE)

from common import IP_LIST, do_ping
import time

z = []
#for i in range(0, len(IP_LIST)):
for ip in IP_LIST:
p = do_ping(ip)
z.append((p, ip))

for p, ip in z:
print time.asctime(), "WAITING FOR", ip
p.wait()
print time.asctime(), ip, "RETURNED", p.returncode

jmjones@dinkgutsy:thread_discuss$ python nothread.py
Sat Apr 19 06:45:43 2008 DOING PING FOR google.com
Sat Apr 19 06:45:43 2008 DOING PING FOR yahoo.com
Sat Apr 19 06:45:43 2008 DOING PING FOR yelp.com
Sat Apr 19 06:45:43 2008 DOING PING FOR amazon.com
Sat Apr 19 06:45:43 2008 DOING PING FOR freebase.com
Sat Apr 19 06:45:43 2008 DOING PING FOR clearink.com
Sat Apr 19 06:45:43 2008 DOING PING FOR ironport.com
Sat Apr 19 06:45:43 2008 WAITING FOR google.com
Sat Apr 19 06:45:47 2008 google.com RETURNED 0
Sat Apr 19 06:45:47 2008 WAITING FOR yahoo.com
Sat Apr 19 06:45:47 2008 yahoo.com RETURNED 0
Sat Apr 19 06:45:47 2008 WAITING FOR yelp.com
Sat Apr 19 06:45:47 2008 yelp.com RETURNED 0
Sat Apr 19 06:45:47 2008 WAITING FOR amazon.com
Sat Apr 19 06:45:57 2008 amazon.com RETURNED 1
Sat Apr 19 06:45:57 2008 WAITING FOR freebase.com
Sat Apr 19 06:45:57 2008 freebase.com RETURNED 0
Sat Apr 19 06:45:57 2008 WAITING FOR clearink.com
Sat Apr 19 06:45:57 2008 clearink.com RETURNED 0
Sat Apr 19 06:45:57 2008 WAITING FOR ironport.com
Sat Apr 19 06:46:58 2008 ironport.com RETURNED 0

Note

As a disclaimer for the following threading examples, note that they are somewhat complex examples, because the same thing can be done using subprocess.Popen. subprocess.Popen is a great choice if you need to launch a bunch of processes and then wait for a response. If you need to communicate with each process, then using subprocess.Popen with a thread would be appropriate. The point in showing multiple examples is to highlight that concurrency is often full of choices with trade-offs. It is often difficult to say one model fits all, whether it be threads, or processes, or asynchronous libraries such as stackless or twisted. The following is the most efficient way to ping a large pool of IP addresses.

Now that we have the equivalent of Hello World out of the way for threading, let’s actually do something a real systems administrator would appreciate. Let’s take our example and slightly modify it to create a small script to ping a network for responses. This is a starter kit for a general network tool. See Example 10-18.

Example 10-18. Threaded ping sweep

#!/usr/bin/env python
from threading import Thread
import subprocess
from Queue import Queue

num_threads = 3
queue = Queue()
ips = ["10.0.1.1", "10.0.1.3", "10.0.1.11", "10.0.1.51"]
def pinger(i, q):
"""Pings subnet"""
while True:
ip = q.get()
print "Thread %s: Pinging %s" % (i, ip)
ret = subprocess.call("ping -c 1 %s" % ip,
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)
if ret == 0:
print "%s: is alive" % ip
else:
print "%s: did not respond" % ip
q.task_done()

for i in range(num_threads):

worker = Thread(target=pinger, args=(i, queue))
worker.setDaemon(True)
worker.start()

for ip in ips:
queue.put(ip)

print "Main Thread Waiting"
queue.join()
print "Done"

When we run this reasonably simple piece of code, we get this output:

[ngift@Macintosh-6][H:10432][J:0]# python ping_thread_basic.py
Thread 0: Pinging 10.0.1.1
Thread 1: Pinging 10.0.1.3
Thread 2: Pinging 10.0.1.11
Main Thread Waiting
10.0.1.1: is alive
Thread 0: Pinging 10.0.1.51
10.0.1.3: is alive
10.0.1.51: is alive
10.0.1.11: did not respond
Done

This example deserves to be broken down into understandable pieces, but first a little explanation is in order. Using threads to develop a ping sweep of a subnet is about as good of an example of using threads as it gets. A “normal” Python program that did not use threads would take up to N * (average response time per ping). There are two ping states: a response state and a timeout state. A typical network would be a mixture of responses and timeouts.

This means that if you wrote a ping sweep application that sequentially examined a Class C network with 254 addresses, it could take up to 254 * (~ 3 seconds). That comes out to 12.7 minutes. If you use threads, we can reduce that to a handful of seconds. That is why threads are important for network programming. Now, let’s take this one step further and think about a realistic environment. How many subnets exist in a typical data center? 20? 30? 50? Obviously, this sequential program becomes unrealistic very quickly, and threads are an ideal match.

Now, we can revisit our simple script and look at some of the implementation details. The first thing to examine are the modules that were imported. The two to look at in particular are threading and queue. As we mentioned in the very first example, using threading without queues makes it more complex than many people can realistically handle. It is a much better idea to always use the queuing module if you find you need to use threads. Why? Because the queue module also alleviates the need to explicitly protect data with mutexes because the queue itself is already protected internally by a mutex.

Imagine you are a farmer/scientist living in the Middle Ages. You have noticed that a group of crows, commonly referred to as a “murder,” (please consult Wikipedia for the reasons why), attack your crops in groups of 20 or more.

Because these crows are quite smart, it is almost impossible to scare them all away by throwing rocks, as you can throw, at most, a rock every 3 seconds, and the group of crows numbers, at times, up to 50. To scare away all of the crows, it can take up to several minutes, at least, by which time significant damage is done to your crops. As a student of math and science, you understand that the solution to this problem is simple. You need to create a queue of rocks in a basket, and then allocate several workers to grab rocks out of this basket and throw them at the crows all at once.

Using this new strategy, if you allocated 30 workers to pull rocks from the basket and throw rocks at the crows, you could throw a rock at 50 crows in less than 10 seconds. This is the basic formula for threads and queuing in Python as well. You give a pool of workers something to do, and when the queue is empty, the job is over.

Queues act as a way to delegate a task to a “pool” of workers in a centralized manner. One of the most important parts of our simple program is the join(). If we look at the docstring, we see that queue.join() states the following:

Namespace:    Interactive
File:        /System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
Queue.py
Definition:    Queue.Queue.join(self)
Docstring:
Blocks until all items in the Queue have been gotten and processed.

The count of unfinished tasks goes up whenever an item is added to the
queue. The count goes down whenever a consumer thread calls task_done()
to indicate the item was retrieved and all work on it is complete.

When the count of unfinished tasks drops to zero, join() unblocks.

A join is a way to control the main thread from exiting the program before the other threads get a chance to finish working on items in a queue. To go back to our farmer metaphor, it would be like the farmer dropping his basket of rocks and leaving while the workers lined up ready to throw rocks. In our example, if we comment out the queue.join() line, we can see the negative repercussions of our actions: First, we comment out the queue.join line:

print "Main Thread Waiting"
#By commenting out the join, the main program exits before threads have a chance
to run
#queue.join()
print "Done"

Next, we watch our nice script barf. See Example 10-19.

Example 10-19. Example of main thread exiting before worker threads

[ngift@Macintosh-6][H:10189][J:0]# python ping_thread_basic.py
Main Thread Waiting
Done
Unhandled exception in thread started by
Error in sys.excepthook:

Original exception was:

With that background theory on threads and queue out of the way, here is the walkthrough of that code step by step. In this portion, we hardcode values that would normally be passed into a more generic program. The num_threads is the number of worker threads, the queue is an instance of queue, and finally, the ips, are a list of IP addresses that we will eventually place into a queue:

num_threads = 3
queue = Queue()
ips = ["10.0.1.1", "10.0.1.3", "10.0.1.11", "10.0.1.51"]

This is the function that does all of the work in the program. This function is run by each thread everytime an “ip” is pulled from the queue. Notice that a new IP address is popped off a stack just like it is in a list. Doing this allows us to take an item until the queue is empty. Finally, notice that q.task_done() is called at the end of this while loop; this is significant because it tells the join() that it has completed what it pulled from the queue. Or, in plain English, it says the job is done. Let’s look at the docstring for Queue.Queue.task_done:

File:        /System/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/
Queue.py
Definition:    Queue.Queue.task_done(self)
Docstring:
Indicate that a formerly enqueued task is complete.

Used by Queue consumer threads. For each get() used to fetch a task,
a subsequent call to task_done() tells the queue that the processing
on the task is complete.

If a join() is currently blocking, it will resume when all items
have been processed (meaning that a task_done() call was received
for every item that had been put() into the queue).

Raises a ValueError if called more times than there were items
placed in the queue.

From the docstring, we can see that there is a relationship between q.get(), q.task_done(), and finally, q.join(). It is almost like a start, a middle, and an end to a story:

def pinger(i, q):
"""Pings subnet"""
while True:
ip = q.get()
print "Thread %s: Pinging %s" % (i, ip)
ret = subprocess.call("ping -c 1 %s" % ip,
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)
if ret == 0:
print "%s: is alive" % ip
else:
print "%s: did not respond" % ip
q.task_done()

If we look below, we are using a simple for loop as a controller that is orchestrating the spawning of a thread pool. Notice that this thread pool will just sit and “block,” or wait, until something is placed in the queue. It is not until the next section that anything even happens.

There is one subtle suprise lurking in our program that will be sure to catch you off guard. Notice the use of the setDaemon(True). If this is not set before the start method is called, our program will hang indefinitely.

The reason is fairly subtle, and that is because a program will only exit if daemon threads are running. You may have noticed that in the pinger function, we used an infinite loop. Since threads never die, it’s imperative to declare them as daemon threads. To see this happen, just comment out the worker.start() line and see what happens. To cut to the chase, the program will hang around indefinitely without the setting of the threads to a daemonic flag. You should test this out for yourself, as it will take away part of the magic of the process:

for i in range(num_threads):
worker = Thread(target=pinger, args=(i, queue))
worker.setDaemon(True)
worker.start()

By this point in our program, we have an angry pool of three threads waiting to do our bidding. They just need to have items placed in their queue, as that sends a signal to our threads to grab an item and do what we told it, in this case, ping an IP address:

for ip in ips:
queue.put(ip)

Finally, this one critical line sandwiched in the middle of two print statements is what ultimately has control of the program. Calling join on a queue, as we discussed earlier, will cause the main thread of the program to wait until the queue is empty. This is why threads and queue are like chocolate and peanut butter. Each is great alone, but together, they make an especially tasty treat.

print "Main Thread Waiting"
queue.join()
print "Done"

To really understand threads and queues, we need to take our example a step further and create another thread pool and another queue. In our first example, we ping a list of IP addresses that a thread pool grabs from a queue. In this next example, we will have our first pool of threads place valid IP addresses that respond to a ping into a second queue.

Next, our second pool of threads will take the IP addresses from the first queue and then perform an arping and return the IP address along with the Mac address if it can find it. Let’s see how this looks. See Example 10-20.

Example 10-20. Multiple queues with multiple thread pools

#!/usr/bin/env python
#This requires Python2.5 or greater
from threading import Thread
import subprocess
from Queue import Queue
import re

num_ping_threads = 3
num_arp_threads = 3
in_queue = Queue()
out_queue = Queue()
ips = ["10.0.1.1", "10.0.1.3", "10.0.1.11", "10.0.1.51"]

def pinger(i, iq, oq):
"""Pings subnet"""
while True:
ip = iq.get()
print "Thread %s: Pinging %s" % (i, ip)
ret = subprocess.call("ping -c 1 %s" % ip,
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)
if ret == 0:
#print "%s: is alive" % ip
#place valid ip address in next queue
oq.put(ip)
else:
print "%s: did not respond" % ip
iq.task_done()

def arping(i, oq):
"""grabs a valid IP address from a queue and gets macaddr"""
while True:
ip = oq.get()
p = subprocess.Popen("arping -c 1 %s" % ip,
shell=True,
stdout=subprocess.PIPE)
out = p.stdout.read()

#match and extract mac address from stdout
result = out.split()
pattern = re.compile(":")
macaddr = None
for item in result:
if re.search(pattern, item):
macaddr = item
print "IP Address: %s | Mac Address: %s " % (ip, macaddr)
oq.task_done()

#Place ip addresses into in queue
for ip in ips:
in_queue.put(ip)

#spawn pool of ping threads
for i in range(num_ping_threads):

worker = Thread(target=pinger, args=(i, in_queue, out_queue))
worker.setDaemon(True)
worker.start()

#spawn pool of arping threads
for i in range(num_arp_threads):

worker = Thread(target=arping, args=(i, out_queue))
worker.setDaemon(True)
worker.start()

print "Main Thread Waiting"
#ensures that program does not exit until both queues have been emptied
in_queue.join()
out_queue.join()

print "Done"

If we run this code, here is what the output looks like:

python2.5 ping_thread_basic_2.py
Main Thread Waiting
Thread 0: Pinging 10.0.1.1
Thread 1: Pinging 10.0.1.3
Thread 2: Pinging 10.0.1.11
Thread 0: Pinging 10.0.1.51
IP Address: 10.0.1.1 | Mac Address: [00:00:00:00:00:01]
IP Address: 10.0.1.51 | Mac Address: [00:00:00:80:E8:02]
IP Address: 10.0.1.3 | Mac Address: [00:00:00:07:E4:03]
10.0.1.11: did not respond
Done

To implement this solution, we only slightly extended the behavior of our first example by adding another pool of threads and queue. This is an important technique to have in your own personal toolkit, as using the queue module makes using threads a lot easier and safer. Arguably, it could even be called necessary.

Timed Delay of Threads with threading.Timer

Python has another threading feature that comes in handy for systems administration tasks. It is quite trivial to run the timed execution of a function inside of a thread by using threading.Timer. Example 10-21 is contrived.

Example 10-21. Thread timer

#!/usr/bin/env python
from threading import Timer
import sys
import time
import copy

#simple error handling
if len(sys.argv) != 2:
print "Must enter an interval"
sys.exit(1)

#our function that we will run
def hello():
print "Hello, I just got called after a %s sec delay" % call_time

#we spawn our time delayed thread here
delay = sys.argv[1]
call_time = copy.copy(delay)    #we copy the delay to use later
t = Timer(int(delay), hello)
t.start()

#we validate that we are not blocked, and that the main program continues
print "waiting %s seconds to run function" % delay
for x in range(int(delay)):
print "Main program is still running for %s more sec" % delay
delay = int(delay) - 1
time.sleep(1)

And if we run this code, we can see that the main thread, or program, continues to run, while a timed delay has been triggered for our function:

[ngift@Macintosh-6][H:10468][J:0]# python thread_timer.py 5
waiting 5 seconds to run function
Main program is still running for 5 more sec
Main program is still running for 4 more sec
Main program is still running for 3 more sec
Main program is still running for 2 more sec
Main program is still running for 1 more sec
Hello, I just got called after a 5 sec delay

Threaded Event Handler

Because this is a book about systems administration, let’s use our previous technique for a realistic application. In this example, we take our delayed thread trick and mix in an event loop that watches two directories for changes in filenames. We could get really sophisticated and examine file modification times, but in the spirit of keeping examples simple, we will look at how this event loop looks for a registered event, and if the event is triggered, then an action method is called in a delayed thread.

This module could be abstracted quite easily into a more generic tool, but for now, Example 10-22 is hardcoded to keep two directories in sync if they fall out of sync by using rsync -av --delete in a delayed background thread.

Example 10-22. Threaded directory synchronization tool

#!/usr/bin/env python
from threading import Timer
import sys
import time
import copy
import os
from subprocess import call

class EventLoopDelaySpawn(object):
"""An Event Loop Class That Spawns a Method in a Delayed Thread"""

def __init__(self, poll=10,
wait=1,
verbose=True,
dir1="/tmp/dir1",
dir2="/tmp/dir2"):

self.poll = int(poll)
self.wait = int(wait)
self.verbose = verbose
self.dir1 = dir1
self.dir2 = dir2

def poller(self):
"""Creates Poll Interval"""
time.sleep(self.poll)
if self.verbose:
print "Polling at %s sec interval" % self.poll

def action(self):
if self.verbose:
print "waiting %s seconds to run Action" % self.wait
ret = call("rsync -av --delete %s/ %s" % (self.dir1, self.dir2), shell=True)

def eventHandler(self):
#if two directories contain same file names
if os.listdir(self.dir1) != os.listdir(self.dir2):
print os.listdir(self.dir1)
t = Timer((self.wait), self.action)
t.start()
if self.verbose:
print "Event Registered"
else:
if self.verbose:
print "No Event Registered"

def run(self):
"""Runs an event loop with a delayed action method"""
try:
while True:
self.eventHandler()
self.poller()

except Exception, err:
print "Error: %s " % err

finally:
sys.exit(0)

E = EventLoopDelaySpawn()
E.run()

The observant reader may be thinking that the delay is not strictly necessary, and this is true. The delay can create some added benefit, however. If you add a delay for, say, five seconds, you could tell the thread to cancel if you discovered another event, such as if your master directory was accidentally deleted. A thread delay is a great mechanism to create conditional future operations that can still be canceled.

Processes

Threads are not the only way to deal with concurrency in Python. In fact, processes have some advantages to threads in that they will scale to multiple processors, unlike threads in Python. Because of the GIL, or global interpreter lock, only one thread can truly run at one time, and it is limited to a single processor. However, to make heavy use of the CPU with Python code, threads are not a good option. In such cases, it’s better to use separate processes.

If a problem requires the use of multiple processors, then processes are a fine choice. Additionally, there are many libraries that just will not work with threads. For example, the current Net-SNMP library for Python is synchronous, so writing concurrent code requires the use of forked processes.

While threads share global state, processes are completely independent, and communication with a process requires a bit more effort. Talking to processes through pipes can be a little difficult; fortunately, however, there is a processing library that we will discuss in great detail here. There has been some talk of integrating the processing library into the standard library in Python, so it would be useful to understand.

In an earlier note, we mentioned an alternate method of using subprocess.Popen to spawn multiple processes. For many situations, this an excellent and very simple choice to execute code in parallel. If you refer to Chapter 13, you can take a look at an example of where we did this in creating a tool that spawned many dd processes.

Processing Module

So what is this processing module we have hinted at, anyway? As of the printing of this book, “processing is a package for the Python language which supports the spawning of processes using the API of the standard library’s threading module…” One of the great things about the processing module is that it maps to the threading API, more or less. This means that you don’t have to learn a new API to fork processes instead of threads. Visit http://pypi.python.org/pypi/processing to find out more about the processing module.

Note

As we mentioned earlier, things are never simple with concurrency. This example could be considered inefficient as well, because we could have just used subprocess.Popen, instead of forking with the processing module, and then running subprocess.call. In the context of a larger application, however, there are some benefits to using the queue type API, and as such, it serves as a reasonable comparison to the threading example earlier. There is some talk of merging the processing module into Subprocess, as Subprocess currently lacks the ability to manage a flock of processes like the processing module does. This request was made in the original PEP, or Python Enhancement Proposal, for Subprocess: http://www.python.org/dev/peps/pep-0324/.

Now that we have some background on the processing module, let’s take a look at Example 10-23.

Example 10-23. Introduction to processing module

#!/usr/bin/env python
from processing import Process, Queue
import time

def f(q):
x = q.get()
print "Process number %s, sleeps for %s seconds" % (x,x)
time.sleep(x)
print "Process number %s finished" % x
q = Queue()

for i in range(10):
q.put(i)
i = Process(target=f, args=[q])
i.start()

print "main process joins on queue"
i.join()
print "Main Program finished"

If we look at the output, we see the following:

[ngift@Macintosh-7][H:11199][J:0]# python processing1.py
Process number 0, sleeps for 0 seconds
Process number 0 finished
Process number 1, sleeps for 1 seconds
Process number 2, sleeps for 2 seconds
Process number 3, sleeps for 3 seconds
Process number 4, sleeps for 4 seconds
main process joins on queue
Process number 5, sleeps for 5 seconds
Process number 6, sleeps for 6 seconds
Process number 8, sleeps for 8 seconds
Process number 7, sleeps for 7 seconds
Process number 9, sleeps for 9 seconds
Process number 1 finished
Process number 2 finished
Process number 3 finished
Process number 4 finished
Process number 5 finished
Process number 6 finished
Process number 7 finished
Process number 8 finished
Process number 9 finished
Main Program finished

All this program does is tell each process to sleep as long as the number of the processes. As you can see, it is a clean and straightforward API.

Now that we have the equivalent of a Hello World out of the way for the processing module, we can do something more interesting. If you remember in the threading section, we wrote a simple threaded subnet discovery script. Because the processing API is very similar to the threading API, we can implement an almost identical script using processes instead of threads. See Example 10-24.

Example 10-24. Processed-based ping sweep

#!/usr/bin/env python
from processing import Process, Queue, Pool
import time
import subprocess
from IPy import IP
import sys

q = Queue()
ips = IP("10.0.1.0/24")
def f(i,q):
while True:
if q.empty():
sys.exit()
print "Process Number: %s" % i
ip = q.get()
ret = subprocess.call("ping -c 1 %s" % ip,
shell=True,
stdout=open('/dev/null', 'w'),
stderr=subprocess.STDOUT)
if ret == 0:
print "%s: is alive" % ip
else:
print "Process Number: %s didn’t find a response for %s " % (i, ip)

for ip in ips:
q.put(ip)

#q.put("192.168.1.1")

for i in range(50):
p = Process(target=f, args=[i,q])
p.start()

print "main process joins on queue"
p.join()
print "Main Program finished"

This code looks remarkably similar to the threaded code we reviewed earlier. If we take a look at the output, we will see something similar as well:

[snip]
10.0.1.255: is alive
Process Number: 48 didn't find a response for 10.0.1.216
Process Number: 47 didn't find a response for 10.0.1.217
Process Number: 49 didn't find a response for 10.0.1.218
Process Number: 46 didn't find a response for 10.0.1.219
Main Program finished
[snip]
[ngift@Macintosh-7][H:11205][J:0]#

This snippet of code bears some explanation. Even though the API is similar, it is slightly different. Notice that each process runs inside of a infinite loop, grabbing items from the queue. In order to tell the processes to “go away” with the processing module, we create a conditional statement that looks at whether the queue is empty. Each of the 50 threads first checks to see if the queue is empty, and if it is, then it “poisons” itself, by running sys.exit.

If the queue still has things in it, then the process happily grabs the item, in this case, an IP address, and goes along with the job it was assigned, in this case, pinging the IP address. The main program uses a join, just like we do in a threading script, and joins on the queue until it is empty. After all of the worker processes die and the queue is empty, the next print statement gets run, stating the program is finished.

With an API as simple to use as the processing module, forking instead of threading is a relative no-brainer. In Chapter 7, we discussed a practical implementation of using the processing module with Net-SNMP, which has synchronous bindings to Python.

Scheduling Python Processes

Now that we have covered the gamut of ways to deal with processes in Python, we should talk about ways to schedule these processes. Using good old-fashioned cron is highly suitable for running processes in Python.

One of the nice new features of cron in many POSIX systems is the advent of scheduled directories. This is the only way we use cron anymore, as it is very convenient to just drop a python script in one of the four default directories: /etc/cron.daily, /etc/cron.hourly, /etc/cron.monthly, and /etc/cron.weekly.

Quite a few sysadmins have, at one point in their life, written the good-old-fashioned disk usage email. You place a Bash script in /etc/cron.daily and it looks something like this:

df -h | mail -s "Nightly Disk Usage Report" staff@example.com

You then put that script in /etc/cron.daily/diskusage.sh and the email looks something like this.

From:     guru-python-sysadmin@example.com
Subject:     Nightly Disk Usage Report
Date:     February 24, 2029 10:18:57 PM EST
To:     staff@example.com

Filesystem            Size  Used Avail Use% Mounted on
/dev/hda3              72G   16G   52G  24% /
/dev/hda1              99M   20M   75M  21% /boot
tmpfs                1010M     0 1010M   0% /dev/shm

There is a better way than this. Even cron jobs can benefit from Python scripts instead of Bash or Perl. In fact, cron and Python go quite well together. Let’s take our Bash example and “Pythonize” it. See Example 10-25.

Example 10-25. Cron-based disk report email Python

import smtplib
import subprocess
import string

p = subprocess.Popen("df -h", shell=True, stdout=subprocess.PIPE)
MSG = p.stdout.read()
FROM = "guru-python-sysadmin@example.com"
TO = "staff@example.com"
SUBJECT = "Nightly Disk Usage Report"
msg = string.join((
"From: %s" % FROM,
"To: %s" % TO,
"Subject: %s" % SUBJECT,
"",
MSG), "\r\n")
server = smtplib.SMTP('localhost')
server.sendmail(FROM, TO, msg)
server.quit()

This is a trivial recipe to create an automated cron-based disk report, but for many tasks, it should work just fine. Here is a walk through of what this handful of Python does. First, we use subprocess.Popen to read the stdout of df. Next, we create variables for From, To, and Subject. Then, we join those strings together to create the message. That is the most difficult part of the script. Finally, we set the outgoing smtp mail server to use localhost, and then pass the variables we set earlier into server.sendmail().

A typical way to use this script would be to simply place it in /etc/cron.daily/nightly_disk_report.py.

If you are new to Python, you may want to use this script as boilerplate code to get something fun working rather quickly. In Chapter 4, we went into even greater detail on creating email messages, so you should refer to that chapter for more advice.

daemonizer

Dealing with daemons is a given for anyone who has spent more than a cursory amount of time on Unix. Daemons do everything from handling requests to sending files to a printer (such as lpd), fielding HTTP requests, and serving up files (such as Apache’s httpd).

So, what is a daemon? A daemon is often thought of as a background task that doesn’t have a controlling terminal. If you are familiar with Unix job control, you may think that running a command with an & at the end of it will make it a daemon. Or perhaps starting a process and then hitting Ctrl-z and then issuing the bg command will make a daemon. Both of these will background the process, but neither of them breaks the process free from your shell process and disassociates them from the controlling terminal (probably of your shell process as well). So, these are the three signs of a daemon: running in the background, being dislocated from the process that started it, and having no controlling terminal. Backgrounding a process with normal shell job control will only accomplish the first of these.

Following is a piece of code that defines a function named daemonize() that causes the calling code to become a daemon in the sense that we discussed in the previous paragraph. This function was extracted from the “Forking a Daemon Process on Unix” recipe in David Ascher’s Python Cookbook, Second Edition, pages 388–389 (O’Reilly). This code follows pretty closely to the steps that Richard Stevens laid out in his book UNIX Network Programming: The Sockets Networking API (O’Reilly) for the “proper” way of daemonizing a process. For anyone not familiar with the Stevens book, it is typically regarded as the reference for Unix network programming as well as how to make a daemon process under Unix. See Example 10-26.

Example 10-26. Daemonize function

import sys, os
def daemonize (stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
# Perform first fork.
try:
pid = os.fork( )
if pid > 0:
sys.exit(0) # Exit first parent.
except OSError, e:
sys.stderr.write("fork #1 failed: (%d) %s\n" % (e.errno, e.strerror))
sys.exit(1)
# Decouple from parent environment.
os.chdir("/")
os.umask(0)
os.setsid( )
# Perform second fork.
try:
pid = os.fork( )
if pid > 0:
sys.exit(0) # Exit second parent.
except OSError, e:
sys.stderr.write("fork #2 failed: (%d) %s\n" % (e.errno, e.strerror))
sys.exit(1)
# The process is now daemonized, redirect standard file descriptors.
for f in sys.stdout, sys.stderr: f.flush( )
si = file(stdin, 'r')
so = file(stdout, 'a+')
se = file(stderr, 'a+', 0)
os.dup2(si.fileno( ), sys.stdin.fileno( ))
os.dup2(so.fileno( ), sys.stdout.fileno( ))
os.dup2(se.fileno( ), sys.stderr.fileno( ))

The first thing that this code does is to fork() a process. fork()ing makes a copy of the running process where the copy is considered the “child” process and the original is considered the “parent” process. When the child process forks off, the parent is free to exit. We check for what the pid is after the fork. If the pid is positive, it means that we’re in the parent process. If you have never fork()ed a child process, this may seem a bit odd to you. After the call to os.fork() completes, there will be two copies of the same process running. Both then check the return code of the fork() call, which returns 0 in the child and the process ID in the parent. Whichever process has a non-zero return code, which will only be the parent, exits. If an exception occurs at this point, the process exits. If you called this script from an interactive shell (such as Bash), you would now have your prompt back because the process you started would have just terminated. But the child process of the process you started (i.e., the grandchild process) lives on.

The next three things the process does is to change directory to / (os.chdir("/"), set its umask to 0 (os.umask(0), and create a new session (os.setsid()). Changing directory to / puts the daemon process in a directory that will always exist. An added benefit of changing to / is that your long-running process won’t tie up your ability to unmount a filesystem if it happens to be in a directory of a filesystem you are trying to unmount. The next thing that the process does is to change its file mode creation mask to most permissive. If the daemon needs to create files with group-read and group-write permissions, an inherited mask with more restrictive permissions might have ill effects. The last of these three actions (os.setsid()) is perhaps the least familiar to most people. The setsid call does a number of things. First, it causes the process to become a session leader of a new session. Next, it causes the process to become a process group leader of a new process group. Finally, and perhaps most important for the purposes of daemonization, it causes the process to have no controlling terminal. The fact that it has no controlling terminal means that the process cannot fall victim to unintentional (or even intentional) job control actions from some terminal. This is important to having an uninterrupted, long-running process like a daemon.

But the fun doesn’t stop there. After the call to os.setsid(), there is another forking that takes place. The first fork and setsid set the stage for this second fork; they detach from any controlling terminal and set the process as a session leader. Another fork means that the resulting process cannot be a session leader. This means that the process cannot acquire a controlling terminal. This second fork is not necessary, but is more of a precaution. Without the final fork, the only way that the process could acquire a controlling terminal is if it directly opened a terminal device without using a O_NOCTTY flag.

The last thing that happens here is some file cleanup and readjustment. Standard output and error (sys.stdout and sys.stderr) are flushed. This ensures that information intended for those streams actually make it there. This function allows the caller to specify files for stdin, stdout, and stderr. The defaults for all three are /dev/null. This code takes either the user specified or default stdin, stdout, and stderr and sets the process’s standard input, output, and error to these files, respectively.

So, how do you use this daemonizer? Assuming the daemonizer code is in a module named daemonize.py, Example 10-27 is a sample script to use it.

Example 10-27. Using a daemonizer

from daemonize import daemonize
import time
import sys

def mod_5_watcher():
start_time = time.time()
end_time = start_time + 20
while time.time() < end_time:
now = time.time()
if int(now) % 5 == 0:
sys.stderr.write('Mod 5 at %s\n' % now)
else:
sys.stdout.write('No mod 5 at %s\n' % now)
time.sleep(1)

if __name__ == '__main__':
daemonize(stdout='/tmp/stdout.log', stderr='/tmp/stderr.log')
mod_5_watcher()

This script first daemonizes and specifies that /tmp/stdout.log should be used for standard output and /tmp/stderr.log should be used for standard error. It then watches the time for the next 20 seconds, sleeping 1 second in between checking the time. If the time, denoted in seconds, is divisible by five, we write to standard error. If the time is not divisible by five, we write to standard output. Since the process is using /tmp/stdout.log for standard output and /tmp/stderr.log for standard error, we should be able to see the results in those files after running this example.

After running this script, we immediately saw a new prompt silently appear:

jmjones@dinkgutsy:code$ python use_daemonize.py
jmjones@dinkgutsy:code$

And here are the result files from running the example:

jmjones@dinkgutsy:code$ cat /tmp/stdout.log
No mod 5 at 1207272453.18
No mod 5 at 1207272454.18
No mod 5 at 1207272456.18
No mod 5 at 1207272457.19
No mod 5 at 1207272458.19
No mod 5 at 1207272459.19
No mod 5 at 1207272461.2
No mod 5 at 1207272462.2
No mod 5 at 1207272463.2
No mod 5 at 1207272464.2
No mod 5 at 1207272466.2
No mod 5 at 1207272467.2
No mod 5 at 1207272468.2
No mod 5 at 1207272469.2
No mod 5 at 1207272471.2
No mod 5 at 1207272472.2
jmjones@dinkgutsy:code$ cat /tmp/stderr.log
Mod 5 at 1207272455.18
Mod 5 at 1207272460.2
Mod 5 at 1207272465.2
Mod 5 at 1207272470.2

This is a really simple example of writing a daemon, but hopefully it gets the basic concepts across. You could use this daemonizer to write directory watchers, network monitors, network servers, or anything else you can imagine that runs for a long (or unspecified amount of) time.

Summary

Hopefully, this chapter demonstrated just how mature and powerful Python is at dealing with processes. Python has an elegant and sophisticated threading API, but it is always good to remember about the GIL. If you are I/O bound, then often this is not an issue, but if you require multiple processors, then using processes is a good choice. Some people think processes are better than using threads even if the GIL did not exist. The main reason for this is that debugging threaded code can be a nightmare.

Finally, it would be a good idea to get familiar with the Subprocess module if you are not already. Subprocess is a one-stop shop for dealing with, well, subprocesses.

Chapter 11. Building GUIs

When informed people consider the duties of a system administrator, building GUI applications probably does not come to mind at all. However, there are times when you will need to build a GUI application, or by building a GUI app your life will be easier than if you didn’t. We’re using GUI in the broad sense here to mean both traditional GUI applications using toolkits such as GTK and QT, as well as web-based applications.

This chapter will focus on PyGTK, curses, and the Django web framework. We’ll start off with the basics of GUI building, then move on to creating a fairly simple application using PyGTK, then the same app using curses and Django. Finally, we’ll show you how Django can, with very little code, work as a fairly polished frontend to a database.

GUI Building Theory

When you write a console utility, you often expect it to run and complete without user intervention. This is definitely the case when scripts are run from cron and at, anyway. But when you write a GUI utility, you expect that a user will have to provide some input in order to make things happen and exercise your utility. Think for a moment about your experiences with GUI applications such as web browsers, email clients, and word processors. You run the application somehow. The application performs some sort of initialization, perhaps loading some configuration and putting itself into some known state. But then, in general, the application just waits for the user to do something. Of course, there are examples of applications executing seemingly on their own, such as Firefox automatically checking for updates without the explicit request or consent of the user, but that’s another story.

What is the application waiting for? How does it know what to do when the user does something? The application is waiting for an event to happen. An event is just something that happens within the application, specifically to one of the GUI components such as a button being pressed or a checkbox being selected. And the application “knows” what to do when these events happen because the programmer associated certain events with certain pieces of code. The “pieces of code” that are associated with certain events, are referred to as event handlers. One of the jobs of a GUI toolkit is to call the right event handler when the associated event occurs. To be a little more precise, the GUI toolkit provides an “event loop” that quietly loops around, waits for events to happen, and when they do, it handles them appropriately.

Behavior is event driven. When you code your GUI application, you decide how you want your application to behave when a user does certain things. You set up event handlers that the GUI toolkit calls when the user triggers events.

That describes the behavior of an application, but what about the form? Meaning, how do you get the buttons, text fields, labels, and checkboxes on an application? The answer to this question can vary a bit. There may be a GUI builder for the GUI toolkit that you choose. A GUI builder lays out the various components such as buttons, labels, checkboxes, etc. for a GUI application. For example, if you are working on a Mac and choose to write a Cocoa app, Interface Builder is available to lay the GUI components out for you. Or, if you are using PyGTK on Linux, you can use Glade. Or, if you are using PyQT, you can use QT Designer.

GUI builders can be helpful, but sometimes you may want more control of your GUI than the builder offers. In those cases, it is not difficult to lay out a GUI “by hand” by writing a little code. In PyGTK, each type of GUI component corresponds to a Python class. For example, a window is an object of the class gtk.Window. And a button is an object of the class gtk.Button. In order to create a simple GUI app that has a window and a button, you instantiate objects of classes gtk.Window and gtk.Button and add the button to the window. If you want the button to do something when it is clicked, you have to specify an event handler for the “clicked” event for the button.

Building a Simple PyGTK App

We’ll create a simple piece of code which uses the already-mentioned gtk.Window and gtk.Button classes. Following is a simple GUI application that doesn’t do anything useful except show some of the basic tenets of GUI programming.

Before being able to run this example or write your own PyGTK app, you’ll have to install PyGTK. This is pretty simple if you’re running a relatively modern Linux distribution. It even looks pretty easy for Windows. If you’re running Ubuntu, it should already be installed. If there isn’t a binary distribution for your platform, you can expect pain. See Example 11-1.

Example 11-1. Simple PyGTK application with one window and one button

#!/usr/bin/env python

import pygtk
import gtk
import time

class SimpleButtonApp(object):
"""This is a simple PyGTK app that has one window and one button.
When the button is clicked, it updates the button's label with the current time.
"""

def __init__(self):
#the main window of the application
self.window = gtk.Window(gtk.WINDOW_TOPLEVEL)

#this is how you "register" an event handler. Basically, this
#tells the gtk main loop to call self.quit() when the window "emits"
#the "destroy" signal.
self.window.connect("destroy", self.quit)

#a button labeled "Click Me"
self.button = gtk.Button("Click Me")

#another registration of an event handler. This time, when the
#button "emits" the "clicked" signal, the 'update_button_label'
#method will get called.
self.button.connect("clicked", self.update_button_label, None)

#The window is a container. The "add" method puts the button
#inside the window.
self.window.add(self.button)

#This call makes the button visible, but it won't become visible
#until its container becomes visible as well.
self.button.show()

#Makes the container visible
self.window.show()

def update_button_label(self, widget, data=None):
"""set the button label to the current time

This is the handler method for the 'clicked' event of the button
"""
self.button.set_label(time.asctime())

def quit(self, widget, data=None):
"""stop the main gtk event loop

When you close the main window, it will go away, but if you don't
tell the gtk main event loop to stop running, the application will
continue to run even though it will look like nothing is really
happening.
"""
gtk.main_quit()

def main(self):
"""start the gtk main event loop"""
gtk.main()

if __name__ == "__main__":
s = SimpleButtonApp()
s.main()

The first thing you probably noticed in this example is that the main class inherits from object rather than some GTK class. Creating a GUI application in PyGTK is not necessarily an object-oriented exercise. You will certainly have to instantiate objects, but you don’t have to create your own custom classes. However, for anything more than a trivial example such as what we are creating, we strongly recommend creating your own custom class. The main benefit to creating your own class for a GUI application is that all your GUI components (windows, buttons, checkboxes) wind up all attached to the same object, which allows easy access to those components from elsewhere in the application.

Since we chose to create a custom class, the first place to look to start understanding what is going on is in the constructor (the __init__() method). In fact, in this example, you can see what is going on by focusing on the constructor. This example is pretty well commented, so we won’t duplicate an explanation of everything here, but we will give a recap. We created two GUI objects: a gtk.Window and a gtk.Button. We put the button in the window, since the window is a container object. We also created event handlers for the window and the button for the destroy and clicked events, respectively. If you run this code, it will display a window with a button labeled “Click Me.” Every time you click the button, it will update the button’s label with the current time. Figures 11-1 and 11-2 are screenshots of the application before and after clicking the button.

Figure 11-1. Simple PyGTK app—before clicking the button

Figure 11-2. Simple PyGTK app—after clicking the button

Building an Apache Log Viewer Using PyGTK

Now that we have covered the basics of GUI building in general and of using PyGTK specifically, the following is an example of building something a little more useful with PyGTK; we’re going to walk through creating an Apache logfile viewer. The functionality we are going to include in this application is as follows:

- Select and open specified logfile

- View line number, remote host, status, and bytes sent at a glance

- Sort loglines by line number, remote host, status, or bytes sent

This example builds on the Apache log parsing code that we wrote in Chapter 3. Example 11-2 is the source code for the logfile viewer.

Example 11-2. PyGTK Apache log viewer

#!/usr/bin/env python

import gtk
from apache_log_parser_regex import dictify_logline

class ApacheLogViewer(object):
"""Apache log file viewer which sorts on various pieces of data"""

def __init__(self):
#the main window of the application
self.window = gtk.Window(gtk.WINDOW_TOPLEVEL)
self.window.set_size_request(640, 480)
self.window.maximize()

#stop event loop on window destroy
self.window.connect("destroy", self.quit)

#a VBox is a container that holds other GUI objects primarily for layout
self.outer_vbox = gtk.VBox()

#toolbar which contains the open and quit buttons
self.toolbar = gtk.Toolbar()

#create open and quit buttons and icons
#add buttons to toolbar
#associate buttons with correct handlers
open_icon = gtk.Image()
quit_icon = gtk.Image()
open_icon.set_from_stock(gtk.STOCK_OPEN, gtk.ICON_SIZE_LARGE_TOOLBAR)
quit_icon.set_from_stock(gtk.STOCK_QUIT, gtk.ICON_SIZE_LARGE_TOOLBAR)
self.open_button = gtk.ToolButton(icon_widget=open_icon)
self.quit_button = gtk.ToolButton(icon_widget=quit_icon)
self.open_button.connect("clicked", self.show_file_chooser)
self.quit_button.connect("clicked", self.quit)
self.toolbar.insert(self.open_button, 0)
self.toolbar.insert(self.quit_button, 1)

#a control to select which file to open
self.file_chooser = gtk.FileChooserWidget()
self.file_chooser.connect("file_activated", self.load_logfile)

#a ListStore holds data that is tied to a list view
#this ListStore will store tabular data of the form:
#line_numer, remote_host, status, bytes_sent, logline
self.loglines_store = gtk.ListStore(int, str, str, int, str)

#associate the tree with the data...
self.loglines_tree = gtk.TreeView(model=self.loglines_store)
#...and set up the proper columns for it
self.add_column(self.loglines_tree, 'Line Number', 0)
self.add_column(self.loglines_tree, 'Remote Host', 1)
self.add_column(self.loglines_tree, 'Status', 2)
self.add_column(self.loglines_tree, 'Bytes Sent', 3)
self.add_column(self.loglines_tree, 'Logline', 4)

#make the area that holds the apache log scrollable
self.loglines_window = gtk.ScrolledWindow()

#pack things together
self.window.add(self.outer_vbox)
self.outer_vbox.pack_start(self.toolbar, False, False)
self.outer_vbox.pack_start(self.file_chooser)
self.outer_vbox.pack_start(self.loglines_window)
self.loglines_window.add(self.loglines_tree)

#make everything visible
self.window.show_all()
#but specifically hide the file chooser
self.file_chooser.hide()

def add_column(self, tree_view, title, columnId, sortable=True):
column = gtk.TreeViewColumn(title, gtk.CellRendererText() , text=columnId)
column.set_resizable(True)
column.set_sort_column_id(columnId)
tree_view.append_column(column)

def show_file_chooser(self, widget, data=None):
"""make the file chooser dialog visible"""
self.file_chooser.show()

def load_logfile(self, widget, data=None):
"""load logfile data into tree view"""
filename = widget.get_filename()
print "FILE-->", filename
self.file_chooser.hide()
self.loglines_store.clear()
logfile = open(filename, 'r')
for i, line in enumerate(logfile):
line_dict = dictify_logline(line)
self.loglines_store.append([i + 1, line_dict['remote_host'],
line_dict['status'], int(line_dict['bytes_sent']), line])
logfile.close()

def quit(self, widget, data=None):
"""stop the main gtk event loop"""
gtk.main_quit()

def main(self):
"""start the gtk main event loop"""
gtk.main()

if __name__ == "__main__":
l = ApacheLogViewer()
l.main()

In the PyGTK Apache Log Viewer example, the main class, ApacheLogViewer, only derives from object. There is nothing special about our main object; it just happens to be where we are hanging all of the pieces and actions of the GUI.

Next, and jumping to the __init__() method, we create a window object. Something a little different about this example from the previous, “simple” example is that we specify sizing requirements for this window. We initially specify that this window should be displayed at 640×480 and then specify that it should be maximized. Setting the sizing parameters twice was intentional. 640×480 is a reasonable starting, so this isn’t a bad default. While 640×480 is a fine size, bigger is better, so we maximized the window. It turns out that setting 640×480 (or some other size of your preference) first is probably a good practice. According to the PyGTK documentation, the window manager may not honor the maximize() request. Also, the user can unmaximize the window after it is maximized, so you may want to specify the size when it is unmaximized.

After creating and sizing the window, we create a VBox. This is a “vertical box,” which is simply a container object. GTK has the concept of using vertical (VBox) and horizontal (HBox) boxes for laying out widgets on a window. The idea behind these boxes is that you “pack” them with widgets relative either to their beginning (which is the top for VBoxes and left for HBoxes) or their end. If you don’t know what a widget is, it’s simply a GUI component such as a button or text box. By using these boxes, you can lay out the widgets on a window pretty much any way you can imagine. Since boxes are containers, they can contain other boxes, so feel free to pack one box into another.

After adding the VBox to the window, we add the toolbar and tool buttons. The toolbar itself is another container and provides methods for adding components to itself. We create the icons for the buttons, create the buttons, and attach the event handlers to the buttons. Finally, we add the buttons to the toolbar. Just as with pack_start() on VBox, we use insert() on the ToolBar to add widgets.

Next, we create a file chooser widget that we use to navigate to the logfile to process and then associate it with an event handler. This part is very straightforward, but we will readdress it in a moment.

After creating the file chooser, we create the list component that will contain the loglines. This component comes in two pieces: the data piece (which is a ListStore), and the piece you interact with (which is a TreeView). We create the data piece first by defining what data types we want in which columns. Next, we create the display component and associate the data component with it.

After creating the list component, we create one final container, a scrollable window, and then pack everything together. We pack the toolbar, file chooser, and the scrollable window into the VBox we created earlier. We put the list piece, which will contain the loglines, into the scrollable window so that if there are more than a handful of lines, we can scroll through them.

Finally, we make things visible and invisible. We make the main window visible with the show_all() call. This call also makes all children visible. Given how we have created this GUI application, we want the file chooser to be invisible until we click the “open” button. So, we make the file chooser control invisible when it is created.

When you launch this application, you can see that it meets our initial requirements. We are able to select and open specified logfiles. Each of the line number, remote host, status, and bytes pieces of data have their own columns in the list control, so we can easily surmise those pieces of data just by glancing at each line. And, we can sort on any of those columns by simply clicking on the corresponding column header.

Building an Apache Log Viewer Using Curses

curses is a library that facilitates the creation of interactive text-based applications. Unlike GUI toolkits, curses does not follow an event handling and callback approach. You are responsible for getting input from the user and then doing something with it, whereas in GTK, the widget handles getting input from the user and the toolkit calls a handler function when an event occurs. Another difference between curses and GUI toolkits is that with GUI toolkits you are adding widgets to some container and letting the toolkit deal with drawing and refreshing the screen. With curses, you are typically painting text directly on the screen.

Example 11-3 is the Apache log viewer again, implemented using the curses module from the Python Standard Library.

Example 11-3. curses Apache log viewer

#!/usr/bin/env python

"""
curses based Apache log viewer

Usage:

curses_log_viewer.py logfile

This will start an interactive, keyboard driven log viewing application. Here
are what the various key presses do:

u/d   - scroll up/down
t     - go to the top of the log file
q     - quit
b/h/s - sort by bytes/hostname/status
r     - restore to initial sort order

"""

import curses
from apache_log_parser_regex import dictify_logline
import sys
import operator

class CursesLogViewer(object):
def __init__(self, logfile=None):
self.screen = curses.initscr()
self.curr_topline = 0
self.logfile = logfile
self.loglines = []

def page_up(self):
self.curr_topline = self.curr_topline - (2 * curses.LINES)
if self.curr_topline < 0:
self.curr_topline = 0
self.draw_loglines()

def page_down(self):
self.draw_loglines()

def top(self):
self.curr_topline = 0
self.draw_loglines()

def sortby(self, field):
#self.loglines = sorted(self.loglines, key=operator.itemgetter(field))
self.loglines.sort(key=operator.itemgetter(field))
self.top()

def set_logfile(self, logfile):
self.logfile = logfile
self.load_loglines()

def load_loglines(self):
self.loglines = []
logfile = open(self.logfile, 'r')
for i, line in enumerate(logfile):
line_dict = dictify_logline(line)
self.loglines.append((i + 1, line_dict['remote_host'],
line_dict['status'], int(line_dict['bytes_sent']), line.rstrip()))
logfile.close()
self.draw_loglines()

def draw_loglines(self):
self.screen.clear()
status_col = 4
bytes_col = 6
remote_host_col = 16
status_start = 0
bytes_start = 4
remote_host_start = 10
line_start = 26
logline_cols = curses.COLS - status_col - bytes_col - remote_host_col - 1
for i in range(curses.LINES):
c = self.curr_topline
try:
curr_line = self.loglines[c]
except IndexError:
break
self.screen.addstr(i, status_start, str(curr_line[2]))
self.screen.addstr(i, bytes_start, str(curr_line[3]))
self.screen.addstr(i, remote_host_start, str(curr_line[1]))
#self.screen.addstr(i, line_start, str(curr_line[4])[logline_cols])
self.screen.addstr(i, line_start, str(curr_line[4]), logline_cols)
self.curr_topline += 1
self.screen.refresh()

def main_loop(self, stdscr):
stdscr.clear()
self.load_loglines()
while True:
c = self.screen.getch()
try:
c = chr(c)
except ValueError:
continue
if c == 'd':
self.page_down()
elif c == 'u':
self.page_up()
elif c == 't':
self.top()
elif c == 'b':
self.sortby(3)
elif c == 'h':
self.sortby(1)
elif c == 's':
self.sortby(2)
elif c == 'r':
self.sortby(0)
elif c == 'q':
break

if __name__ == '__main__':
infile = sys.argv[1]
c = CursesLogViewer(infile)
curses.wrapper(c.main_loop)

In Example 11-3, we created a single class, CursesLogViewer, in order to structure our code. In the constructor, we create a curses screen and initialize a few variables. We instantiate CursesLogViewer in the “main” of our program and pass in the logfile that we want to view. We could have set an option in the application for browsing to a file and selecting it, but it would have been considerably more effort than the file browser in the PyGTK implementation of the log viewer. Besides, since users will be at a shell to run this application, it won’t be abnormal to expect them to navigate to the file from the command line and pass it in as they start the application. After instantiating CursesLogViewer, we pass its main_loop() method to the curses function wrapper(). The curses function wrapper() sets the terminal to a state that makes it ready for a curses application to use it, calls the function, then sets the terminal back to normal before returning.

The main_loop() method acts as a rudimentary event loop. It sits waiting for a user to enter input at the keyboard. When a user enters input, the loop dispatches the proper method (or at least to the proper behavior). Pressing the u or d keys will scroll up or down, respectively, by calling the page_up() or page_down() methods, respectively. The page_down() method simply calls draw_loglines(), which paints the loglines on the terminal, starting with the current top line. As each line is drawn to the screen, the current top line moves to the next log line. Since draw_loglines() only draws as many loglines as will fit on the screen, the next time it is called, it will start drawing the next log line on the top line of the screen. So, repeatedly calling draw_loglines() will have the visual effect of scrolling down through a logfile. The page_up() method will set the current top line two pages up and then redraw the loglines by calling draw_loglines(). This has the visual effect of scrolling up through a logfile. The reason that we set the current top line two pages up in page_up() is that when we draw a page, the current top line is really at the bottom of the screen. This is really set this way in anticipation of scrolling down.

The next class of behavior for our application is sorting. We have built functionality in to sort by hostname, status, and number of bytes sent in a request. Invoking any of the sort behaviors results in a call to sortby(). The sortby() method sorts the loglines list for our CursesLogViewer object on the specified field, and then calls the top() method. The top() method sets the current top line to the first line in the loglines list, and then draws the next page of loglines (which will be the first page).

The final event handler for our application is quit. The quit method simply breaks out of the “event loop” and lets the main_loop() method return to the curses wrapper() function for further terminal cleanup.

While the number of lines of code for the PyGTK app and the curses app are comparable, the curses app felt like more work. Perhaps it was having to create our own event loop. Or perhaps it was having to, in a sense, create our own widgets. Or perhaps it was “painting” text directly on the terminal screen that made it feel like more work. However, there are times when knowing how to put together a curses app will benefit you.

Figure 11-3 shows the curses log viewer sorting records by bytes transferred.

Figure 11-3. Apache log listing

One improvement we could have made on this application is the ability to reverse the sort order of whichever sort method is currently active. This would be a very simple change to make, but we’ll leave that to the reader. Another improvement would be to view the entire contents of a log line as we scroll past it. This should also be a moderately simple change to make, but we’ll leave it as an exercise for the reader as well.

Web Applications

To say that the Web is huge is an understatement. The Web is teeming with applications that people rely on daily. Why are there so many applications available on the Web? First, a web application is potentially universally accessible. This means that when a web application is deployed, anyone with access to it can just point their browser at a URL and use it. Users don’t have to download and install anything except for the browser (which they likely already have installed) unless you are using browser plug-ins like Flash. The primary appeal of this point is for the user. Second, web applications are potentially unilaterally upgradeable for the whole user base. This means that one party (the owner of the application) can upgrade the entire user base without the other party (the user) having to do anything. This is really only true when you are not relying on features that may not be in the user’s current environment. For example, if your upgrade relies on a feature in a newer version of Flash than what the current user base is required to install, this benefit may fly right out the window. But, when it works, this point is appealing to both parties, although the users are less likely to be as conscious of it. Third, the browser is pretty much a universal deployment platform. There are some cross-browser compatibility issues, but for the most part, if you are not using special plug-ins, a web application that works in one browser on one operating system will mostly work in another browser on another operating system. This point is appealing to both parties as well. Just a little more work on the development side will get the application working in multiple browser environments. And the user enjoys using the application where he chooses.

So how is this relevant for you as a system administrator? All the reasons that we have posited regarding building GUIs in general apply to building web applications. One benefit of web applications for system administrators is that the web application can have access to the filesystem and process table of the machine on which it runs. This particular property of web applications makes a web application an excellent solution for system, application, and user monitoring and reporting mechanisms. And that class of problems is in the domain of the system administrator.

Hopefully, you can see the benefit, though it may be useful for you only occasionally, of building a web application for yourself or your users. But what can you use to build a web application? Since this is a book on Python, we will, of course, recommend a Python solution. But which one? One of the criticisms of Python is that it has as many different web application frameworks as a year has days. At the moment, the four dominant choices are TurboGears, Django, Pylons, and Zope. Each of these four has its own benefits, but we felt that Django fit the subject of this book particularly well.

Django

Django is a full-stack web application framework. It contains a templating system, database connectivity by way of an object-relational mapper, and, of course, Python itself for writing the logic pieces of the application. Related to being a “full stack” framework, Django also follows a Model-View-Template (MVT) approach. This Model-View-Template approach is similar, if not identical, to a common approach called Model-View-Controller (MVC). Both are ways of developing applications so that the pieces of the application are not unnecessarily comingled. The database code is separated into an area referred to in both approaches as the “model.” The business logic is separated into an area referred to as the “view” in MVT and the “controller” in MVC. And the presentation is separated into an area referred to as the “template” in MVT and the “view” in MVC.

Apache Log Viewer Application

In the following example, which consists of several pieces of code, we will create another implementation of the Apache log viewer similar to the PyGTK implementation. Since we are going to be opening logfiles to allow a user to view and sort them, we really won’t need a database, so this example is devoid of any database connectivity. Before we walk through the example code, we will show you how to set up a project and application in Django.

You can download the Django code from http://www.djangoproject.com/. At the time of this writing, the latest release was 0.96. The recommended version to install, however, is from the development trunk. Once you’ve downloaded it, just install with the normal python setup.py install command. After installation, you will have the Django libraries in your site-packages directory and a script django-admin.py in your scripts directory. Typically, on *nix systems, the scripts directory will be the same directory that your python executable file lives in.

After installing Django, you need to create a project and an application. Projects contain one or more applications. They also act as the center for configuration for the overall web application (not to be confused with the Django application) that you are building. Django applications are smaller pieces of functionality that can be reused in different projects. For our Apache log viewing application, we created a project called “dj_apache” by running django-admin.py startproject dj_apache. This step created a directory and a handful of files. Example 11-4 is a tree view of the new project.

Example 11-4. Tree view of a Django project

jmjones@dinkbuntu:~/code$ tree dj_apache
dj_apache
|-- __init__.py
|-- manage.py
|-- settings.py
`-- urls.py

0 directories, 4 files

Now that we have a project, we can give it an application. We first navigate into the dj_apache directory, and then create an application with django-admin.py startapp logview. This will create a logview directory in our dj_apache directory and a few files. Example 11-5 is a tree view of all the files and directories we now have.

Example 11-5. Tree view of a Django application

jmjones@dinkbuntu:~/tmp$ tree dj_apache/
dj_apache/
|-- __init__.py
|-- logview
|   |-- __init__.py
|   |-- models.py
|   `-- views.py
|-- manage.py
|-- settings.py
`-- urls.py

You can see that the application directory (logview) contains models.py and views.py. Django follows the MVT convention, so these files help break the overall application up into its corresponding components. The file models.py contains the database layout, so it falls into the model component of the MVT acronym. The views.py contains the logic behind the application, so it falls into the view component of the acronym.

That leaves us without the template component of the acronym. The template component contains the presentation layer of the overall application. There are a few ways we can get Django to see our templates, but for Example 11-6, we will create a templates directory under the logview directory.

Example 11-6. Adding a templates directory

jmjones@dinkbuntu:~/code$ mkdir dj_apache/logview/templates
jmjones@dinkbuntu:~/code$ tree dj_apache/
dj_apache/
|-- __init__.py
|-- logview
|   |-- __init__.py
|   |-- models.py
|   |-- templates
|   `-- views.py
|-- manage.py
|-- settings.py
`-- urls.py

2 directories, 7 files

Now, we are ready to start fleshing out the application. The first thing we will do is decide how we want our URLs to work. This is a pretty basic application, so the URLs will be pretty straightforward. We want to list the logfiles and view them. Since our functionality is so simple and limited, we will let “/” list the logfiles to open and "/viewlog/some_sort_method/some_log_file" view the specified logfile using the specified sort method. In order to associate a URL with some activity, we have to update the urls.py file in the project top-level directory. Example 11-7 is the urls.py for our log viewer application.

Example 11-7. Django URL config (urls.py)

from django.conf.urls.defaults import *

urlpatterns = patterns('',
(r'^$', 'dj_apache.logview.views.list_files'),
(r'^viewlog/(?P<sortmethod>.*?)/(?P<filename>.*?)/$',
'dj_apache.logview.views.view_log'),
)

The URL config file is pretty clear and fairly simple to figure out. This config file relies heavily on regular expressions to map URLs that match a given regular expression to a view function that exactly matches a string. We are mapping the URL “/” to the function "dj_apache.logview.views.list_files". We are also mapping all URLs matching the regular expression '^viewlog/(?P<sortmethod>.*?)/(?P<filename>.*?)/$' to the view function "dj_apache.logview.views.view_log". When a browser connects to a Django application and sends a request for a certain resource, Django looks through urls.py for an item whose regular expression matches the URL, then dispatches the request to the matching view function.

The source file in Example 11-8 contains both of the view functions for this application along with a utility function.

Example 11-8. Django view module (views.py)

# Create your views here.

from django.shortcuts import render_to_response

import os
from apache_log_parser_regex import dictify_logline
import operator

log_dir = '/var/log/apache2'

def get_log_dict(logline):
l = dictify_logline(logline)
try:
l['bytes_sent'] = int(l['bytes_sent'])
except ValueError:
bytes_sent = 0
l['logline'] = logline
return l

def list_files(request):
file_list = [f for f in os.listdir(log_dir) if
os.path.isfile(os.path.join(log_dir, f))]
return render_to_response('list_files.html', {'file_list': file_list})

def view_log(request, sortmethod, filename):
logfile = open(os.path.join(log_dir, filename), 'r')
loglines = [get_log_dict(l) for l in logfile]
logfile.close()
try:
loglines.sort(key=operator.itemgetter(sortmethod))
except KeyError:
pass
return render_to_response('view_logfile.html', {'loglines': loglines,
'filename': filename})

The list_files() function lists all files in the directory specified by log_dir and passes that list to the list_files.html template. That’s really all that happens in list_files(). This function is configurable by changing the value of log_dir. Another option for configuring this is to put the log directory in the database somehow. If we put the value of the log directory in the database, we could change the value without having to restart the application.

The view_log() function accepts as arguments the sort method and the logfile name. Both of these parameters were extracted from the URL by way of regular expression in the urls.py file. We named the regular expression groups for the sort method and filename in urls.py, but we didn’t have to. Arguments are passed into the view function from the URL in the same sequence that they are found in their respective groups. It is good practice, though, to use named groups in the URL regular expression so you can easily tell what parameters you are extracting from a URL as well as what a URL should look like.

The view_log() function opens the logfile whose filename comes in from the URL. It then uses the Apache log parsing library from earlier examples to convert each log line into a tuple in the format of status, remote host, bytes_sent, and the log line itself. Then view_log() sorts the list of tuples based on the sort method that was passed in from the URL. Finally, view_log() passes this list into the view_logfile.html template for formatting.

The only thing left is to create the templates that we have told the view functions to render to. In Django, templates can inherit from other templates, thereby improving code reuse and making it simple to establish a uniform look and feel among pages. The first template we’ll build is a template the two other templates will inherit from. This template will set a common look and feel for the other two templates in the application. That’s why we are starting with it. This is base.html. See Example 11-9.

Example 11-9. Django base template (base.html)

<html>
<head>
<title>{% block title %}Apache Logviewer - File Listing{% endblock %}</title>
</head>
<body>
<div><a href="/">Log Directory</a></div>
{% block content %}Empty Content Block{% endblock %}
</body>
</html>

This is a very simple base template. It is perhaps the simplest HTML page you can get. The only items of interest are the two “block” sections: “content” and “title.” When you define a “block” section in a parent template, a child template can override the parent block with its own content. This allows you to set default content on a part of a page and allow the child template to override that default. The “title” block allows the child pages to set a value which will show up in their page’s title tag. The “content” block is a common convention for updating the “main” section of a page while allowing the rest of the page to remain unchanged.

Example 11-10 is a template that will simply list the files in the specified directory.

Example 11-10. Django file listing template (list_files.html)

{% extends "base.html" %}

{% block title %}Apache Logviewer - File Listing{% endblock %}

{% block content %}
<ul>
{% for f in file_list %}
<li><a href="/viewlog/linesort/{{ f }}/" >{{ f }}</a></li>
{% endfor %}
</ul>
{% endblock %}

Figure 11-4 shows what the file listing page looks like.

Figure 11-4. Apache log listing

In this template, we state that we are extending “base.html.” This allows us to get everything defined in “base.html” and plug in code into any defined code blocks and override their behavior. We do exactly that with the “title” and “content” blocks. In the “content” block, we loop over a variable file_list that was passed into the template. For each item in file_list, we create a link that will result in opening and parsing the logfile.

The template in Example 11-11 is responsible for creating the pages that the link in the previous Example 11-10 takes the user to. It displays the detail of the specified logfile.

Example 11-11. Django file listing template (view_log file.html)

{% extends "base.html" %}

{% block title %}Apache Logviewer - File Viewer{% endblock %}

{% block content %}
<table border="1">
<tr>
<td><a href="/viewlog/status/{{ filename }}/">Status</a></td>
<td><a href="/viewlog/remote_host/{{ filename }}/">Remote Host</a></td>
<td><a href="/viewlog/bytes_sent/{{ filename }}/">Bytes Sent</a></td>
<td><a href="/viewlog/linesort/{{ filename }}/">Line</a></td>
</tr>
{% for l in loglines %}
<tr>
<td>{{ l.status }}</td>
<td>{{ l.remote_host }}</td>
<td>{{ l.bytes_sent }}</td>
<td><pre>{{ l.logline }}</pre></td>
</tr>
{% endfor %}
</table>
{% endblock %}

The template in Example 11-11 inherits from the base template mentioned earlier and creates a table in the “content” area. The table header details the contents of each column: status, remote host, bytes sent, and the log line itself. In addition to detailing the column contents, the header allows users to specify how to sort the logfile. For example, if a user clicks on the “Bytes Sent” column header (which is simply a link), the page will reload and the code in the view will sort the loglines by the “bytes sent” column. Clicking on any column header except for “Line” will sort the loglines by that column in ascending order. Clicking on “Line” will put the loglines back in their original order.

Figure 11-5 shows the application viewed in Line order, and Figure 11-6 shows the application viewed in Bytes Sent order.

Figure 11-5. Django Apache log viewer—line order

Figure 11-6. Django Apache log viewer—bytes sent order

This was a very simple web application built using Django. And actually, this is a pretty atypical application as well. Most Django applications are going to be connected to a database of some sort. Improvements that could have been made include sorting all fields in reverse order, filtering loglines based on a specific status code or remote host, filtering loglines based on greater than or less than criteria for bytes sent, combining filters with one another, and putting AJAXy touches on it. Rather than walking through any of those improvements, we’ll just leave that as an exercise for the willing reader.

Simple Database Application

We mentioned that the previous Django example varied from the norm of Django applications in that it did not use a database. While the following example will be more in line with how people are using Django, the focus will be slightly different. When people build a Django application that connects to a database, they often write templates to display data from a database, as well as forms to validate and process user input. This example will show how to create a database model using Django’s object-relational mappers and how to write templates and views to display that data, but the data entry will rely on Django’s built-in admin interface. The purpose of taking this approach is to show you how quickly and easily you can put together a database with a usable frontend to enter and maintain the data.

The application that we are going to walk through creating is an inventory management app for computer systems. Specifically, this application is geared to allow you to add computers to the database with a description of the computer, associate IP addresses with it, state what services are running on it, detail what hardware constitutes the server, and more.

We’ll follow the same steps to create this Django project and application as in the previous Django example. Following are the commands to create the project and the application using the django-admin command-line tool:

jmjones@dinkbuntu:~/code$ django-admin startproject sysmanage
jmjones@dinkbuntu:~/code$ cd sysmanage
jmjones@dinkbuntu:~/code/sysmanage$ django-admin startapp inventory
jmjones@dinkbuntu:~/code/sysmanage$

This created the same sort of directory structure as our Django-based Apache log viewer. Following is a tree view of the directories and files that were created:

jmjones@dinkbuntu:~/code/sysmanage$ cd ../
jmjones@dinkbuntu:~/code$ tree sysmanage/
sysmanage/
|-- __init__.py
|-- inventory
|   |-- __init__.py
|   |-- models.py
|   `-- views.py
|-- manage.py
|-- settings.py
`-- urls.py

After creating the project and app, we need to configure the database we want to connect to. SQLite is a great option, especially if you are testing or developing an app and not rolling it out to production. If more than a few people were going to be hitting the application, we would recommend considering a more robust database such as PostgreSQL. In order to configure the application to use a SQLite database, we change a couple of lines in the settings.py file in the project main directory. Here are the lines we change to configure the database:

DATABASE_ENGINE = 'sqlite3'
DATABASE_NAME = os.path.join(os.path.dirname(__file__), 'dev.db')

We set “sqlite3” as our database engine. The line configuring the location of the database (the DATABASE_NAME option) does something worth noting. Rather than specifying an absolute path to the database file, we configure the database such that it will always be in the same directory as the settings.py file. __file__ holds the absolute path to the settings.py file. Calling os.path.dirname(__file__) gives us the directory that the settings.py file is in. Passing the directory that the file is in and the name of the database file we want to create to os.path.join() will give us the absolute path of the database file that is resilient to the application living in different directories. This is a useful idiom to get into the habit of using for your settings files.

In addition to configuring our database, we need to include the Django admin interface and our inventory application among the applications for this project. Here is the relevant portion of the settings.py file:

INSTALLED_APPS = (
'django.contrib.admin',
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.sites',
'sysmanage.inventory',
)

We added the django.contrib.admin and sysmanage.inventory to the list of installed apps. This means that when we tell Django to create the database for us, it will create tables for all included projects.

Next, we will change the URL mapping so that the this project includes the admin interface. Here is the relevant line from the URL config file:

# Uncomment this for admin:
(r'^admin/', include('django.contrib.admin.urls')),

The tool that created the urls.py created it with a line to include the admin interface, but the line needs to be uncommented. You can see that we have simply removed the # character from the beginning of the line to include the admin URLs config file.

Now that we have configured a database, added the admin and inventory applications, and added the admin interface to the URLs config file, we are ready to start defining the database schema. In Django, each application has its own schema definition. In each application directory, “inventory” in this case, there is a file named models.py that contains definitions for the tables and columns that your application will use. With Django, as well as many other web frameworks that rely on ORMs, it is possible to create and use a database without having to write a single SQL expression. Django’s ORM turns classes into tables and class attributes into columns on those tables. For example, following is a piece of code that defines a table definition in the configured database (this piece of code is part of the larger example that we’ll get into shortly):

class HardwareComponent(models.Model):
manufacturer = models.CharField(max_length=50)
#types include video card, network card...
type = models.CharField(max_length=50)
model = models.CharField(max_length=50, blank=True, null=True)
vendor_part_number = models.CharField(max_length=50, blank=True, null=True)
description = models.TextField(blank=True, null=True)

Notice that the HardwareComponent class inherits from a Django model class. This means that the HardwareComponent class is of the Model type and will behave appropriately. We have given our hardware component a number of attributes: manufacturer, type, model, vendor_part_number, and description. Those attributes are coming from Django. Not that Django supplies some listing of hardware manufacturers, but it does provide the CharField type.

This class definition in the inventory application will create an inventory_hardwarecomponent table with six columns: id, manufacturer, type, model, vendor_part_number, and description. This mostly corresponds with the class definition for the ORM. Actually, it consistently corresponds to the class definition for the ORM. When you define a model class, Django will create a corresponding table the name of which is the application name (lowercased), followed by an underscore, followed by the lowercased class name. Also, if you do not specify otherwise, Django will create an id column on your table that will act as the primary key. Following is the SQL table creation code that corresponds to the HardwareComponent model:

CREATE TABLE "inventory_hardwarecomponent" (
"id" integer NOT NULL PRIMARY KEY,
"manufacturer" varchar(50) NOT NULL,
"type" varchar(50) NOT NULL,
"model" varchar(50) NULL,
"vendor_part_number" varchar(50) NULL,
"description" text NULL
)

If you ever want to see the SQL that Django uses to create your database, simply run, in your project directory, python manage.py sql myapp, where myapp corresponds to the name of your application.

Now that you have been exposed to Django’s ORM, we’ll walk through creating the database model for our system inventory application. Example 11-12 is the model.py for the inventory application.

Example 11-12. Database layout (models.py)

from django.db import models

# Create your models here.

class OperatingSystem(models.Model):
name = models.CharField(max_length=50)
description = models.TextField(blank=True, null=True)

def __str__(self):
return self.name

class Admin:
pass

class Service(models.Model):
name = models.CharField(max_length=50)
description = models.TextField(blank=True, null=True)

def __str__(self):
return self.name

class Admin:
pass

class HardwareComponent(models.Model):
manufacturer = models.CharField(max_length=50)
#types include video card, network card...
type = models.CharField(max_length=50)
model = models.CharField(max_length=50, blank=True, null=True)
vendor_part_number = models.CharField(max_length=50, blank=True, null=True)
description = models.TextField(blank=True, null=True)

def __str__(self):
return self.manufacturer

class Admin:
pass

class Server(models.Model):
name = models.CharField(max_length=50)
description = models.TextField(blank=True, null=True)
os = models.ForeignKey(OperatingSystem)
services = models.ManyToManyField(Service)
hardware_component = models.ManyToManyField(HardwareComponent)

def __str__(self):
return self.name

class Admin:
pass

class IPAddress(models.Model):
address = models.TextField(blank=True, null=True)
server = models.ForeignKey(Server)

def __str__(self):
return self.address

class Admin:
pass

We defined five classes for our model: OperatingSystem, Service, HardwareComponent, Server, and IPAddress. The OperatingSystem class will allow us to define, as needed, different operating systems for the servers in which we are taking inventory. We defined this class with a name and description attribute, which is all we really need. It would be better to create an OperatingSystemVendor class and link to it from the OperatingSystem class, but in the interest of simplicity and explicability, we will leave the vendor relation out of it. Each server will have one operating system. We will show you that relationship when we get to the Server.

The Service class allows us to list all potential services that can run on a server. Examples include Apache web server, Postfix mail server, Bind DNS server, and OpenSSH server. As with the OperatingSystem class, this class holds a name and a description attribute. Each server may have many services. We will show you how these classes relate to one another in the Server class.

The HardwareComponent class represents a list of all hardware components that our servers may contain. This will only be interesting if you have either added hardware to the system your vendor supplied you with or if you built your own server from individual components. We defined five attributes for HardwareComponent: manufacturer, type, model, vendor_part_number, and description. As with the vendor for OperatingSystem, we could have created other classes for the hardware manufacturer and type and created relationships to them. But, again, for the sake of simplicity, we chose not to create those relationships.

The Server class is the heart of this inventory system. Each Server instance is a single server that we are tracking. Server is where we tie everything together by establishing relationships to the three previous classes. First of all, we have given each Server a name and description attribute. These are identical to the attributes that we have given the other classes. In order to link to the other classes, we had to specify what kind of relationship Server had to them. Each Server will have only one operating system, so we created a foreign key relationship to OperatingSystem. As virtualization becomes more common, this type of relationship will make less sense, but for now, it serves its purpose. A server may have many services running on it and each type of service may run on many servers, so we created a many to many relationship between Server and Service. Likewise, each server may have many hardware components and each type of hardware component may exist on multiple servers. Therefore, we created another many to many relationship from Server to HardwareComponent.

Finally, IPAddress is a listing of all IP addresses on all servers that we are tracking. We listed this model last to emphasize the relationship that IP addresses have with servers. We gave IPAddress one attribute and one relationship. The address is the attribute and should by convention be in the XXX.XXX.XXX.XXX format. We created a foreign key relationship from IPAddress to Server because one IP address should belong to only one server. Yes, again, this is simplistic, but it serves the purpose of demonstrating how to establish relationships between data components in Django.

Now we are ready to create the sqlite database file. Running python manage.py syncdb in your project directory will create any uncreated tables for all applications you included in your settings.py file. It will also prompt you to create a superuser if it creates the auth tables. Following is the (truncated) output from running python manage.py syncdb:

jmjones@dinkbuntu:~/code/sysmanage$ python manage.py syncdb
Creating table django_admin_log
Creating table auth_message
. . .
Creating many-to-many tables for Server model
Adding permission 'log entry | Can add log entry'
Adding permission 'log entry | Can change log entry'
Adding permission 'log entry | Can delete log entry'

You just installed Django's auth system, which means you don't have any
superusers defined.
Would you like to create one now? (yes/no): yes
Username (Leave blank to use 'jmjones'): E-mail address: none@none.com
Password:
Password (again): Superuser created successfully.
Adding permission 'message | Can add message'
. . .
Adding permission 'service | Can change service'
Adding permission 'service | Can delete service'
Adding permission 'server | Can add server'
Adding permission 'server | Can change server'
Adding permission 'server | Can delete server'

We are now ready to start the Django development server and explore the admin interface. Following is the command to start the Django development server and the output that command generates:

jmjones@dinkbuntu:~/code/sysmanage$ python manage.py runserver 0.0.0.0:8080
Validating models...
0 errors found

Django version 0.97-pre-SVN-unknown, using settings 'sysmanage.settings'
Development server is running at http://0.0.0.0:8080/
Quit the server with CONTROL-C.

Figure 11-7 shows the login form. Once we log in, we can add servers, hardware, operating systems, and the like. Figure 11-8 shows the Django admin main page and Figure 11-9 shows the “add hardware” form. There is benefit to having a database tool to store and display your data in a consistent, simple, usable manner. Django does a fantastic job of providing a simple, usable interface to a set of data. And if that is all that it did, it would be a useful tool. But that’s just the start of what Django can do. If you can think of a way that a browser can display data, you can very likely get Django to do it. And it is typically not very difficult.

Figure 11-7. Django admin login

For example, if we wanted one page with every type of operating system, hardware component, service, etc., we could do it. And if we wanted to be able to click on each one of those individual items and display a page containing nothing but servers with those individual characteristics, we could do that, too. And if we wanted to be able to click on each one of those servers in the list and have it display detailed information about the server, we could do that as well. Actually, let’s do that. We’ll use those “suggestions” for requirements that we will go by for this application.

Figure 11-8. Django admin main page

First, Example 11-13 is an updated urls.py.

Example 11-13. URL mapping (urls.py)

from django.conf.urls.defaults import *

urlpatterns = patterns('',
# Example:
# (r'^sysmanage/', include('sysmanage.foo.urls')),

# Uncomment this for admin:
(r'^admin/', include('django.contrib.admin.urls')),
(r'^$', 'sysmanage.inventory.views.main'),
(r'^categorized/(?P<category>.*?)/(?P<category_id>.*?)/$',
'sysmanage.inventory.views.categorized'),
(r'^server_detail/(?P<server_id>.*?)/$',
'sysmanage.inventory.views.server_detail'),
)

We added three new lines mapping non-admin URLs to functions. There is really nothing different to see here from what was in the Apache log viewer app. We are mapping regular expressions of URLs to functions and using a little bit of regular expression groupings as well.

Figure 11-9. Django admin add hardware component

The next thing we will do is to add functions to the views module that we declared in the URL mapping file. Example 11-14 is the views module.

Example 11-14. Inventory views (views.py)

# Create your views here.

from django.shortcuts import render_to_response
import models

def main(request):
os_list = models.OperatingSystem.objects.all()
svc_list = models.Service.objects.all()
hardware_list = models.HardwareComponent.objects.all()
return render_to_response('main.html', {'os_list': os_list,
'svc_list': svc_list, 'hardware_list': hardware_list})

def categorized(request, category, category_id):
category_dict = {'os': 'Operating System',
'svc': 'Service', 'hw': 'Hardware'}
if category == 'os':
server_list = models.Server.objects.filter(os__exact=category_id)
category_name = models.OperatingSystem.objects.get(id=category_id)
elif category == 'svc':
server_list = \
models.Server.objects.filter(services__exact=category_id)
category_name = models.Service.objects.get(id=category_id)
elif category == 'hw':
server_list = \
models.Server.objects.filter(hardware_component__exact=category_id)
category_name = models.HardwareComponent.objects.get(id=category_id)
else:
server_list = []
return render_to_response('categorized.html', {'server_list': server_list,
'category': category_dict[category], 'category_name': category_name})

def server_detail(request, server_id):
server = models.Server.objects.get(id=server_id)
return render_to_response('server_detail.html', {'server': server})

Just as we added three URL mappings to the urls.py file, so we also added three functions to the views.py file. The first is main(). This function simply takes a list of all the different OSes, hardware components, and services and passes them into the main.html template.

In Example 11-14, we created a templates directory in the application folder. We will do the same thing here:

jmjones@dinkbuntu:~/code/sysmanage/inventory$ mkdir templates
jmjones@dinkbuntu:~/code/sysmanage/inventory$

Example 11-15 is the “main.html” template that the main() view function is passing data into.

Example 11-15. Main Template (main.html)

{% extends "base.html" %}

{% block title %}Server Inventory Category View{% endblock %}

{% block content %}
<div>
<h2>Operating Systems</h2>
<ul>
{% for o in os_list %}
<li><a href="/categorized/os/{{ o.id }}/" >{{ o.name }}</a></li>
{% endfor %}
</ul>
</div>
<div>
<h2>Services</h2>
<ul>
{% for s in svc_list %}
<li><a href="/categorized/svc/{{ s.id }}/" >{{ s.name }}</a></li>
{% endfor %}
</ul>
</div>
<div>
<h2>Hardware Components</h2>
<ul>
{% for h in hardware_list %}
<li><a href="/categorized/hw/{{ h.id }}/" >{{ h.manufacturer }}</a></li>
{% endfor %}
</ul>
</div>
{% endblock %}

This template is pretty straightforward. It divides up the page into three parts, one for each category that we want to see. For each category, it itemizes the entries that the category has along with a link to see all servers that have the specified category item.

When a user clicks on one of those links, it will take them to the next view function, categorized().

The main template passes a category (being one of os for Operating System, hw for Hardware Component, and svc for Service) and a category ID (i.e., the specific component that the user clicked on, such as “3Com 905b Network Card”) into the categorized() view function. The categorized() function takes these arguments and retrieves a list of all servers from the database that have the selected component. After querying the database for the proper information, the categorized() function passes its information on to the “categorized.html” template. Example 11-16 shows the contents of the “categorized.html” template.

Example 11-16. Categorized Template (categorized.html)

{% extends "base.html" %}

{% block title %}Server List{% endblock %}

{% block content %}
<h1>{{ category }}::{{ category_name }}</h1>
<div>
<ul>
{% for s in server_list %}
<li><a href="/server_detail/{{ s.id }}/" >{{ s.name }}</a></li>
{% endfor %}
</ul>
</div>
{% endblock %}

The “categorized.html” template displays a list of all the servers that categorized() passed in to it.

The user can then click on a link to individual servers, which will take her to the server_detail() view function. The server_detail() view function takes a server id parameter, retrieves data about that server from the database, and passes that data on to the “server_detail.html” template.

The “server_detail.html” template shown in Example 11-17 is perhaps the longest of the templates, but it is very simple. Its job is to display the individual pieces of data for the server, such as what OS the server is running, what pieces of hardware the server has, what services are running on the server, and what IP addresses the server has.

Example 11-17. Server detail template (server_detail.html)

{% extends "base.html" %}

{% block title %}Server Detail{% endblock %}

{% block content %}
<div>
Name: {{ server.name }}
</div>
<div>
Description: {{ server.description }}
</div>
<div>
OS: {{ server.os.name }}
</div>
<div>
<div>Services:</div>
<ul>
{% for service in server.services.all %}
<li>{{ service.name }}</li>
{% endfor %}
</ul>
</div>
<div>
<div>Hardware:</div>
<ul>
{% for hw in server.hardware_component.all %}
<li>{{ hw.manufacturer }} {{ hw.type }} {{ hw.model }}</li>
{% endfor %}
</ul>
</div>
<div>
<div>IP Addresses:</div>
<ul>
{% for ip in server.ipaddress_set.all %}
<li>{{ ip.address }}</li>
{% endfor %}
</ul>
</div>
{% endblock %}

And that is an example of how to build a pretty simple database application using Django. The admin interface provides a friendly means of populating the database and with just a few more lines of code, we were able to create custom views of sorting and navigating the data, as shown in Figures 11-10, 11-11, and 11-12.

Figure 11-10. System management application main page

Figure 11-11. System management application CentOS category

Figure 11-12. System management application server detail

Conclusion

While building GUI applications doesn’t seem to fit the traditional responsibilities of a system administrator, it can prove to be an invaluable skill. Sometimes, you may need to build some simple application for one of your users. Other times, you may need to build a simple application for yourself. Still other times, you may realize that you don’t need it, but it might make some task go along just a little bit more smoothly. Once you’re comfortable building GUI applications, you may be surprised at how often you find yourself building them.

Chapter 12. Data Persistence

Data persistence, in a simple, generic sense, is saving data for later use. This implies that the data, once saved for later, will survive if the process that saved it terminates. This is typically accomplished by converting the data to some format and then writing that data to disk. Sometimes, the format is human readable, such as XML or YAML. Other times, the format is not usable directly by humans, such as a Berkeley DB file (bdb) or a SQLite database.

What kind of data might you need to save for later? Perhaps you have a script that keeps track of the last modified date of the files in a directory and you need to run it occasionally to see which files have changed since the last time you ran it. The data about the files is something you want to save for later, where later is the next time you run the script. You could store this data in some kind of persistent data file. In another scenario, you have one machine that has potential network issues and you decide to run a script every 15 minutes to see how quickly it pings a number of other machines on the network. You could store the ping times in a persistent data file for later use. Later in this case has more to do with when you plan on examining the data, rather than when the program that gathered the data needs access to it.

We will be breaking this discussion of serialization into two categories: simple and relational.

Simple Serialization

There are a number of ways of storing data to disk for later use. We are calling “simple serialization” the process of saving data to disk without saving the relationships between the pieces of data. We’ll discuss the difference between simple and relational in the relational section.

Pickle

The first, and perhaps the most basic “simple serialization” mechanism for Python is the standard library pickle module. If you think of pickling in the agricultural or culinary sense, the idea is to preserve a food item, put it into a jar, and use it later. The culinary concept translates nicely to what happens with the pickle module. With the pickle module, you take an object, write it to disk, exit your Python process, come back later, start your Python process again, read your object back from disk, and then interact with it.

What can you pickle? Here is a list taken from the Python Standard Library documentation on pickle that lists types of objects that are pickleable:

- None, true, and false

- Integers, long integers, floating-point numbers, complex numbers

- Normal and Unicode strings

- Tuples, lists, sets, and dictionaries containing only pickleable objects

- Functions defined at the top level of a module

- Built-in functions defined at the top level of a module

- Classes that are defined at the top level of a module

- Instances of such classes whose __dict__ or __setstate__() is pickleable

Here is how to serialize your object to disk using the pickle module:

In [1]: import pickle

In [2]: some_dict = {'a': 1, 'b': 2}

In [3]: pickle_file = open('some_dict.pkl', 'w')

In [4]: pickle.dump(some_dict, pickle_file)

In [5]: pickle_file.close()

And here is what the pickled file looks like:

jmjones@dinkgutsy:~$ ls -l some_dict.pkl
-rw-r--r-- 1 jmjones jmjones 30 2008-01-20 07:13 some_dict.pkl
jmjones@dinkgutsy:~$ cat some_dict.pkl
(dp0
S'a'
p1
I1
sS'b'
p2
I2

You could learn the pickle file format and create one manually, but we wouldn’t recommend it.

Here is how to unpickle a pickle file:

In [1]: import pickle

In [2]: pickle_file = open('some_dict.pkl', 'r')

In [3]: another_name_for_some_dict = pickle.load(pickle_file)

In [4]: another_name_for_some_dict
Out[4]: {'a': 1, 'b': 2}

Notice that we didn’t name the object that we unpickled the same thing that we named it before it was pickled. Remember that a name is just a way of referring to an object.

It’s interesting to note that there need not be a one-to-one relationship between your objects and your pickle files. You can dump as many objects to a single pickle file as you have hard drive space for or your filesystem allows, whichever comes first. Here is an example of dumping a number of dictionary objects to a single pickle file:

In [1]: list_of_dicts = [{str(i): i} for i in range(5)]

In [2]: list_of_dicts
Out[2]: [{'0': 0}, {'1': 1}, {'2': 2}, {'3': 3}, {'4': 4}]

In [3]: import pickle

In [4]: pickle_file = open('list_of_dicts.pkl', 'w')

In [5]: for d in list_of_dicts:
...:     pickle.dump(d, pickle_file)
...:
...:

In [6]: pickle_file.close()

We created a list of dictionaries, created a writable file object, iterated over the list of dictionaries, and serialized each one to the pickle file. Notice that this is the exact same method that we used to write one object to a pickle file in an earlier example, only without the iterating and the multiple dump() calls.

Here is an example of unpickling and printing the objects from the pickle file that contains multiple objects:

In [1]: import pickle

In [2]: pickle_file = open('list_of_dicts.pkl', 'r')

In [3]: while 1:
...:     try:
...:         print pickle.load(pickle_file)
...:     except EOFError:
...:         print "EOF Error"
...:         break
...:
...:
{'0': 0}
{'1': 1}
{'2': 2}
{'3': 3}
{'4': 4}
EOF Error

We created a readable file object pointing at the file created in the previous example and kept trying to load a pickle object from the file until we hit an EOFError. You can see that the dictionaries that we got out of the pickle file are the same (and in the same order) as the files we stuffed into the pickle file.

Not only can we pickle simple built-in types of objects, but we can also pickle objects of types that we ourselves have created. Here is a module that we’ll use for the next two examples. This module contains a custom class that we’ll be pickling and unpickling:

#!/usr/bin/env python

class MyClass(object):
def __init__(self):
self.data = []
def __str__(self):
return "Custom Class MyClass Data:: %s" % str(self.data)
def add_item(self, item):
self.data.append(item)

Here is a module that imports the module with the custom class and pickles a custom object:

#!/usr/bin/env python

import pickle
import custom_class

my_obj = custom_class.MyClass()
my_obj.add_item(1)
my_obj.add_item(2)
my_obj.add_item(3)

pickle_file = open('custom_class.pkl', 'w')
pickle.dump(my_obj, pickle_file)
pickle_file.close()

In this example, we imported the module with the custom class, instantiated an object from the custom class, added a few items to the object, then serialized it. Running this module gives no resulting output.

Here is a module that imports the module with the custom class and then loads the custom object from the pickle file:

#!/usr/bin/env python

import pickle
import custom_class

pickle_file = open('custom_class.pkl', 'r')
my_obj = pickle.load(pickle_file)
print my_obj
pickle_file.close()

Here is the output from running the unpickling file:

jmjones@dinkgutsy:~/code$ python custom_class_unpickle.py
Custom Class MyClass Data:: [1, 2, 3]

It is not necessary for the unpickling code to explicitly import the custom class you are unpickling. However, it is necessary for the unpickling code to be able to find the module that the custom class is in. Following is a module that doesn’t import the custom class module:

#!/usr/bin/env python

import pickle
##import custom_class ##commented out import of custom class

pickle_file = open('custom_class.pkl', 'r')
my_obj = pickle.load(pickle_file)
print my_obj
pickle_file.close()

Here is the output from running the nonimporting module:

jmjones@dinkgutsy:~/code$ python custom_class_unpickle_noimport.py
Custom Class MyClass Data:: [1, 2, 3]

And here is the output from running the same module after copying it (and the pickle file) to another directory and running from there:

jmjones@dinkgutsy:~/code/cantfind$ python custom_class_unpickle_noimport.py
Traceback (most recent call last):
File "custom_class_unpickle_noimport.py", line 7, in <module>
my_obj = pickle.load(pickle_file)
File "/usr/lib/python2.5/pickle.py", line 1370, in load
return Unpickler(file).load()
File "/usr/lib/python2.5/pickle.py", line 858, in load
dispatch[key](self)
File "/usr/lib/python2.5/pickle.py", line 1090, in load_global
klass = self.find_class(module, name)
File "/usr/lib/python2.5/pickle.py", line 1124, in find_class
__import__(module)
ImportError: No module named custom_class

The last line of this traceback shows an import error because pickle failed to load our custom module. Pickle will try to find the module that your custom class is in and import it so that it can return you an object of the same type as you initially pickled.

All of the previous examples on pickle work fine, but there is an option that we haven’t mentioned yet. pickle uses the default protocol when pickling an object-like pickle.dump(object_to_pickle, pickle_file). The protocol is the format specification for how the file is serialized. The default protocol uses the almost human readable format that we showed earlier. Another protocol choice is a binary format. You may want to consider using the binary protocol if you notice that pickling your objects is taking a substantial amount of time. Here is a comparison of using the default protocol and the binary protocol:

In [1]: import pickle

In [2]: default_pickle_file = open('default.pkl', 'w')

In [3]: binary_pickle_file = open('binary.pkl', 'wb')

In [4]: d = {'a': 1}

In [5]: pickle.dump(d, default_pickle_file)

In [6]: pickle.dump(d, binary_pickle_file, -1)

In [7]: default_pickle_file.close()

In [8]: binary_pickle_file.close()

The first pickle file we created (named default.pkl) will contain the pickle data in its default nearly human-readable format. The second pickle file we created (named binary.pkl) will contain the pickle data in a binary format. Notice that we opened default.pkl in normal write mode ('w'), but we opened binary.pkl in binary writable mode ('wb'). The only difference between the call to dump between these objects is the call to the binary dump has one more argument: a -1 that signifies that the “highest” protocol, which currently is a binary protocol, will be used.

Here is a hex dump of the binary pickle file:

jmjones@dinkgutsy:~/code$ hexcat binary.pkl
00000000 - 80 02 7d 71  00 55 01 61  71 01 4b 01  73 2e        ..}q.U.aq.K.s.

And here is a hex dump of the default pickle file:

jmjones@dinkgutsy:~/code$ hexcat default.pkl
00000000 - 28 64 70 30  0a 53 27 61  27 0a 70 31  0a 49 31 0a  (dp0.S'a'.p1.I1.
00000010 - 73 2e                                               s.

That is really unnecessary since we can just cat it out and will be able to read the contents of the file. Here are the plain contents of the default pickle file:

jmjones@dinkgutsy:~/code$ cat default.pkl
(dp0
S'a'
p1
I1
s.

cPickle

In the Python Standard Library, there is another implementation of the Pickle library that you should consider using. It is called cPickle. As the name implies, cPickle was implemented in C. As with our suggestion regarding using binary files, if you notice that pickling your objects is taking a while, you may want to consider trying the cPickle module. The syntax is identical for cPickle as for “regular” pickle.

shelve

Another persistence option is the shelve module. shelve provides an easy, usable interface to object persistence that simplifies multiple object persistence. By that we mean storing multiple objects in the same persistent object store and then easily getting them back. Storing objects in the shelve persistent data store is similar to simply using a Python dictionary. Here is an example of opening a shelve file, serializing data to it, then reopening it and accessing its contents:

In [1]: import shelve

In [2]: d = shelve.open('example.s')

In [3]: d
Out[3]: {}

In [4]: d['key'] = 'some value'

In [5]: d.close()

In [6]: d2 = shelve.open('example.s')

In [7]: d2
Out[7]: {'key': 'some value'}

One difference between using shelve and using a plain dictionary is that you create a shelve object by using shelve.open() rather than instantiating the dict class or using curly braces ({}). Another difference is that with shelve, when you are done with your data, you need to call close() on the shelve object.

Shelve has a couple of tricky points. We already mentioned the first: you have to call close() when you are done with the operation you are working on. If you don’t close() your shelve object, any changes you made to it won’t be persisted. Following is an example of losing your changes by not closing your shelve object. First, we’ll just create and persist our shelve object and exit IPython:

In [1]: import shelve

In [2]: d = shelve.open('lossy.s')

In [3]: d['key'] = 'this is a key that will persist'

In [4]: d
Out[4]: {'key': 'this is a key that will persist'}

In [5]: d.close()

In [6]:
Do you really want to exit ([y]/n)?

Next, we’ll start IPython again, open the same shelve file, create another item, and exit without explicitly closing the shelve object:

In [1]: import shelve

In [2]: d = shelve.open('lossy.s')

In [3]: d
Out[3]: {'key': 'this is a key that will persist'}

In [4]: d['another_key'] = 'this is an entry that will not persist'

In [5]:
Do you really want to exit ([y]/n)?

Now, we’ll start IPython again, reopen the same shelve file, and see what we have:

In [1]: import shelve

In [2]: d = shelve.open('lossy.s')

In [3]: d
Out[3]: {'key': 'this is a key that will persist'}

So, make sure you close() any shelve objects that you have changed and whose data you would like to save.

Another tricky area is around changing mutable objects. Remember that mutable objects are objects whose value can be changed without having to reassign the value to the variable. Here, we create a shelve object, create a key that contains a mutable object (in this case, a list), change the mutable object, then close the shelve object:

In [1]: import shelve

In [2]: d = shelve.open('mutable_lossy.s')

In [3]: d['key'] = []

In [4]: d['key'].append(1)

In [5]: d.close()

In [6]:
Do you really want to exit ([y]/n)?

Since we called close() on the shelve object, we might expect that the value for 'key' is the list [1]. But we would be wrong. Here is the result of opening the previous shelve file and deserializing it:

In [1]: import shelve

In [2]: d = shelve.open('mutable_lossy.s')

In [3]: d
Out[3]: {'key': []}

This isn’t odd or unexpected behavior at all. In fact, it’s in the shelve documentation. The problem is that inline changes to persistent objects aren’t picked up by default. But there are a couple of ways to work around this behavior. One is specific and targeted, and the other is broad and all-encompassing. First, in the specific/targeted approach, you can just reassign to the shelve object like this:

In [1]: import shelve

In [2]: d = shelve.open('mutable_nonlossy.s')

In [3]: d['key'] = []

In [4]: temp_list = d['key']

In [5]: temp_list.append(1)

In [6]: d['key'] = temp_list

In [7]: d.close()

In [8]:
Do you really want to exit ([y]/n)?

When we deserialize our shelved object, here is what we get:

In [1]: import shelve

In [2]: d = shelve.open('mutable_nonlossy.s')

In [3]: d
Out[3]: {'key': [1]}

The list that we created and appended to has been preserved.

Next, the broad and all-encompassing approach: changing the writeback flag for the shelve object. The only parameter we demonstrated passing in to shelve.open() was the filename of the shelve file. There are a few other options, one of which is the writeback flag. If the writeback flag is set to True, any entries of the shelve object that have been accessed are cached in memory and then persisted when close() is called on the shelve object. This can be useful for the case of dealing with mutable objects, but it does have a trade-off. Since the accessed objects will all be cached and then persisted upon close (whether changed or not), memory usage and file sync time will grow proportionately to the number of objects you are accessing on the shelve object. So, if you have a large number of objects you are accessing on a shelve object, you may want to consider not setting the writeback flag to True.

In this next example, we will set the writeback flag to True and manipulate a list inline without reassigning it to the shelve object:

In [1]: import shelve

In [2]: d = shelve.open('mutable_nonlossy.s', writeback=True)

In [3]: d['key'] = []

In [4]: d['key'].append(1)

In [5]: d.close()

In [6]:
Do you really want to exit ([y]/n)?

Now, let’s see if our change was persisted.

In [1]: import shelve

In [2]: d = shelve.open('mutable_nonlossy.s')

In [3]: d
Out[3]: {'key': [1]}

It was persisted as we hoped it would be.

Shelve offers an easy way to work with persistent data. There are a couple of gotchas along the way, but overall, it’s a useful module.

YAML

Depending on who you ask, YAML stands for “YAML ain’t markup language” or “yet another markup language.” Either way, it is a data format that is often used to store, retrieve, and update pieces of data in a plain text layout. This data is often hierarchical. Probably the easiest way to start working with YAML in Python is to easy_install PyYAML. But why use YAML when you have to install it and pickle is built-in? There are two attractive reasons for choosing YAML over pickle. These two reasons don’t make YAML the right choice in all situations, but for certain cases, it can make a lot of sense. First, YAML is human-readable. The syntax feels similar to a config file. If you have cases where editing a config file is a good option, YAML may be a good choice for you. Second, YAML parsers have been implemented in many other languages. If you need to get data between a Python application and an application written in another language, YAML can be a good intermediary solution.

Once you easy_install PyYAML, you can serialize and deserialize YAML data. Here is an example of serializing a simple dictionary:

In [1]: import yaml

In [2]: yaml_file = open('test.yaml', 'w')

In [3]: d = {'foo': 'a', 'bar': 'b', 'bam': [1, 2,3]}

In [4]: yaml.dump(d, yaml_file, default_flow_style=False)

In [5]: yaml_file.close()

This example is pretty easy to follow, but let’s walk through it anyway. The first thing we do is import the YAML module (named yaml). Next, we create a writable file that we will later use to store the YAML in. Next, we create a dictionary (named d) that contains the data that we want to serialize. Then, we serialize the dictionary (named d) using the dump() function from the yaml module. The parameters that we pass to dump() are the dictionary that we are serializing, the YAML output file, and a parameter that tells the YAML library to write the output in block style rather than the default style, pieces of which look like a string conversion of the data object that we are serializing.

Here is what the YAML file looks like:

jmjones@dinkgutsy:~/code$ cat test.yaml
bam:
- 1
- 2
- 3
bar: b
foo: a

If we want to deserialize the file, we perform the inverse operations as what we performed in the dump() example. Here is how we get the data back out of the YAML file:

In [1]: import yaml

In [2]: yaml_file = open('test.yaml', 'r')

In [3]: yaml.load(yaml_file)
Out[3]: {'bam': [1, 2, 3], 'bar': 'b', 'foo': 'a'}

As with the dump() example, we first have to import the YAML module (yaml). Next we create a YAML file. This time, we create a readable file object from the YAML file on disk. Finally, we call the load() function from the yaml module. load() then returns back a dictionary that is equivalent to the input dictionary.

When using the yaml module, you will probably find yourself cyclically creating data, dumping it to disk, then loading it back up, and so on.

You may not need to dump your YAML data out to a human-readable format, so let’s walk through serializing the same dictionary from the previous example in non-block mode. Here is how to dump the same dictionary as before in nonblock mode:

In [1]: import yaml

In [2]: yaml_file = open('nonblock.yaml', 'w')

In [3]: d = {'foo': 'a', 'bar': 'b', 'bam': [1, 2,3]}

In [4]: yaml.dump(d, yaml_file)

In [5]: yaml_file.close()

Here is what the YAML file looks like: jmjones@dinkgutsy:~/code$ cat nonblock.yaml bam: [1, 2, 3] bar: b foo: a. This looks pretty similar to the block-mode format except for the list value for bam. The differences appear when there is some level of nesting and some array-like data structure like a list or dictionary. Let’s compare a couple of examples to show those differences. But before we do, it will be easier to walk through these examples if we don’t have to keep showing the output of catting the YAML files. The file argument in the dump() function of the yaml module is optional. (Actually, the PyYAML documentation refers to the “file” object as a “stream” object, but doesn’t really matter much.) If you leave out the “file” or “stream” argument, dump() will write the serialized object to standard out. So, in the following example, we will leave out the file object and print out the YAML results.

Here is a comparison of a few data structures using the block style serialization and non-block style serialization. The examples that have default_flow_style use the block formatting and the examples that don’t have default_flow_style do not use the block formatting:

In [1]: import yaml

In [2]: d = {'first': {'second': {'third': {'fourth': 'a'}}}}

In [3]: print yaml.dump(d, default_flow_style=False)
first:
second:
third:
fourth: a

In [4]: print yaml.dump(d)
first:
second:
third: {fourth: a}

In [5]: d2 = [{'a': 'a'}, {'b': 'b'}, {'c': 'c'}]

In [6]: print yaml.dump(d2, default_flow_style=False)
- a: a
- b: b
- c: c

In [7]: print yaml.dump(d2)
- {a: a}
- {b: b}
- {c: c}

In [8]: d3 = [{'a': 'a'}, {'b': 'b'}, {'c': [1, 2, 3, 4, 5]}]

In [9]: print yaml.dump(d3, default_flow_style=False)
- a: a
- b: b
- c:
- 1
- 2
- 3
- 4
- 5

In [10]: print yaml.dump(d3)
- {a: a}
- {b: b}
- c: [1, 2, 3, 4, 5]

What if you want to serialize a custom class? The yaml module behaves nearly identically to the pickle regarding custom classes. The following example will even use the same custom_class module that we used in the pickle custom_class example.

Here is a Python module that imports the custom_class module, creates an object from MyClass, adds some items to the object, and then serializes it:

#!/usr/bin/env python

import yaml
import custom_class

my_obj = custom_class.MyClass()
my_obj.add_item(1)
my_obj.add_item(2)
my_obj.add_item(3)

yaml_file = open('custom_class.yaml', 'w')
yaml.dump(my_obj, yaml_file)
yaml_file.close()

When we run the previous module, here is the output we see:

jmjones@dinkgutsy:~/code$ python custom_class_yaml.py
jmjones@dinkgutsy:~/code$

Nothing. That means that everthing went well.

Here is the inverse of the previous module:

#!/usr/bin/env python

import yaml
import custom_class

yaml_file = open('custom_class.yaml', 'r')
my_obj = yaml.load(yaml_file)
print my_obj
yaml_file.close()

This script imports the yaml and custom_class modules, creates a readable file object from the YAML file created previously, loads the YAML file into an object, and prints the object.

When we run it, we see the following:

jmjones@dinkgutsy:~/code$ python custom_class_unyaml.py
Custom Class MyClass Data:: [1, 2, 3]

This is identical to the output from the unpickling example that we ran earlier in this chapter, so the behavior is what we would expect to see.

ZODB

Another option for serializing data is Zope’s ZODB module. ZODB stands for “Zope Object Database.” Simple usage of ZODB feels pretty similar to serializing to pickle or YAML, but ZODB has the ability to grow with your needs. For example, if you need atomicity in your operations, ZODB provides transactions. And if you need a more scalable persistent store, you can use ZEO, Zope’s distributed object store.

ZODB could have possibly gone into the “relational persistence” section rather than the “simple persistence” section. However, this object database doesn’t exactly fit the mold of what we’ve come to recognize as a relational database over the years even though you can easily establish relationships among objects. Also, we’re only displaying some of the more basic features of ZODB, so in our examples, it looks more like shelve than a relational database. So, we decided to keep ZODB in the “simple persistence” section.

Regarding installation of ZODB, it is as simple of a matter as doing easy_install ZODB3. The ZODB module has a number of dependencies but easy_install resolves them well, downloads everything it needs, and installs them.

For an example of simple use of ZODB, we’ll create a ZODB storage object and add a dictionary and a list to it. Here is the code for serializing the dictionary and list:

#!/usr/bin/env python

import ZODB
import ZODB.FileStorage
import transaction

filestorage = ZODB.FileStorage.FileStorage('zodb_filestorage.db')
db = ZODB.DB(filestorage)
conn = db.open()

root = conn.root()
root['list'] = ['this', 'is', 'a', 'list']
root['dict'] = {'this': 'is', 'a': 'dictionary'}

transaction.commit()
conn.close()

ZODB requires a couple more lines of code to start working with it than we’ve seen with pickle or YAML, but once you have a persistent store created and initialized, usage is pretty similar to the other options. This example is pretty self-explanatory, especially given the other examples of data persistence. But we’ll walk through it quickly, anyway.

First, we import a couple of ZODB modules, namely ZODB, ZODB.FileStorage, and transaction. (We’ll engage in just a little hair splitting at this point. Providing a module for import that does not contain an identifying prefix seems awkward. It seems to us that the transaction module that we import above should be prefixed by ZODB. Regardless, this is how it is, so you’ll just want to be aware of that. Now we’ll move on.) Next, we create a FileStorage object by specifying the database file to use for it. Then, we create a DB object and connect it to the FileStorage object. Then we open() the database object and get root node of it. From there, we can update the root object with our data structures, which we do with an impromptu list and dictionary. We then commit the changes we have made with transaction.commit() and then close the database connection with conn.close().

Once you’ve created a ZODB data storage container (such as the file storage object in this example) and have committed some data to it, you may want to get that data back out. Here is an example of opening the same database up, but reading the data rather than writing it:

#!/usr/bin/env python

import ZODB
import ZODB.FileStorage

filestorage = ZODB.FileStorage.FileStorage('zodb_filestorage.db')
db = ZODB.DB(filestorage)
conn = db.open()

root = conn.root()
print root.items()

conn.close()

And if we run this code after running the code that populates the database, here is the output we would see:

jmjones@dinkgutsy:~/code$ python zodb_read.py
No handlers could be found for logger "ZODB.FileStorage"
[('list', ['this', 'is', 'a', 'list']), ('dict', {'this': 'is', 'a': 'dictionary'})]

Just as we’ve shown how to serialize custom classes for other data persistence frameworks, we’ll show how to do so with ZODB. We will diverge, however, from using the same MyClass example (and we’ll explain why later). Just as with the other frameworks, you just define a class, create an object from it, and then tell the serialization engine to save it to disk. Here is the custom class that we’ll be using this time:

#!/usr/bin/env python

import persistent

class OutOfFunds(Exception):
pass

class Account(persistent.Persistent):
def __init__(self, name, starting_balance=0):
self.name = name
self.balance = starting_balance
def __str__(self):
return "Account %s, balance %s" % (self.name, self.balance)
def __repr__(self):
return "Account %s, balance %s" % (self.name, self.balance)
def deposit(self, amount):
self.balance += amount
return self.balance
def withdraw(self, amount):
if amount > self.balance:
raise OutOfFunds
self.balance -= amount
return self.balance

This is a very simple account class for managing financial funds. We defined an OutOfFunds exception that we will explain later. The Account class subclasses persistent.Persistent. (Regarding persistent, we could go into the same rant about the propriety of providing a meaningful prefix to a module that people are going to be using. How does a glance at this code inform the reader that ZODB code is being used here? It doesn’t. But we won’t go into that rant again.) Subclassing from persistent.Persistent does some magic behind the scenes to make it easier for ZODB to serialize this data. In the class definition, we created custom __str__ and __repr__ string converters. You’ll get to see those in action later. We also created deposit() and withdraw() methods. Both methods update the object attribute balance positively or negatively, depending on which method was called. The withdraw() method checks if there is enough money in the balance attribute before it subtracts funds. If there is not enough money in balance, the withdraw() method will raise the OutOfFunds exception that we mentioned earlier. Both deposit() and withdraw return the resulting balance after either adding or subtracting funds.

Here is a bit of code that will serialize the custom class that we just walked through:

#!/usr/bin/env python

import ZODB
import ZODB.FileStorage
import transaction
import custom_class_zodb

filestorage = ZODB.FileStorage.FileStorage('zodb_filestorage.db')
db = ZODB.DB(filestorage)
conn = db.open()

root = conn.root()
noah = custom_class_zodb.Account('noah', 1000)
print noah
root['noah'] = noah
jeremy =  custom_class_zodb.Account('jeremy', 1000)
print jeremy
root['jeremy'] = jeremy

transaction.commit()
conn.close()

This example is nearly identical to the previous ZODB example in which we serialized a dictionary and a list. However, we are importing our own module, creating two objects from a custom class, and serializing those two objects to the ZODB database. Those two objects are a noah account and a jeremy account, both of which are created with a balance of 1000 (presumably $1,000.00 USD, but we didn’t identify any currency units).

Here is this example’s output:

jmjones@dinkgutsy:~/code$ python zodb_custom_class.py
Account noah, balance 1000
Account jeremy, balance 1000

And if we run the module that displays the contents of a ZODB database, here is what we see:

jmjones@dinkgutsy:~/code$ python zodb_read.py
No handlers could be found for logger "ZODB.FileStorage"
[('jeremy', Account jeremy, balance 1000), ('noah', Account noah, balance 1000)]

Our code not only created the objects as we expected, but it also saved them to disk for later use.

How do we open the database up and change data for different accounts? This code would be pretty useless if it didn’t allow us to do that. Here is a piece of code that will open the database previously created and transfer 300 (presumably dollars) from the noah account to the jeremy account:

#!/usr/bin/env python

import ZODB
import ZODB.FileStorage
import transaction
import custom_class_zodb

filestorage = ZODB.FileStorage.FileStorage('zodb_filestorage.db')
db = ZODB.DB(filestorage)
conn = db.open()

root = conn.root()
noah = root['noah']
print "BEFORE WITHDRAWAL"
print "================="
print noah
jeremy = root['jeremy']
print jeremy
print "-----------------"

transaction.begin()
noah.withdraw(300)
jeremy.deposit(300)
transaction.commit()

print "AFTER WITHDRAWAL"
print "================"
print noah
print jeremy
print "----------------"

conn.close()

Here is the output from running this script:

jmjones@dinkgutsy:~/code$ python zodb_withdraw_1.py
BEFORE WITHDRAWAL
=================
Account noah, balance 1000
Account jeremy, balance 1000
-----------------
AFTER WITHDRAWAL
================
Account noah, balance 700
Account jeremy, balance 1300
----------------

And if we run our ZODB database printint script, we can see if the data was saved:

jmjones@dinkgutsy:~/code$ python zodb_read.py
[('jeremy', Account jeremy, balance 1300), ('noah', Account noah, balance 700)]

The noah account went from 1000 to 700 and the jeremy account went from 1000 to 1300.

The reason that we wanted to diverge from the MyClass custom class example was to show a little bit about transactions. One of the canonical examples for demonstrating how transactions work is with a bank account. If you want to be able to ensure that funds are successfully transfered from one account to another without losing or gaining funds, transactions are probably the first approach to look at. Here is a code example that uses transactions in a loop in order to show that no money is lost:

#!/usr/bin/env python

import ZODB
import ZODB.FileStorage
import transaction
import custom_class_zodb

filestorage = ZODB.FileStorage.FileStorage('zodb_filestorage.db')
db = ZODB.DB(filestorage)
conn = db.open()

root = conn.root()
noah = root['noah']
print "BEFORE TRANSFER"
print "==============="
print noah
jeremy = root['jeremy']
print jeremy
print "-----------------"

while True:
try:
transaction.begin()
jeremy.deposit(300)
noah.withdraw(300)
transaction.commit()
except custom_class_zodb.OutOfFunds:
print "OutOfFunds Error"
print "Current account information:"
print noah
print jeremy
transaction.abort()
break

print "AFTER TRANSFER"
print "=============="
print noah
print jeremy
print "----------------"

conn.close()

This is a slight modification of the previous transfer script. Instead of only transferring once, it transfers 300 from the noah account to the jeremy account until there isn’t enough money left to transfer. At the point that there are insufficient funds to transfer, it will print out a notice that an exception has occurred and the current account information. It will then abort() the transaction and break from the loop. The script also prints account information before and after the transaction loop. If the transactions worked, both the before and after account details should total 2000, since both accounts started with 1000 each.

Here is a result of running the script:

jmjones@dinkgutsy:~/code$ python zodb_withdraw_2.py
BEFORE TRANSFER
===============
Account noah, balance 700
Account jeremy, balance 1300
-----------------
OutOfFunds Error
Current account information:
Account noah, balance 100
Account jeremy, balance 2200
AFTER TRANSFER
==============
Account noah, balance 100
Account jeremy, balance 1900
----------------

In the “before” snapshot, noah has 700 and jeremy has 1300 for a total of 2000. When the OutOfFunds exception occurs, noah has 100 and jeremy has 2200 for a total of 2300. In the “after” snapshot, noah has 100 and jeremy has 1900 for a total of 2000. So during the exception, before the transaction.abort(), there was an additional 300 that would have been unexplained. But aborting the transaction fixed that problem.

ZODB feels like a solution between the simple and the relational. It is straightforward in its approach. An object that you serialize to disk corresponds to an object in memory both before serialization and after deserialization. But, it has some advanced features like transactions. ZODB is an option worth considering if you want the straightforwardness of simple object mapping, but you may need to grow into more advanced features later.

In summary of simple persistence, sometimes all you need is to simply save and store Python objects for later use. All the options we laid out here are excellent. Each one has its strengths and weaknesses. If you need this at some point, you will have to investigate which one will work best for you and your project.

Relational Serialization

Sometimes simple serialization isn’t enough. Sometimes you need the power of relational analysis. Relational serialization refers to either serializing Python objects and relationally connecting them with other Python objects or storing relational data (for example, in a relational database) and providing a Python object-like interface to that data.

SQLite

Sometimes it’s helpful to store and deal with data in a more structured and relational way. What we’re talking about here is the family of information stores referred to as relational databases, or RDBMSs. We assume that you have used a relational database such as MySQL, PostgreSQL, or Oracle before. If so, you should have no problem with this section.

According to the SQLite website, SQLite “is a software library that implements a self-contained, serverless, zero-configuration, transactional SQL database engine.” So what does all that mean? Rather than the database running in a separate server process from your code, the database engine runs in the same process as your code and you access it as a library. The data lives in a file rather than in multiple directories scattered across multiple filesystems. And rather than having to configure which hostname, port, username, password, etc. to connect to, you just point your code at the database file that the SQLite library creates. This statement also means that SQLite is a fairly featureful database. In a nutshell, the statement identifies two main benefits of using SQLite: it’s easy to use and it will do much of the same work that a “real” database will do. Another benefit is that it is ubiquitous. Most major operating systems and programming languages offer support for SQLite.

Now that you know why you may want to consider using it, let’s look at how to use it. We pulled the following table definitions from the Django example in Chapter 11. Assume we have a file named inventory.sql that contains the following data:

BEGIN;
CREATE TABLE "inventory_ipaddress" (
"id" integer NOT NULL PRIMARY KEY,
"address" text NULL,
"server_id" integer NOT NULL
)
;
CREATE TABLE "inventory_hardwarecomponent" (
"id" integer NOT NULL PRIMARY KEY,
"manufacturer" varchar(50) NOT NULL,
"type" varchar(50) NOT NULL,
"model" varchar(50) NULL,
"vendor_part_number" varchar(50) NULL,
"description" text NULL
)
;
CREATE TABLE "inventory_operatingsystem" (
"id" integer NOT NULL PRIMARY KEY,
"name" varchar(50) NOT NULL,
"description" text NULL
)
;
CREATE TABLE "inventory_service" (
"id" integer NOT NULL PRIMARY KEY,
"name" varchar(50) NOT NULL,
"description" text NULL
)
;
CREATE TABLE "inventory_server" (
"id" integer NOT NULL PRIMARY KEY,
"name" varchar(50) NOT NULL,
"description" text NULL,
"os_id" integer NOT NULL REFERENCES "inventory_operatingsystem" ("id")
)
;
CREATE TABLE "inventory_server_services" (
"id" integer NOT NULL PRIMARY KEY,
"server_id" integer NOT NULL REFERENCES "inventory_server" ("id"),
"service_id" integer NOT NULL REFERENCES "inventory_service" ("id"),
UNIQUE ("server_id", "service_id")
)
;
CREATE TABLE "inventory_server_hardware_component" (
"id" integer NOT NULL PRIMARY KEY,
"server_id" integer NOT NULL REFERENCES "inventory_server" ("id"),
"hardwarecomponent_id" integer
NOT NULL REFERENCES "inventory_hardwarecomponent" ("id"),
UNIQUE ("server_id", "hardwarecomponent_id")
)
;
COMMIT;

We can create a SQLite database with the following command-line argument:

jmjones@dinkgutsy:~/code$ sqlite3 inventory.db < inventory.sql

Assuming, of course, that you have SQLite installed. With Ubuntu and Debian systems, installing is as easy as apt-get install sqlite3. On Red Hat systems, all you have to do is yum install sqlite. For other distributions of Linux that may not have it installed, other UNIXes, or for Windows, you can download source and precompiled binaries at http://www.sqlite.org/download.html.

Assuming you have SQLite installed and have a database created, we’ll proceed with “connecting” to the database and populating it with some data. Here is all it takes to connect to a SQLite database:

In [1]: import sqlite3

In [2]: conn = sqlite3.connect('inventory.db')

All we had to do was import the SQLite library and then call connect() on the sqlite3 module. Connect() returns a connection object, which we referred to as conn and which we will use in the remainder of the example. Next, we execute a query on the connection object to insert data into the database:

In [3]: cursor = conn.execute("insert into inventory_operatingsystem (name,
description) values ('Linux', '2.0.34 kernel');")

The execute() method returns a database cursor object, so we decided to refer to it as cursor. Notice that we only provided values for the name and description fields and left out a value for the id field, which is the primary key. We’ll see in a moment what value it gets populated with. Since this is an insert rather than a select, we would not expect a result set from the query, so we’ll just look at the cursor and fetch any results it may be holding:

In [4]: cursor.fetchall()
Out[4]: []

Nothing, as we expected. Now, we’ll commit and move on:

In [5]: conn.commit()

In [6]:

Really, we shouldn’t have to commit this insert. We would expect this change to flush when we close the database connection at the latest. But it never hurts to explicitly commit() a change when you know that you want it committed.

Now that we can create and populate a database using SQLite, let’s get that data back out. First, we’ll fire up an IPython prompt, import sqlite3, and create a connection to our database file:

In [1]: import sqlite3

In [2]: conn = sqlite3.connect('inventory.db')

Now, we’ll execute a select query and get a cursor to the results:

In [3]: cursor = conn.execute('select * from inventory_operatingsystem;')

Finally, we’ll fetch the results from the cursor:

In [4]: cursor.fetchall()
Out[4]: [(1, u'Linux', u'2.0.34 kernel')]

This is the data that we plugged in above. Both the name and the description fields are unicode. And the id field is populated with an integer. Typically, when you insert data into a database and do not specify a value for the primary key, the database will populate it for you and automatically increment the value with the next unique value for that field.

Now that you are familiar with the basic methods of dealing with a SQLite database, doing joins, updating data, and doing more complex things becomes mostly an academic exercise. SQLite is a great format in which to store data, especially if the data is only going to be accessed by one script at a time, or only a few users at a time. In other words, the format is great for fairly small uses. However, the interface that the sqlite3 module provides is arcane.

Storm ORM

While a plain SQL interface to a database is all you really need to retrieve, update, insert, and delete data from a database, it is often convenient to have access to the data without diverting from the simplicity of Python. A trend regarding database access over the last few years has been to create an object-oriented representation of the data that is stored within a database. This trend is called an Object-RelationalMapping (or ORM). An ORM is different from merely providing an object-oriented interface to the database. In an ORM, an object in the programming language can correspond to a single row for a single table of a database. Tables connected with a foreign key relationship can even be accessed as an attribute of that object.

Storm is an ORM that was recently released as open source by Canonical, the company responsible for the creation of the Ubuntu distribution of Linux. Storm is a relative newcomer to the database arena for Python, but it has already developed a noticeable following and we expect it to become one of the front-running Python ORMs.

We will now use Storm to access the data in the database defined earlier in the SQLite” section. The first thing we have to do is to create a mapping to the tables of which we are interested. Since we’ve already accessed the inventory_operatingsystem table and added an entry to it, we’ll continue accessing only that table. Here is what a mapping in Storm looks like:

import storm.locals

class OperatingSystem(object):
__storm_table__ = 'inventory_operatingsystem'
id = storm.locals.Int(primary=True)
name = storm.locals.Unicode()
description = storm.locals.Unicode()

This is a pretty normal class definition. There doesn’t appear to be weird, magical things going on. There is no subclass other than the built-in object type. There are a number of class-level attributes being defined. The one slightly odd-looking thing is the class attribute of __storm_table__. This lets Storm know which table that objects of this type should be accessing. While it seems pretty simple, straightforward, and non-magical, there is at least a little bit of magic in the mix. For example, the name attribute is mapped to the name column of the inventory_operatingsystem table and the description attribute is mapped to the description column of the inventory_operatingsystem table. How? Magic. Any attribute that you assign to a Storm mapping class is automatically mapped to a column that shares its name in the table designated by the __storm_table__ attribute.

What if you don’t want the description attribute of your object mapped to the description column? Simple. Pass in a name keyword argument to the storm.locals.Type that you are using. For example, changing the description attribute to this: dsc = storm.locals.Unicode(name='description') connects OperatingSystem objects to the same columns (namely, name and description). However, rather than referring to the description as mapped_object.description, you would refer to it as mapped_object.dsc.

Now that we have a mapping of a Python class to a database table, let’s add another row to the database. To go along with our ancient Linux distribution with a 2.0.34 kernel, we’ll add Windows 3.1.1 to the operating system table:

import storm.locals
import storm_model
import os

operating_system = storm_model.OperatingSystem()
operating_system.name = u'Windows'
operating_system.description = u'3.1.1'

db = storm.locals.create_database('sqlite:///%s' % os.path.join(os.getcwd(),
'inventory.db'))

store = storm.locals.Store(db)
store.add(operating_system)
store.commit()

In this example, we imported the storm.locals, storm_model, and os modules. Then, we instantiated an OperatingSystem object and assigned values to the name and description attributes. (Notice that we used unicode values for these attributes.) Then we created a database object by calling the create_database() function and passing it the path to our SQLite database file, inventory.db. While you might think that the database object is what we would use to add data to the database, it isn’t, at least not directly. We first had to create a Store object by passing the database into its constructor. After we did that, we were able to add the operating_system object to the store object. Finally, we called commit() on the store to finalize the addition of this operating_system to the database.

We also want to see that the data we inserted does in fact find its way into the database. Since this is a SQLite database, we could just use the sqlite3 command-line tool. If we did that, we would have less reason to write code to retrieve data from the database using Storm. So, here is a simple utility to retrieve all records from the inventory_operatingsystem table and print it out (albeit in rather ugly fashion):

import storm.locals
import storm_model
import os

db = storm.locals.create_database('sqlite:///%s' % os.path.join(os.getcwd(),
'inventory.db'))

store = storm.locals.Store(db)

for o in store.find(storm_model.OperatingSystem):
print o.id, o.name, o.description

The first several lines of code in this example are strikingly similar to the first several lines of the previous example. Part of the reason for that is that we copied and pasted the code from one file to the other. Never mind that, though. The bigger reason is that both examples require some common setup steps before they can “talk” to the database. We have the same import statements here as in the previous example. We have a db object that was returned from the create_database() function. We have a store object created by passing the db object to the Store constructor. But now, rather than adding an object to the store, we’re calling the find() method of the store object. This particular call to find() (i.e., store.find(storm_model.OperatingSystem)) returns a result set of all storm_model.OperatingSystem objects. Because we mapped the OperatingSystem class to the inventory_operatingsystem table, Storm will look up all the relevant records in the inventory_operatingsystem table and create OperatingSystem objects from them. For each OperatingSystem object, we print out the id, name, and description attributes. These attributes map to the column values in the database that share the same names for each record.

We should have one record already in the database from the earlier example in the SQLite” section. Let’s see what happens when we run the retrieve script. We would expect it to display one record even though that record was not inserted using the Storm library:

jmjones@dinkgutsy:~/code$ python storm_retrieve_os.py
1 Linux 2.0.34 kernel

This is exactly what we expected to happen. Now, what happens when we run the add script and then the retrieve script? It should show the old entry that was in the database from earlier (the 2.0.34 Linux kernel) as well as the newly inserted entry (Windows 3.1.1):

jmjones@dinkgutsy:~/code$ python storm_add_os.py
jmjones@dinkgutsy:~/code$ python storm_retrieve_os.py
1 Linux 2.0.34 kernel
2 Windows 3.1.1

Again, this was exactly what we expected.

But what if we want to filter the data? Supposed we only want to see operating system entries that started with the string “Lin.” Here is a piece of code to do just that:

import storm.locals
import storm_model
import os

db = storm.locals.create_database('sqlite:///%s' % os.path.join(os.getcwd(),
'inventory.db'))

store = storm.locals.Store(db)

for o in store.find(storm_model.OperatingSystem,
storm_model.OperatingSystem.name.like(u'Lin%')):
print o.id, o.name, o.description

This example is identical to the previous example that uses store.find() except that this one passes in a second parameter to store.find(): a search criteria. Store.find(storm_model.OperatingSystem,storm_model.OperatingSystem.name.like(u'Lin%')) tells Storm to look for all OperatingSystem objects that have a name that starts with the unicode value Lin. For each value that is in the result set, we print it out identically to the previous example.

And when you run it, you will see something like this:

jmjones@dinkgutsy:~/code$ python storm_retrieve_os_filter.py
1 Linux 2.0.34 kernel

This database still has the “Windows 3.1.1” entry, but it was filtered out because “Windows” does not begin with “Lin.”

SQLAlchemy ORM

While Storm is gaining an audience and building a community, SQLAlchemy appears to be the dominate ORM in Python at the moment. Its approach is similar to that of Storm. That could probably be better said as, “Storm’s approach is similar to that of SQLAlchemy,” since SQLAlchemy was first. Regardless, we’ll walk through the same inventory_operatingsystem example for SQLAlchemy that we finished for Storm.

Here is the table and object definition for the inventory_operatingsystem table:

#!/usr/bin/env python

import os
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, Text, VARCHAR, MetaData
from sqlalchemy.orm import mapper
from sqlalchemy.orm import sessionmaker

engine = create_engine('sqlite:///%s' % os.path.join(os.getcwd(),
'inventory.db'))

metadata = MetaData()
os_table = Table('inventory_operatingsystem', metadata,
Column('id', Integer, primary_key=True),
Column('name', VARCHAR(50)),
Column('description', Text()),
)

class OperatingSystem(object):
def __init__(self, name, description):
self.name = name
self.description = description

def __repr__(self):
return "<OperatingSystem('%s','%s')>" % (self.name, self.description)

mapper(OperatingSystem, os_table)
Session = sessionmaker(bind=engine, autoflush=True, transactional=True)
session = Session()

The biggest difference between our Storm and SQLAlchemy example table definition code is that SQLAlchemy uses an additional class other than the table class and then maps the two together.

Now that we have a definition of our table, we can write a piece of code to query all records from that table:

#!/usr/bin/env python

from sqlalchemy_inventory_definition import session, OperatingSystem

for os in session.query(OperatingSystem):
print os

And if we run it now, after populating some data from the previous examples, we’ll see this:

$ python sqlalchemy_inventory_query_all.py <OperatingSystem('Linux','2.0.34 kernel')>
<OperatingSystem('Windows','3.1.1')>
</OperatingSystem></OperatingSystem>

If we want to create another record, we can easily do so by just instantiating an OperatingSystem object and adding it to the session:

#!/usr/bin/env python

from sqlalchemy_inventory_definition import session, OperatingSystem

ubuntu_710 = OperatingSystem(name='Linux', description='2.6.22-14 kernel')
session.save(ubuntu_710)
session.commit()

That will add another Linux kernel to the table, this time a more current kernel. Running our query all script again gives us this output:

$ python sqlalchemy_inventory_query_all.py
<OperatingSystem('Linux','2.0.34 kernel')>
<OperatingSystem('Windows','3.1.1')>
<OperatingSystem('Linux','2.6.22-14 kernel')>

Filtering results is pretty simple in SQLAlchemy as well. For example, if we wanted to filter out all the OperatingSystems whose names start with “Lin,” we could write a script like this:

#!/usr/bin/env python

from sqlalchemy_inventory_definition import session, OperatingSystem

for os in session.query(OperatingSystem).filter(OperatingSystem.name.like('Lin%')):
print os

And we would see output like this:

$ python sqlalchemy_inventory_query_filter.py
<OperatingSystem('Linux','2.0.34 kernel')>
<OperatingSystem('Linux','2.6.22-14 kernel')>

This was just a brief overview of what SQLAlchemy can do. For more information on using SQLAlchemy, visit the website at http://www.sqlalchemy.org/. Or you can check out Essential SQLAlchemy by Rick Copeland (O’Reilly).

Celebrity Profile: SQLAlchemy: Mike Bayer

Michael Bayer is a NYC-based software contractor with a decade of experience dealing with relational databases of all shapes and sizes. After writing many homegrown database abstraction layers in such languages as C, Java and Perl, and finally, after several years of practice working with a huge multi-server Oracle system for Major League Baseball, he wrote SQLAlchemy as the “ultimate toolset” for generating SQL and dealing with databases overall. The goal is to contribute toward a world-class one-of-a-kind toolset for Python, helping to make Python the universally popular programming platform it deserves to be.

Summary

In this chapter, we addressed a number of different tools that allow you to store your data for later use. Sometimes you’ll need something simple and lightweight like the pickle module. Other times, you’ll need something more full-featured like the SQLAlchemy ORM. As we’ve shown, with Python, you have plenty of options from very simple to complex and powerful.

Chapter 13. Command Line

Introduction

The command line has a special relationship with the sysadmin. No other tool carries the same level of significance or prestige as the command line. A complete mastery of the art of the command line is a rite of passage for most systems administrators. Many sysadmins think less of other sysadmins that use a “GUI” and call GUI administration a crutch. This may not be completely fair, but it is a commonly held belief for true mastery of the art of system’s administration.

For the longest time, Unix systems embraced the philosophy that the command line interface (CLI) was far superior to any GUI that could be developed. In a recent turn of events, it seems like Microsoft has also gotten back to its roots. Jeffrey Snover, architect of Windows Powershell, said, “It was a mistake to think that GUIs ever would, could, or even should, eliminate CLIs.”

Even Windows, which has had the poorest CLI of any modern OS for decades, now recognizes the value of the CLI in its current Windows PowerShell implementation. We will not be covering Windows in this book, but it is a very interesting fact that cements just how important mastering the command line and command-line tool creation really is.

There is more to the story, though, than just mastering prebuilt Unix command-line tools. To really become a master at the command line, you need to create your own tools, and this may be the sole reason you picked up this book in the first place. Don’t worry, this chapter won’t dissapoint you. After finishing it, you will be a master of creating command-line tools in Python.

It was a purposeful decision to make the last chapter of the book focus on creating command-line tools. The reason for this was to first expose you to a wide assortment of techniques in Python, and then to finally teach you how to harness all of these skills to summon your full power to create command-line tool masterpieces.

Basic Standard Input Usage

The simplest possible introduction to creating a command-line tool revolves around knowing that the sys module is able to process command-line arguments via sys.argv. Example 13-1 shows quite possibly the simplest command-line tool.

Example 13-1. sysargv.py

#!/usr/bin/env python

import sys
print sys.argv

These two lines of code return to standard out, whatever you type on the command line after executing the command:

./sysargv.py

['./sysargv.py']

and

./sysargv.py foo

returns to standard out

['./sysargv.py', 'test']

and

./sysargv.py foo bad for you

returns to standard out

['./sysargv.py', 'foo', 'bad', 'for', 'you']

Let’s be a little more specific and slightly change the code to count the number of command-line arguments in Example 13-2.

Example 13-2. sysargv.py

#!/usr/bin/env python
import sys

#Python indexes start at Zero, so let's not count the command itself which is
#sys.argv[0]

num_arguments = len(sys.argv) - 1
print sys.argv, "You typed in ", num_arguments, "arguments"

You might be thinking, “Wow, this is pretty easy, all I have to do now is reference sys.argv arguments by number and write some logic to connect them.” Well, you’re right, it is pretty easy to do that. Let’s add some features to our command-line application. One final thing we can do is send an error message to standard out if there are no arguments passed to the command line. See Example 13-3.

Example 13-3. sysargv-step2.py

#!/usr/bin/env python
import sys

num_arguments = len(sys.argv) - 1

#If there are no arguments to the command, send a message to standard error.
if num_arguments == 0:
sys.stderr.write('Hey, type in an option silly\n')

else:
print sys.argv, "You typed in ", num_arguments, "arguments"

Using sys.argv to create command-line tools is quick and dirty but is often the wrong choice. The Python Standard Library includes the optparse module, which handles all of the messy and uncomfortable parts of creating a quality command-line tool. Even for tiny “throwaway” tools, it is a better choice to use optparse than sys.argv, as often “throwaway” tools have a habit of growing into production tools. In the coming sections, we will explore why, but the short answer is that a good option parsing module handles the edge cases for you.

Introduction to Optparse

As we mentioned in the previous section, even small scripts can benefit from using optparse to handle option handling. A fun way to get started with optparse is to code up a “Hello World” example that handles options and arguments. Example 13-4 is our Hello World example.

Example 13-4. Hello World optparse

#!/usr/bin/env python
import optparse

def main():
p = optparse.OptionParser()
p.add_option('--sysadmin', '-s', default="BOFH")
options, arguments = p.parse_args()
print 'Hello, %s' % options.sysadmin

if __name__ == '__main__':
main()

When we run this, we the get the following different kinds of outputs:

$ python hello_world_optparse.py
Hello, BOFH

$ python hello_world_optparse.py --sysadmin Noah
Hello, Noah

$ python hello_world_optparse.py --s Jeremy
Hello, Jeremy

$ python hello_world_optparse.py --infinity Noah
Usage: hello_world_optparse.py [options]

hello_world_optparse.py: error: no such option: --infinity

In our small script, we saw that we could set both short -s, and long --sysadmin options, as well as default values. Finally, we saw the power of the built-in error handling that optparse delivers when we wrongly entered an option, readability, that did not exist for Perl.

Simple Optparse Usage Patterns

No Options Usage Pattern

In the previous section, we mentioned that even for small scripts optparse can be useful. Example 13-5 is a simple optparse usage pattern in which we don’t even take options but still take advantage of the power of optparse.

Example 13-5. ls command clone

#!/usr/bin/env python
import optparse
import os

def main():
p = optparse.OptionParser(description="Python  'ls' command clone",
prog="pyls",
version="0.1a",
usage="%prog [directory]")
options, arguments = p.parse_args()
if len(arguments) == 1:
path = arguments[0]
for filename in os.listdir(path):
print filename
else:
p.print_help()
if __name__ == '__main__':
main()

In this example, we reimplement the ls command in Python, except we only take an argument, the path to perform the ls on. We don’t even use options, but can still utilize the power of optparse, by relying on it to handle the flow of our program. First we provide some implementation deals this time when we make an instance of optparse, and add a usage value that instructs the potential user of our tool how to execute it properly. Next, we check to make sure the number of arguments is exactly one; if there are more or less arguments than one, we use the built-in help message p.print_help() to display the instructions on how to use the tool again. Here is what it looks like when run correctly first by running it against our current directory or “.”:

$ python no_options.py .
.svn
hello_world_optparse.py
no_options.py

Next we look at what happens when we don’t enter any options:

$ python no_options.py
Usage: pyls [directory]

Python 'ls' command clone

Options:
--version   show program's version number and exit
-h, --help  show this help message and exit

What is interesting about this is we defined this behavior with the p.print_help() call if the arguments were not exactly one. This is exactly the same as if we entered --help:

$ python no_options.py --help
Usage: pyls [directory]

Python 'ls' command clone

Options:
--version   show program's version number and exit
-h, --help  show this help message and exit

And because we defined a --version option, we can see that output as well:

$ python no_options.py --version
0.1a

In this example, optparse was helpful even on simple “throwaway” scripts that you might be tempted to toss.

True/False Usage Pattern

Using an option to set a True or False statement in your program is quite useful. The classic example of this involves setting both a --quiet, which supresses all standard out, and a --verbose, which triggers extra output. Example 13-6 is what this looks like.

Example 13-6. Adding and subtracting verbosity

#!/usr/bin/env python
import optparse
import os

def main():
p = optparse.OptionParser(description="Python 'ls' command clone",
prog="pyls",
version="0.1a",
usage="%prog [directory]")
p.add_option("--verbose", "-v", action="store_true",
help="Enables Verbose Output",default=False)
options, arguments = p.parse_args()
if len(arguments) == 1:
if options.verbose:
print "Verbose Mode Enabled"
path = arguments[0]
for filename in os.listdir(path):
if options.verbose:
print "Filename: %s " % filename
elif options.quiet:
pass
else:
print filename
else:
p.print_help()
if __name__ == '__main__':
main()

By using a --verbose, we have effectively set levels of verbosity for stdout. Let’s take a look at each level of verbosity in action. First, here is the normal way:

$ python true_false.py /tmp
.aksusb
alm.log
amt.log
authTokenData
FLEXnet
helloworld
hsperfdata_ngift
ics10003
ics12158
ics13342
icssuis501
MobileSync.lock.f9e26440fe5adbb6bc42d7bf8f87c1e5fc61a7fe
summary.txt

Next, here is our --verbose mode:

$ python true_false.py --verbose /tmp
Verbose Mode Enabled
Filename: .aksusb
Filename: alm.log
Filename: amt.log
Filename: authTokenData
Filename: FLEXnet
Filename: helloworld
Filename: hsperfdata_ngift
Filename: ics10003
Filename: ics12158
Filename: ics13342
Filename: icssuis501
Filename: MobileSync.lock.f9e26440fe5adbb6bc42d7bf8f87c1e5fc61a7fe
Filename: summary.txt

When we set the --verbose option, it makes options.verbose become True, and as a result, our conditional statement gets executed that prints “Filename:” in front of the actual filename. Notice in our script that we set default=False and action="store_true", this effectively says in English, by default be False, but if someone specifies this --option, set the option value to become True. This is the essence of using True/False options with optparse.

Counting Options Usage Pattern

In a typical Unix command-line tool, for example, tcpdump, if you specify -vvv, you will get extra verbose output as opposed to just using -v or -vv. You can do the same thing with optparse by adding a count for each time an option is specified. For example, if you wanted to add the same level of verbosity in your tool, it would look like Example 13-7.

Example 13-7. Counting Options Usage pattern

#!/usr/bin/env python
import optparse
import os

def main():
p = optparse.OptionParser(description="Python 'ls' command clone",
prog="pyls",
version="0.1a",
usage="%prog [directory]")
p.add_option("-v", action="count", dest="verbose")
options, arguments = p.parse_args()
if len(arguments) == 1:
if options.verbose:
print "Verbose Mode Enabled at Level: %s" % options.verbose
path = arguments[0]
for filename in os.listdir(path):
if options.verbose == 1:
print "Filename: %s " % filename
elif options.verbose ==2 :
fullpath = os.path.join(path,filename)
print "Filename: %s | Byte Size: %s" % (filename,
os.path.getsize(fullpath))
else:
print filename
else:
p.print_help()
if __name__ == '__main__':
main()

By using an auto-incremented count design pattern, we can make sure of just one option, yet do three different things. The first time we call -v, it sets options.verbose to 1, and if we use --v it sets options.verbose to 2. In our actual program, with no options we just print out the filename, with -v we print out the word Filename with the filename, and then finally, whew, with -vv we print out the byte size as well as the filename. This is our output with -vv specified:

$ python verbosity_levels_count.py -vv /tmp
Verbose Mode Enabled at Level: 2
Filename: .aksusb | Byte Size: 0
Filename: alm.log | Byte Size: 1403
Filename: amt.log | Byte Size: 3038
Filename: authTokenData | Byte Size: 32
Filename: FLEXnet | Byte Size: 170
Filename: helloworld | Byte Size: 170
Filename: hsperfdata_ngift | Byte Size: 102
Filename: ics10003 | Byte Size: 0
Filename: ics12158 | Byte Size: 0
Filename: ics13342 | Byte Size: 0
Filename: ics14183 | Byte Size: 0
Filename: icssuis501 | Byte Size: 0
Filename: MobileSync.lock.f9e26440fe5adbb6bc42d7bf8f87c1e5fc61a7fe | Byte Size: 0
Filename: summary.txt | Byte Size: 382

Choices Usage Pattern

Sometimes it’s just easier to present a few choices for an option. In our last example, we created options for --verbose and --quiet, but we could also just make them choices that get selected from a --chatty option. Using our previous example, Example 13-8 is what it looks like when it is reworked to use options.

Example 13-8. Choices Usage pattern

#!/usr/bin/env python
import optparse
import os

def main():
p = optparse.OptionParser(description="Python 'ls' command clone",
prog="pyls",
version="0.1a",
usage="%prog [directory]")
p.add_option("--chatty", "-c", action="store", type="choice",
dest="chatty",
choices=["normal", "verbose", "quiet"],
default="normal")
options, arguments = p.parse_args()
print options
if len(arguments) == 1:
if options.chatty == "verbose":
print "Verbose Mode Enabled"
path = arguments[0]
for filename in os.listdir(path):
if options.chatty == "verbose":
print "Filename: %s " % filename
elif options.chatty == "quiet":
pass
else:
print filename
else:
p.print_help()
if __name__ == '__main__':
main()

If we run this command without an option like we did in the previous example, we get this error:

$ python choices.py --chatty
Usage: pyls [directory]

pyls: error: --chatty option requires an argument

And if we give the wrong argument to the option, we get another error that tells us the available options:

$ python choices.py --chatty=nuclear /tmp
Usage: pyls [directory]

pyls: error: option --chatty: invalid choice: 'nuclear' (choose from 'normal',
'verbose', 'quiet')

One of the handy aspects of using choices is that it prevents relying on the user to enter the correct argument for your command. The user can only select from choices you have determined. Finally, here is what the command looks like when run correctly:

$ python choices.py --chatty=verbose /tmp
{'chatty': 'verbose'}
Verbose Mode Enabled
Filename: .aksusb
Filename: alm.log
Filename: amt.log
Filename: authTokenData
Filename: FLEXnet
Filename: helloworld
Filename: hsperfdata_ngift
Filename: ics10003
Filename: ics12158
Filename: ics13342
Filename: ics14183
Filename: icssuis501
Filename: MobileSync.lock.f9e26440fe5adbb6bc42d7bf8f87c1e5fc61a7fe
Filename: summary.txt

If you notice, the output at the top has “chatty” as the key and “verbose” as the value. In our example above, we put a print statement for options to show you what they look like to our program. Finally, here is one final example of using --chatty with a quiet choice:

$ python choices.py --chatty=quiet /tmp
{'chatty': 'quiet'}

Option with Multiple Arguments Usage Pattern

By default, an option with optparse can only take one argument, but it is possible to set the number to something else. Example 13-9 is a contrived example in which we make a version ls that displays the output of two directories at once.

Example 13-9. Listing of two directories

#!/usr/bin/env python
import optparse
import os

def main():
p = optparse.OptionParser(description="Lists contents of two directories",
prog="pymultils",
version="0.1a",
usage="%prog [--dir dir1 dir2]")
p.add_option("--dir", action="store", dest="dir", nargs=2)
options, arguments = p.parse_args()
if options.dir:
for dir in options.dir:
print "Listing of %s:\n" % dir
for filename in os.listdir(dir):
print filename
else:
p.print_help()
if __name__ == '__main__':
main()

If we look at the output of this command with the only argument for the --dir option, we get this error:

[ngift@Macintosh-8][H:10238][J:0]# python multiple_option_args.py --dir /tmp ↴
Usage: pymultils [--dir dir1 dir2]

pymultils: error: --dir option requires 2 arguments

With the correct number of arguments for our --dir option, we get this:

pymultils: error: --dir option requires 2 arguments
[ngift@Macintosh-8][H:10239][J:0]# python multiple_option_args.py --dir /tmp
/Users/ngift/Music
Listing of /tmp:

.aksusb
FLEXnet
helloworld
hsperfdata_ngift
ics10003
ics12158
ics13342
ics14183
ics15392
icssuis501
MobileSync.lock.f9e26440fe5adbb6bc42d7bf8f87c1e5fc61a7fe
summary.txt
Listing of /Users/ngift/Music:

.DS_Store
.localized
iTunes

Unix Mashups: Integrating Shell Commands into Python Command-Line Tools

In Chapter 10, we looked at many of the common ways to use the subprocess module. Creating new command-line tools by either wrapping existing command-line tools with Python and changing their API, or mixing one or more Unix command-line tools with Python offers an interesting approach to examine. It is trivial to wrap an existing command-line tool with Python and change the behavior to meet your specific needs. You may choose to integrate a configuration file that holds some of the arguments for some of the options you use, or you may choose to create defaults for others. Regardless of the requirement, you can use subprocess and optparse to change a native Unix tools behavior without much trouble.

Alternately, mixing a command-line tool with pure Python can lead to interesting tools that are not easily created in C or Bash. How about mixing the dd command with threads and queues, tcpdump with Python’s regular expression library, or perhaps a customized version of rsync? These Unix 2.0 “mashups” are very similar to their Web 2.0 cousins. By mixing Python with Unix tools, new ideas are created, and problems are solved in different ways. In this section, we explore some of these techniques.

Kudzu Usage Pattern: Wrapping a Tool in Python

Sometimes you find yourself using a command-line tool that isn’t exactly what you want it to be. It might require too many options, or the argument order is reversed from the way you want to use it. With Python, it is trivial to change the behavior of a tool and make it do whatever you really want it to do. We like to call this the “Kudzu” design pattern. If you are not familiar with Kudzu, it was a fast-growing vine imported from Japan to the southern United States. Kudzu often engulfs and surrounds natural habit and creates an alternate landscape. With Python, you can do the same to your Unix environment if you so choose.

For this example, we are going to wrap the snmpdf command with Python to simplify its use. First let’s take a look at what it looks like when we run snmpdf normally:

[ngift@Macintosh-8][H:10285][J:0]# snmpdf -c public -v 2c example.com
Description              size (kB)            Used       Available Used%
Memory Buffers             2067636          249560         1818076   12%
Real Memory                2067636         1990704           76932   96%
Swap Space                 1012084              64         1012020    0%
/                         74594112        17420740        57173372   23%
/sys                             0               0               0    0%
/boot                       101086           20041           81045   19%

If you are not familiar with the snmpdf, it is meant to be run remotely on a system that has SNMP enabled and configured to allow access to the disk section of the MIB tree. Often, command-line tools that deal with the SNMP protocol have many options, which make them difficult to use. To be fair, the tool creators had to design something that would work with SNMP versions 1, 2, and 3, plus a whole assortment of other issues. What if you don’t care about this, though, and you are a very lazy person. You want to make your own “Kudzu” version of snmpdf that takes only a machine as an argument. Sure, we can do that; Example 13-10 is what it looks like.

Note

Often, when you wrap a Unix tool in Python to alter the behavior of the tool, it becomes more lines of code than if you altered it with Bash. Ultimately, though, we feel this is a win because it allows you to use the richer Python toolset to extend this tool as you see fit. In addition, you can test this code the same way you test the rest of the tools you write, so often this extra code is the right way to go for the long haul.

Example 13-10. Wrapping SNMPDF command with Python

#!/usr/bin/env python
import optparse
from subprocess import call

def main():
p = optparse.OptionParser(description="Python wrapped snmpdf command",
prog="pysnmpdf",
version="0.1a",
usage="%prog machine")
p.add_option("-c", "--community", help="snmp community string")
p.add_option("-V", "--Version", help="snmp version to use")
p.set_defaults(community="public",Version="2c")
options, arguments = p.parse_args()
SNMPDF = "snmpdf"
if len(arguments) == 1:
machine = arguments[0]
#Our new snmpdf action
call([SNMPDF, "-c", options.community, "-v",options.Version, machine])
else:
p.print_help()
if __name__ == '__main__':
main()

This script runs in at about twenty lines of code, yet it makes our life much easier. Using some of the magic of optparse to help us, we created options that had default arguments that matched out needs. For example, we set a SNMP version option to be version 2 by default, as we know our data center uses only version 2 right now. We also set the community string to “public,” because that is what it is set to in our research and development lab, for example. One of the nice things about doing it with optparse and not a hardcoded script is that we have the flexibility to change our options without changing the script.

Notice that the default arguments were set using the set_defaults method, which allows us to set all defaults for a command-line tool in one spot. Also, notice the use of subprocess.call. We embedded the old options, such as -c, and then wrapped the new values that come in from optparse, or options.community in this case, to fill things in. Hopefully, this technique highlights some of the “Kudzu” power of Python to engulf a tool and change it to meet our needs.

Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior

In our last example, we made snmpdf quite a bit easier to use, but we didn’t change the basic behavior of the tool. The output of both tools was identical. Another approach we can use is to not only engulf a Unix tool, but to then change the basic behavior of the tool with Python as well.

In the next example, we use Python’s generators in a functional programming style to filter the results of our snmpdf command to search for critical information, and then append a "CRITICAL" flag to it. Example 13-11 shows what it looks like.

Example 13-11. Altering the SNMPDF command with generators

#!/usr/bin/env python
import optparse
from subprocess import Popen, PIPE
import re

def main():
p = optparse.OptionParser(description="Python wrapped snmpdf command",
prog="pysnmpdf",
version="0.1a",
usage="%prog machine")
p.add_option("-c", "--community", help="snmp community string")
p.add_option("-V", "--Version", help="snmp version to use")
p.set_defaults(community="public",Version="2c")
options, arguments = p.parse_args()
SNMPDF = "snmpdf"
if len(arguments) == 1:
machine = arguments[0]

#We create a nested generator function
def parse():
"""Returns generator object with line from snmpdf"""
ps = Popen([SNMPDF, "-c", options.community,
"-v",options.Version, machine],
stdout=PIPE, stderr=PIPE)
return ps.stdout

#Generator Pipeline To Search For Critical Items
pattern = "9[0-9]%"
outline = (line.split() for line in parse()) #remove carriage returns
flag = (" ".join(row) for row in outline if re.search(pattern, row[-1]))
#patt search, join strings in list if match
for line in flag: print "%s CRITICAL" % line
#Sample Return Value
#Real Memory 2067636 1974120 93516 95% CRITICAL

else:
p.print_help()
if __name__ == '__main__':
main()

If we run our new “altered” version of snmpdf we get this output on test machine:

[ngift@Macintosh-8][H:10486][J:0]# python snmpdf_alter.py localhost
Real Memory 2067636 1977208 90428 95% CRITICAL

We now have a completely different script that will only generate output if a value in snmpdf is 90 percent or higher, which we have signified as critical. We could run this in a cron job nightly against a few hundred machines, and then send an email if there is a return value from our script. Alternately, we could extend this script a little further and search for usage levels of 80 percent, 70 percent, and generate warnings if they reach those levels as well. It would also be trivial to integrate this with Google App Engine, for example, so that you could create a web application that monitors the disk usage in an infrastructure.

In looking at the code itself, there are a few things to point out that make it different than our previous example. The first difference is the use of subprocess.Popen instead of using subprocess.call. If you find yourself wanting to parse the output of a Unix command-line tool, then subprocess.Popen is what you want to do. Note also, that we used stdout.readlines(), which returns a list instead of a string. This is important later on when we take this output and funnel it through a series of generator expressions.

In the Generator pipeline section, we funnel our generator objects into two expressions to find a match for the critical search criteria we set. As we stated before, we could easily add a couple more generator lines similar to the flag expression, to get results for thresholds in 70 percent and 80 percent ranges.

Note

This tool is perhaps more complex than you would want to implement into a production tool. A better idea might be to break it down into several smaller generic pieces that you import. That being said, it works to illustrate our example.

Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

Our last example was reasonably cool, but another interesting way to change the behavior of existing Unix tools is to make them spawn multiple copies in an efficient way. Sure, it is a little freaky, but hey, sometimes you need to be creative to get your job done. This is one of the parts of being a sysadmin that is fun, sometimes you have to do crazy things to solve a problem in production.

In the data chapter, we created a test script that created image files using the dd command running in parallel. Well, let’s take that idea and run with it, and make a permanent command-line tool we can reuse over and over again. At the very least, we will have something to hammer disk I/O time when we are testing a new file server. See Example 13-12.

Example 13-12. Multi dd command

from subprocess import Popen, PIPE
import optparse
import sys

class ImageFile():
"""Created Image Files Using dd"""
def __init__(self, num=None, size=None, dest=None):
self.num = num
self.size = size
self.dest = dest

def createImage(self):
"""creates N 10mb identical image files"""
value = "%sMB " % str(self.size/1024)
for i in range(self.num):
try:
cmd = "dd if=/dev/zero of=%s/file.%s bs=1024 count=%s"\
% (self.dest,i,self.size)
Popen(cmd, shell=True, stdout=PIPE)
except Exception, err:
sys.stderr.write(err)

def controller(self):
"""Spawn Many dd Commands"""
p = optparse.OptionParser(description="Launches Many dd",
prog="Many dd",
version="0.1",
usage="%prog [options] dest")
p.add_option('-n', '--number', help='set many dd',
type=int)
p.add_option('-s', '--size', help='size of image in bytes',
type=int)
p.set_defaults(number=10,
size=10240)
options, arguments = p.parse_args()
if len(arguments) == 1:
self.dest = arguments[0]
self.size = options.size
self.num = options.number
#runs dd commands
self.createImage()

def main():
start = ImageFile()
start.controller()

if __name__ == "__main__":
main()

Now if we run our multi dd command, we can set the byte size of the file, the path, and the total number of files/processes. Here is what it looks like:

$ ./subprocess_dd.py /tmp/
$ 10240+0 records in
10240+0 records out
10485760 bytes transferred in 1.353665 secs (7746199 bytes/sec)
10240+0 records in
10240+0 records out
10485760 bytes transferred in 1.793615 secs (5846160 bytes/sec)
10240+0 records in
10240+0 records out
10485760 bytes transferred in 2.664616 secs (3935186 bytes/sec)

...output supressed for space....

One immediate use for this hybrid tool would be in testing the disk I/O performance of a high-speed Fibre SAN, or NAS device. With a bit of work, you could add hooks for generation of PDF reports, and email the results. It would be good to point out that the same thing could be accomplished with threads as well, if threads seemed to fit the problem you needed to solve.

Integrating Configuration Files

Integrating a configuration file into a command-line tool can make all the difference in terms of usability and future customization. It is a bit odd to talk about usability and the command line, because often it is only brought up for GUI or web tools. This is unfortunate, as a command-line tool deserves the same attention to usability that a GUI tool does.

A configuration file can also be a useful way to centralize the way a command-line tool runs on multiple machines. The configuration file could be shared out via an NFS mount, and then hundreds of machines could read this configuration file from a generic command-line tool you created. Alternately, you may have some sort of configuration management system in place, and you could distribute configuration files to tools you created as well.

The Python Standard Library has an excellent module, ConfigParser, for reading and writing configuration files using the .ini syntax. It turns out that the .ini format is a nice medium to read and write simple configuration data, without having to resort to XML, and without locking the person editing the file into knowing the Python language. Please refer to the previous chapter for a more detailed look at using the ConfigParser module as well.

Note

Be sure that you do not get in the habit of depending on the order of items in the config file. Interally, the ConfigParser module uses a dictionary, and as such you will need to refer to it in this way to correctly obtain a mapping.

To get started with integrating configuration files into a command-line tool, we are going to create a “hello world” configuration file. Name the file hello_config.ini and paste this inside:

[Section A]
phrase=Config

Now that we have a simple config file, we can integrate this into our previous Hello World command-line tool in Example 13-13.

Example 13-13. Hello config file command-line tool

#!/usr/bin/env python
import optparse
import ConfigParser

def readConfig(file="hello_config.ini"):
Config = ConfigParser.ConfigParser()
Config.read(file)
sections = Config.sections()
for section in sections:
#uncomment line below to see how this config file is parsed
#print Config.items(section)
phrase = Config.items(section)[0][1]
return phrase

def main():
p = optparse.OptionParser()
p.add_option('--sysadmin', '-s')
p.add_option('--config', '-c', action="store_true")
p.set_defaults(sysadmin="BOFH”)

options, arguments = p.parse_args()
if options.config:
options.sysadmin = readConfig()
print 'Hello, %s' % options.sysadmin

if __name__ == '__main__':
main()

If we run this tool without any options, we get a default value of BOFH just like the original “hello world” program:

[ngift@Macintosh-8][H:10543][J:0]# python hello_config_optparse.py
Hello, BOFH

If we select --config file, though, we parse our configuration file and get this response:

[ngift@Macintosh-8][H:10545][J:0]# python hello_config_optparse.py --config
Hello, Config

Note

Most of the time you will probably want to set a default path for a --config option and allow someone to customize the location where the file gets read. You can do that as follows instead of just storing the option to be default_true:

p.add_option('--config', '-c',
help='Path to read in config file')

If this was a bigger, and actually useful program, we could turn it over to someone without knowledge of Python. It would allow them to customize it by changing the value to parser=Config to be something else without having to touch the code. Even if they do have knowledge of Python, though, it is often nice to not have to enter the same options over and over on the command line, yet keep the tool flexible.

Summary

The standard library Optparse and ConfigParser modules are very easy to work with and have been around for quite some time, so they should be available on most systems you run into. If you find yourself needing to write a lot of command-line tools, it might be worth exploring on your own some of the advanced abilities of optparse, such as using callbacks and extending optparse itself. You also might be interested in looking at a few related modules that do not appear in the standard library such as: CommandLineApp (http://www.doughellmann.com/projects/CommandLineApp/), Argparse (http://pypi.python.org/pypi/argparse), and ConfigObj (http://pypi.python.org/pypi/ConfigObj).

Chapter 14. Pragmatic Examples

Managing DNS with Python

Managing a DNS server is a fairly straightforward task compared to, say, an Apache configuration file. The real problem that afflicts data centers and web hosting providers, though, is performing programatic large-scale DNS changes. It turns out that Python does quite a good job in this regard with a module called dnspython. Note there is also also another DNS module named PyDNS, but we will be covering dnspython.

Make sure you refer to the official documentation: http://www.dnspython.org/. There is also a great article on using dnspython here: http://vallista.idyll.org/~grig/articles/.

To get started using dnspython, you will only need to do an easy_install as the package is listed in the Python Package Index.

ngift@Macintosh-8][H:10048][J:0]# sudo easy_install dnspython
Password:
Searching for dnspython
Reading http://pypi.python.org/simple/dnspython/
[output supressed]

Next, we explore the module with IPython, like many other things in the book. In this example, we get the A and MX records for http://oreilly.com:

In [1]: import dns.resolver
In [2]: ip = dns.resolver.query("oreilly.com","A")
In [3]: mail = dns.resolver.query("oreilly.com","MX")
In [4]: for i,p in ip,mail:
....:     print i,p
....:
....:
208.201.239.37 208.201.239.36
20 smtp1.oreilly.com. 20 smtp2.oreilly.com.

In Example 14-1, we assign the “A” record results to ip and the “MX” records to mail. The “A” results are on top, and the “MX” records are on the bottom. Now that we have some idea how it works, let’s write a script that collects the “A” records of a collection of hosts.

Example 14-1. Query a group of hosts

import dns.resolver

hosts = ["oreilly.com", "yahoo.com", "google.com", "microsoft.com", "cnn.com"]

def query(host_list=hosts):
collection = []
for host in host_list:
ip = dns.resolver.query(host,"A”)
for i in ip:
collection.append(str(i))
return collection

if __name__ == "__main__":
for arec in query():
print arec

If we run this script, we get all of the “A” records for these hosts, and it looks like this:

[ngift@Macintosh-8][H:10046][J:0]# python query_dns.py
208.201.239.37
208.201.239.36
216.109.112.135
66.94.234.13
64.233.167.99
64.233.187.99
72.14.207.99
207.46.197.32
207.46.232.182
64.236.29.120
64.236.16.20
64.236.16.52
64.236.24.12

One obvious problem this solves is programmatically testing whether all of your hosts have the correct “A” record that you have on file.

There is quite a bit more that dnspython can do: it can manage DNS zones and perform more complex queries than what we described here. If you are interested in seeing even more examples, please see the URLs referenced earlier.

Using LDAP with OpenLDAP, Active Directory, and More with Python

LDAP is a buzzword at most corporations, and one of the authors even runs an LDAP database to manage his home network. If you are not familiar with LDAP, it stands for Lightweight Directory Access Protocol. One of the best definitions we have heard of LDAP comes from Wikipedia, “an application protocal for querying and modifying directory services running over TCP/IP.” One example of a service is authentication, which is by far the most popular use for the protocol. Examples of directory dervers that support the LDAP protocol are Open Directory, Open LDAP, Red Hat Directory Server, and Active Directory. The python-ldap API suports communication with both OpenLDAP and Active Directory.

There is a Python API to LDAP called python-ldap, and it includes in its API support an object-oriented wrapper with OpenLDAP 2.x. There is also support for other LDAP- related items, including processing LDIF files and LDAPv3. To get started, you will need to download the package from the python-ldap sourceforge project here: http://python-ldap.sourceforge.net/download.shtml.

After you install python-ldap, you will want to first explore the library in IPython. Here is what an interactive session looks like in which we perform both a succesful bind to a public ldap server and then an unsuccesful bind. Getting into the specifics of setting up and configuring LDAP is beyond the scope of this book, but we can start testing the python-ldap API using the University of Michigan’s public LDAP server.

In [1]: import ldap

In [2]: l = ldap.open("ldap.itd.umich.edu")

In [3]: l.simple_bind()
Out[3]: 1

That simple bind tells us we are successful, but let’s look at a failure and see what that looks like as well:

In [5]: try:
....:     l = ldap.open("127.0.0.1")
....: except Exception,err:
....:     print err
....:
....:

In [6]: l.simple_bind()
---------------------------------------------------------------------------
SERVER_DOWN                               Traceback (most recent call last)

/root/&lt;ipython console>

/usr/lib/python2.4/site-packages/ldap/ldapobject.py in simple_bind(self, who, cred,
serverctrls, clientctrls)
167     simple_bind([who='' [,cred='']]) -> int
168     """
--> 169     return self._ldap_call(self._l.simple_bind,who,cred,EncodeControlTuples
(serverctrls),EncodeControlTuples(clientctrls))
170
171   def simple_bind_s(self,who='',cred='',serverctrls=None,clientctrls=None):

/usr/lib/python2.4/site-packages/ldap/ldapobject.py in _ldap_call(self, func, *args,
**kwargs)
92     try:
93       try:
---> 94         result = func(*args,**kwargs)
95       finally:
96         self._ldap_object_lock.release()

SERVER_DOWN: {'desc': "Can't contact LDAP server"}

As we can see, in this example, there is not an LDAP server running, and our code blew up.

Importing an LDIF File

Making a simple bind to a public LDAP directory is not very useful to help you get your job done. Here is an example of doing an asynchronous LDIF import:

import ldap
import ldap.modlist as modlist

ldif = "somefile.ldif"
def create():
l = ldap.initialize("ldaps://localhost:636/")
l.simple_bind_s("cn=manager,dc=example,dc=com","secret")
dn="cn=root,dc=example,dc=com"
rec = {}
rec['objectclass'] = ['top','organizationalRole','simpleSecurityObject']
rec['cn'] = 'root'
rec['userPassword'] = 'SecretHash'
rec['description'] = 'User object for replication using slurpd'
ldif = modlist.addModlist(attrs)
l.add_s(dn,ldif)
l.unbind_s()

Going over this example, we initialize to a local LDAP server first, then create an object class that will map to the LDAP database when we do a mass asynchronous import of an LDIF file. Note that l.add_s is what shows us that we are making an ansynchronous call to the API.

These are the basics for using Python and LDAP together, but you should refer to the resources given at the beginning of the chapter for further information about using python-ldap. Specifically, there are examples that detail LDAPv3; Create, Read, Update, Delete (CRUD); and more.

One final thing to mention is that there is a tool aptly named web2ldap, and it is a Python, web-based frontend for LDAP by the same author of python-ldap. You might consider trying it out as well as an alternative to some of the other web-based management solutions for LDAP. Go to http://www.web2ldap.de/ for the official documentation. It is highly structured around LDAPv3 support.

Apache Log Reporting

Currently, Apache is the web server for approximately 50 percent of the domains on the Internet. The following example is intended to show you an approach for reporting on your Apache logfiles. This example will focus only on one aspect of the information available in your Apache logs, but you should be able to take this approach and apply it to any type of data that is contained in those logs. This approach will also scale well to large data files as well as large numbers of files.

In Chapter 3, we gave a couple of examples of parsing an Apache web server log to extract some information from it. In this example, we’ll reuse the modules we wrote for Chapter 3 to show how to generate a human-readable report from one or more logfiles. In addition to handling all of the logfiles that you specify separately, you can tell this script to consolidate the logfiles together and generate one single report. Example 14-2 shows the code for the script.

Example 14-2. Consolidated Apache logfile reporting

#!/usr/bin/env python

from optparse import OptionParser

def open_files(files):
for f in files:
yield (f, open(f))

def combine_lines(files):
for f, f_obj in files:
for line in f_obj:
yield line

def obfuscate_ipaddr(addr):
return ".".join(str((int(n) / 10) * 10) for n in addr.split('.'))

if __name__ == '__main__':
parser = OptionParser()
parser.add_option("-c", "--consolidate", dest="consolidate", default=False,
action='store_true', help="consolidate log files")
parser.add_option("-r", "--regex", dest="regex", default=False,
action='store_true', help="use regex parser")

(options, args) = parser.parse_args()
logfiles = args

if options.regex:
from  apache_log_parser_regex import generate_log_report
else:
from  apache_log_parser_split import generate_log_report

opened_files = open_files(logfiles)

if options.consolidate:
opened_files = (('CONSOLIDATED', combine_lines(opened_files)),)

for filename, file_obj in opened_files:
print "*" * 60
print filename
print "-" * 60
print "%-20s%s" % ("IP ADDRESS", "BYTES TRANSFERRED")
print "-" * 60
report_dict = generate_log_report(file_obj)
for ip_addr, bytes in report_dict.items():
print "%-20s%s" % (obfuscate_ipaddr(ip_addr), sum(bytes))
print "=" * 60

At the top of the script, we define two functions: open_files() and combine_lines(). Later in the script, both of these functions allow us later to use some mild generator-chaining to simplify the code just a bit. open_files() is a generator function that takes a list (actually, any iterator) of filenames. For each of those filenames, it yields a tuple of the filename and a corresponding open file object. combine_lines() takes an iterable of open file objects as its only argument. It iterates over the file objects with a for loop. For each of those files, it iterates over the lines in the file. And it yields each line that it iterates over. The iterable that we get from combine_lines() is comparable to how file objects are commonly used: iterating over the lines of the file.

Next, we use optparse to parse the command-line arguments from the user. We’re only accepting two arguments, both of them Boolean: consolidate logfiles and use regular expression library. The consolidate option tells the script to treat all the files as one file. In a sense, we wind up concatenating the files together if this option is passed in. But we’ll get to that momentarily. The regex option tells the script to use the regular expression library that we wrote in Chapter 3 rather than the “split” library. Both should offer identical functionality, but the “split” library is faster.

Next, we check whether the regex flag was passed in. If it was, we use the regex module. If not, we use the split module. We really included this flag and import condition to compare the performance of the two libraries. But, we’ll get to the running and performance of this script later.

Then, we call open_files() on our list of file names passed in by the user. As we’ve already mentioned, open_files() is a generator function and yields file objects from the list of filenames that we pass in to it. This means that it doesn’t actually open the file until it yields it back to us. Now that we have an iterable of open file objects, we can do a couple of things with it. We can either iterate over all of the files that we have generated and report on each file, or we can somehow combine the logfiles together and report on them as one file. This is where the combine_lines() function comes in. If the user passed in the “consolidate” flag, the “files” that will be iterated over are actually just a single file-like object: a generator of all the lines in all the files.

So, whether it is a real or combined file, we pass each file to the appropriate generate_log_report() function, which then returns a dictionary of IP addresses and bytes sent to that IP. For each file, we print out some section breaking strings and formatted strings containing the results of generate_log_report(). The output for a run on a single 28 KB logfile looks like this:

************************************************************
access.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         17479
200.80.230.0        45346
200.40.90.110       8276
130.150.250.0       0
70.0.10.140         2115
70.180.0.220        76992
200.40.90.110       23860
190.20.250.190      499
190.20.250.210      431
60.210.40.20        27681
60.240.70.180       20976
70.0.20.120         1265
190.20.250.210      4268
190.50.200.210      4268
60.100.200.230      0
70.0.20.190         378
190.20.250.250      5936
============================================================

The output for three logfiles (actually, the same logfile three times with the same log data duplicated over and over) looks like this:

************************************************************
access.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         17479
200.80.230.0        45346
<snip>
70.0.20.190         378
190.20.250.250      5936
============================================================
************************************************************
access_big.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         1747900
200.80.230.0        4534600
<snip>
70.0.20.190         37800
190.20.250.250      593600
============================================================
************************************************************
access_bigger.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         699160000
200.80.230.0        1813840000
<snip>
70.0.20.190         15120000
190.20.250.250      237440000
============================================================

And the output of all three consolidated together looks like this:

************************************************************
CONSOLIDATED
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         700925379
200.80.230.0        1818419946
<snip>
190.20.250.250      238039536
============================================================

So, how well does this script perform? And what does memory consumption look like? All benchmarks in this section were run on an Ubuntu Gutsy server with an AMD Athlon 64 X2 5400+ 2.8 GHz, 2 GB of RAM, and a Seagate Barracuda 7200 RPM SATA drive. And we were using a roughly 1 GB file:

jmjones@ezr:/data/logs$ ls -l access*log
-rw-r--r-- 1 jmjones jmjones 1157080000 2008-04-18 12:46 access_bigger.log

Here are the run times.

$ time python summarize_logfiles.py --regex access_bigger.log
************************************************************
access_bigger.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         699160000
<snip>
190.20.250.250      237440000
============================================================

real    0m46.296s
user    0m45.547s
sys     0m0.744s

jmjones@ezr:/data/logs$ time python summarize_logfiles.py access_bigger.log
************************************************************
access_bigger.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         699160000
<snip>
190.20.250.250      237440000
============================================================

real    0m34.261s
user    0m33.354s
sys     0m0.896s

For the regular expression version of the data extraction library, it took about 46 seconds. For the version that uses string.split(), it took about 34 seconds. But memory usage was abysmal. It ran up to about 130 MB of memory. The reason for this is that the generate_log_report() keeps a list of bytes transferred for each IP address in the logfile. So, the larger the file, the more memory this script will consume. But we can do something about that. Here is a less memory-hungry version of the parsing library:

#!/usr/bin/env python

def dictify_logline(line):
'''return a dictionary of the pertinent pieces of an apache combined log file

Currently, the only fields we are interested in are remote host and bytes sent,
but we are putting status in there just for good measure.
'''
split_line = line.split()
return {'remote_host': split_line[0],
'status': split_line[8],
'bytes_sent': split_line[9],
}

def generate_log_report(logfile):
'''return a dictionary of format remote_host=>[list of bytes sent]

This function takes a file object, iterates through all the lines in the file,
and generates a report of the number of bytes transferred to each remote host
for each hit on the webserver.
'''
report_dict = {}
for line in logfile:
line_dict = dictify_logline(line)
host = line_dict['remote_host']
#print line_dict
try:
bytes_sent = int(line_dict['bytes_sent'])
except ValueError:
##totally disregard anything we don't understand
continue
report_dict[host] = report_dict.setdefault(host, 0) + bytes_sent
return report_dict

Basically, this one tallies the bytes_sent as it goes rather than making the calling function tally it. Here is a slightly modified summarize_logfiles script with a new option to import the less memory-hungry version of the library:

#!/usr/bin/env python

from optparse import OptionParser

def open_files(files):
for f in files:
yield (f, open(f))

def combine_lines(files):
for f, f_obj in files:
for line in f_obj:
yield line

def obfuscate_ipaddr(addr):
return ".".join(str((int(n) / 10) * 10) for n in addr.split('.'))

if __name__ == '__main__':
parser = OptionParser()
parser.add_option("-c", "--consolidate", dest="consolidate", default=False,
action='store_true', help="consolidate log files")
parser.add_option("-r", "--regex", dest="regex", default=False,
action='store_true', help="use regex parser")
parser.add_option("-m", "--mem", dest="mem", default=False,
action='store_true', help="use mem parser")

(options, args) = parser.parse_args()
logfiles = args

if options.regex:
from  apache_log_parser_regex import generate_log_report
elif options.mem:
from  apache_log_parser_split_mem import generate_log_report
else:
from  apache_log_parser_split import generate_log_report

opened_files = open_files(logfiles)

if options.consolidate:
opened_files = (('CONSOLIDATED', combine_lines(opened_files)),)

for filename, file_obj in opened_files:
print "*" * 60
print filename
print "-" * 60
print "%-20s%s" % ("IP ADDRESS", "BYTES TRANSFERRED")
print "-" * 60
report_dict = generate_log_report(file_obj)
for ip_addr, bytes in report_dict.items():
if options.mem:
print "%-20s%s" % (obfuscate_ipaddr(ip_addr), bytes)
else:
print "%-20s%s" % (obfuscate_ipaddr(ip_addr), sum(bytes))
print "=" * 60

And this actually wound up being a bit faster than the more memory-hungry version:

jmjones@ezr:/data/logs$ time ./summarize_logfiles_mem.py --mem access_bigger.log
************************************************************
access_bigger.log
------------------------------------------------------------
IP ADDRESS          BYTES TRANSFERRED
------------------------------------------------------------
190.40.10.0         699160000
<snip>
190.20.250.250      237440000
============================================================

real    0m30.508s
user    0m29.866s
sys     0m0.636s

Memory consumption held steady at about 4 MB for the duration of this run. This script will handle about 2 GB of logfiles per minute. Theoretically, the file sizes could be indefinite and memory wouldn’t grow like it did with the previous version. However, since this is using a dictionary and each key is a unique IP address, the memory usage will grow with unique IP addresses. If memory consumption becomes a problem, however, you could swap out the dictionary with a persistent database, either relational or even a Berkeley DB.

FTP Mirror

This next example shows how to connect to an FTP server and recursively retrieve all the files on that server starting with some user-specified directory. It also allows you to remove each file after you have retrieved it. You may be wondering, “What is the point of this script? Doesn’t rsync handle all of that?” And the answer is, “Yes, it does.” However, what if rsync is not installed on the server you are working on and you aren’t permitted to install it? (This is unlikely for you as a sysadmin, but it happens.) Or, what if you don’t have SSH or rsync access to the server you’re trying to pull from? It helps to have an alternative. Here is the source code for the mirror script:

#!/usr/bin/env python

import ftplib
import os

class FTPSync(object):
def __init__(self, host, username, password, ftp_base_dir,
local_base_dir, delete=False):
self.host = host
self.username = username
self.password = password
self.ftp_base_dir = ftp_base_dir
self.local_base_dir = local_base_dir
self.delete = delete

self.conn = ftplib.FTP(host, username, password)
self.conn.cwd(ftp_base_dir)
try:
os.makedirs(local_base_dir)
except OSError:
pass
os.chdir(local_base_dir)
def get_dirs_files(self):
dir_res = []
self.conn.dir('.', dir_res.append)
files = [f.split(None, 8)[-1] for f in dir_res if f.startswith('-')]
dirs = [f.split(None, 8)[-1] for f in dir_res if f.startswith('d')]
return (files, dirs)
def walk(self, next_dir):
print "Walking to", next_dir
self.conn.cwd(next_dir)
try:
os.mkdir(next_dir)
except OSError:
pass
os.chdir(next_dir)

ftp_curr_dir = self.conn.pwd()
local_curr_dir = os.getcwd()

files, dirs = self.get_dirs_files()
print "FILES:", files
print "DIRS:", dirs
for f in files:
print next_dir, ':', f
outf = open(f, 'wb')
try:
self.conn.retrbinary('RETR %s' % f, outf.write)
finally:
outf.close()
if self.delete:
print "Deleting", f
self.conn.delete(f)
for d in dirs:
os.chdir(local_curr_dir)
self.conn.cwd(ftp_curr_dir)
self.walk(d)

def run(self):
self.walk('.')

if __name__ == '__main__':
from optparse import OptionParser
parser = OptionParser()
parser.add_option("-o", "--host", dest="host",
action='store', help="FTP host")
parser.add_option("-u", "--username", dest="username",
action='store', help="FTP username")
parser.add_option("-p", "--password", dest="password",
action='store', help="FTP password")
parser.add_option("-r", "--remote_dir", dest="remote_dir",
action='store', help="FTP remote starting directory")
parser.add_option("-l", "--local_dir", dest="local_dir",
action='store', help="Local starting directory")
parser.add_option("-d", "--delete", dest="delete", default=False,
action='store_true', help="use regex parser")

(options, args) = parser.parse_args()
f = FTPSync(options.host, options.username, options.password,
options.remote_dir, options.local_dir, options.delete)
f.run()

This script was a little easier to write by using a class. The constructor takes a number of parameters. To connect and log in, you have to pass it host, username, and password. To get to the appropriate places on the remote server and your local server, you have to pass in ftp_base_dir and local_base_dir. delete is just a flag that specifies whether to delete the file from the remote server once you’ve downloaded it—you can see in the constructor that we set the default value for this to False.

Once we set these values that we received as object attributes, we connect to the specified FTP server and log in. Then, we change to the specified start directory on the server and change to the start directory on the local machine. Before actually changing into the local start directory, we first try to create it. If it exists, we’ll get an OSError exception, which we ignore.

We have three additional methods defined: get_dirs_files(), walk(), and run(). get_dirs_files() determines which files in the current directory are files and which are directories. (By the way, this is expected to only work on Unix servers.) It figures out which are files and which are directories by doing a directory listing looking at the first character of each line of the listing. If the character is d, then it is a directory. If the character is -, then it is a file. This means that we won’t follow symlinks nor deal with block devices.

The next method that we defined is walk(). This method is where the bulk of the work happens. The walk() method takes a single argument: the next directory to visit. Before we go any further, we’ll mention that this is a recursive function. We intend for it to call itself. If any directory contains other directories, they will be walked into. The code in the walk() method first changes directory on the FTP server to the specified directory. Then we change into the directory on the local server, creating it if necessary. Then we store our current positions on both the FTP server and locally into the variables ftp_curr_dir and local_curr_dir for use later. Next, we get the files and directories in this directory from our get_dirs_files() method that we’ve already mentioned. For each of the files in the directory, we retrieve them using the retrbinary() FTP method. We also delete the file if the delete flag was passed in. Next, we change directory to the current directories on the FTP server and FTP server and call walk() to walk into those lower directories. The reason that we change into the current directory again is so that when lower walk() calls return, we can come back up to where we are.

The final method that we defined is run(). run() is simply a convenience method. Calling run() simply calls walk() and passes it the current FTP directory.

We have some very basic error and exception handling in this script. First, we don’t check all the command-line arguments and make sure that at least host, username, and password are passed in. The script will blow up quickly if those aren’t specified. Also, we don’t try to download a file again if an exception happened. Instead, if something causes a download to fail, we’re going to get an exception. The program will terminate in that case. If the script terminates in the middle of a download, the next time you start it up, the script will begin downloading the file again. The upside to this is that it won’t delete a file it has only partially downloaded.

Appendix A. Callbacks

The concept of callbacks and passing functions around may be foreign to you. If so, it is definitely worth digging into so that you understand it well enough to use it, or at the very least, understand what is going on when you see it being used. In Python, functions are “first class,” which means that you can pass them around and treat them as objects—because they really are objects. See Example A-1.

Example A-1. Showing functions as first class

In [1]: def foo():
...:     print foo
...:
...:

In [2]: foo
Out[2]: <function foo at 0x1233270>

In [3]: type(foo)
Out[3]: <type 'function'>

In [4]: dir(foo)
Out[4]:
['__call__',
'__class__',
'__delattr__',
'__dict__',
'__doc__',
'__get__',
'__getattribute__',
'__hash__',
'__init__',
'__module__',
'__name__',
'__new__',
'__reduce__',
'__reduce_ex__',
'__repr__',
'__setattr__',
'__str__',
'func_closure',
'func_code',
'func_defaults',
'func_dict',
'func_doc',
'func_globals',
'func_name']

Simply referring to a function, such as foo in the previous example, does not call it. Referring to a function’s name lets you get at any attributes the function has and to even refer to the function by a different name later. See Example A-2.

Example A-2. Referring to functions by name

In [1]: def foo():
...:     """this is a docstring"""
...:     print "IN FUNCTION FOO"
...:
...:

In [2]: foo
Out[2]: <function foo at 0x8319534>

In [3]: foo.__doc__
Out[3]: 'this is a docstring'

In [4]: bar = foo

In [5]: bar
Out[5]: <function foo at 0x8319534>

In [6]: bar.__doc__
Out[6]: 'this is a docstring'

In [7]: foo.a = 1

In [8]: bar.a
Out[8]: 1

In [9]: foo()
IN FUNCTION FOO

In [10]: bar()
IN FUNCTION FOO

We created a new function foo so that it contained a docstring. We then stated that bar was going to point to the function foo that we just created. In Python, what you usually think of as variables are typically just names that point at (or refer to) some object. The process of associating a name with an object is called “name binding.” So, when we created the function foo, we really created a function object and then bound the name foo to that new function. Using the IPython prompt to see the basic information it can tell us about foo, it reported back that it was a foo function. Interestingly, it said the same thing about the name bar, namely that it was a foo function. We set an attribute a on the function named foo and were able to access it from bar. And, calling both foo and bar produced the same result.

One of the places in this book that we use callbacks is in the chapter on networking, Chapter 5. Passing functions around as in the FTP example in that chapter allows for runtime dynamism and code-time flexibility and can even improve code reuse. Even if you don’t think you’ll ever use it, it’s a thought process worth putting in your brain’s catalog.

Index

A note on the digital index

A link in an index entry is displayed as the section title in which that entry appears. Because some sections have multiple index markers, it is not unusual for an entry to have several links to the same section. Clicking on any link will take you directly to the place in the text in which the marker appears.

Symbols

.py files, Executing Statements in Python (see wrappers)

\ (backslash), Creating strings

escape sequences, list of, Creating strings

$ (dollar sign), Shell Execute

for shell execute variables, Shell Execute

! (exclamation point), Shell Execute, Shell Execute

for shell execute, Shell Execute

!! for shell execute, Shell Execute

%-TAB, Help with Magic Functions

? (question mark) for help, Executing Statements in Python, Help with Magic Functions, pinfo, pinfo, psearch

to obtain object information, pinfo

to search for objects, psearch

?? to obtain object information, pinfo

' (quotation mark, single), Creating strings

creating strings with, Creating strings

" (quotation marks, double), Creating strings

creating strings with, Creating strings

_ (underscore), rehash, psearch, psearch, History results, History results

for results history, History results, History results

__ (in variable names), rehash

__ object, psearch

___ object, psearch

“magic” functions, alias

(see also specific function)

A

Active Directory, using with Python, Using LDAP with OpenLDAP, Active Directory, and More with Python, Importing an LDIF File

active version of package, changing, Change Active Version of Package

alias function, alias, alias, alias

alias table, rehash, rehashx

Amazon Web services (Boto), Amazon Web Services with Boto

Apache config file, hacking (example), Apache Config File Hacking, Apache Config File Hacking

Apache log reporting, Apache Log Reporting, Apache Log Reporting

Apache Log Viewer, building (example), Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses, Apache Log Viewer Application, Apache Log Viewer Application

with curses library, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses

with Django, Apache Log Viewer Application, Apache Log Viewer Application

with PyGTK, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK

Apache logfile, parsing (example), Log Parsing, Log Parsing

appscript project, OS X Scripting APIs

archiving data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

examining TAR file contents, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

ARP protocol, Creating Hybrid SNMP Tools

asr utility, Automatically Re-Imaging Machines

attachments (email), sending, Sending attachments with Python

attrib attribute (Element object), ElementTree

authentication, Authenticating to a Password Protected Site

when installing eggs, Authenticating to a Password Protected Site

authentication (SMTP), Using SMTP authentication

automated information gathering, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email

receiving email, Receiving Email, Receiving Email

automatically re-imaging routines, Automatically Re-Imaging Machines

automation, with IPython shell, Automation and Shortcuts, rep

B

background threading, IPython and Net-SNMP

backslash (\), Creating strings

escape sequences, list of, Creating strings

backups, Introduction, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

examining TAR file contents, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

bar charts, creating, Graphical Images

Bash, Python versus, Why Python?

Bayer, Michael, SQLAlchemy ORM

Bicking, Ian, virtualenv

blocks of code, editing, Magic Edit

bookmark command, bookmark

bookmarks, cd

navigating to bookmarked directories, cd

bootstrapped virtual environment, custom, Creating a Custom Bootstrapped Virtual Environment

Boto (Amazon Web services), Amazon Web Services with Boto

Buildout tool, Buildout, Developing with Buildout, Developing with Buildout

developing with, Developing with Buildout

bzip2 compression, Using tarfile Module to Create TAR Archives

C

callbacks, Callbacks, Callbacks

capitalization, Built-in methods for str data extraction (see case)

case (capitalization), Built-in methods for str data extraction

converting for entire string, Built-in methods for str data extraction

cd command, cd, cd, cd, dhist

-<TAB> option, dhist

-b option, cd

charts, creating, Graphical Images

checksum comparisons, MD5 Checksum Comparisons, MD5 Checksum Comparisons

choices usage pattern (optparse), Choices Usage Pattern

close( ) function (socket module), socket

close() method, Creating files

close() method (shelve), shelve

cloud computing, Cloud Computing, Building a sample Google App Engine application, Amazon Web Services with Boto, Google App Engine, Building a sample Google App Engine application

Amazon Web services with Boto, Amazon Web Services with Boto

cmp() function (filecmp module), Using the filecmp Module

combining strings, Built-in methods for str data extraction

command history, History

command line, Introduction, Summary, Basic Standard Input Usage, Basic Standard Input Usage, Introduction to Optparse, Option with Multiple Arguments Usage Pattern, Unix Mashups: Integrating Shell Commands into Python Command-Line Tools, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes, Integrating Configuration Files, Integrating Configuration Files

basic standard input usage, Basic Standard Input Usage, Basic Standard Input Usage

integrating configuration files, Integrating Configuration Files, Integrating Configuration Files

integrating shell commands, Unix Mashups: Integrating Shell Commands into Python Command-Line Tools, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

optparse, Introduction to Optparse, Option with Multiple Arguments Usage Pattern

community of Python users, Why Python?, IPython

comparing data, Comparing Data, Using os.list, Using the filecmp Module, Using os.list, MD5 Checksum Comparisons, MD5 Checksum Comparisons

files and directory contents, Using the filecmp Module, Using os.list

MD5 checksum comparisons, MD5 Checksum Comparisons, MD5 Checksum Comparisons

comparing strings, Built-in methods for str data extraction

upper() and lower() methods, Built-in methods for str data extraction

compiled regular expressions, re

completion functionality, Tab Completion

complexity of Python functionality, Why Python?

compressing data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

concatenating strings, Built-in methods for str data extraction

concurrency, Threads in Python, Threaded Event Handler, Processes, Processes

processes and, Processes, Processes

threads and, Threads in Python, Threaded Event Handler

conditionals, Why Python?

in Perl and Bash, Why Python?

ConfigParser module, Integrating Configuration Files

configuration files, integrating with command line, Integrating Configuration Files, Integrating Configuration Files

configuration files, when installing eggs, Using Configuration Files

configuring IPython, Configuring IPython

conflicting packages, managing, virtualenv

connect( ) function (socket module), socket

connect() method (ftplib module), ftplib

console scripts, Entry Points and Console Scripts, Entry Points and Console Scripts

__contains__() operator, Built-in methods for str data extraction

context managers, Creating files

counting options usage pattern (optparse), Counting Options Usage Pattern

cPickle library, Pickle, cPickle, cPickle

(see also pickle module)

cron, for running processes, Scheduling Python Processes

cross-platform systems management, Creating a Cross-Platform, Systems Management Tool

currency, Threads in Python, Threaded Event Handler, Processes, Processes

processes and, Processes, Processes

threads and, Threads in Python, Threaded Event Handler

current directory, identifying, pwd

current working directory, installing source in, Install an Unpacked Source Distribution in Current Working Directory

curses library, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses

Apache Log Viewer, building (example), Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses

D

daemonizer, daemonizer, daemonizer

daemons, about, daemonizer

data centers, discovering (SNMP), Discovering a Data Center, Discovering a Data Center, Retrieving Multiple-Values with Net-SNMP, Finding Memory

retrieving multiple values, Retrieving Multiple-Values with Net-SNMP, Finding Memory

data persistence, Data Persistence, Summary, Simple Serialization, ZODB, Pickle, Pickle, cPickle, cPickle, shelve, shelve, YAML, ZODB, ZODB, Relational Serialization, SQLAlchemy ORM, SQLite, SQLite, Storm ORM, Storm ORM, SQLAlchemy ORM, SQLAlchemy ORM

relational serialization, Relational Serialization, SQLAlchemy ORM, SQLite, SQLite, Storm ORM, Storm ORM, SQLAlchemy ORM, SQLAlchemy ORM

SQLAlchemy ORM, SQLAlchemy ORM, SQLAlchemy ORM

SQLite library, SQLite, SQLite

Storm ORM, Storm ORM, Storm ORM

simple serialization, Simple Serialization, ZODB, Pickle, Pickle, cPickle, cPickle, shelve, shelve, YAML, ZODB, ZODB

cPickle library, cPickle, cPickle

pickle module, Pickle, Pickle

shelve module, shelve, shelve

YAML data format, YAML

ZODB module, ZODB, ZODB

data tree, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data

copying with shutil (example), Copying, Moving, Renaming, and Deleting Data

deleting with shutil (example), Copying, Moving, Renaming, and Deleting Data

moving with shutil (example), Copying, Moving, Renaming, and Deleting Data

data wrangler, Introduction

data, working with, Introduction, Using a tarfile Module to Examine the Contents of TAR Files, Using the OS Module to Interact with Data, Using the OS Module to Interact with Data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files, Comparing Data, Using os.list, Merging Data, MD5 Checksum Comparisons, Pattern Matching Files and Directories, Pattern Matching Files and Directories, Wrapping Up rsync, Wrapping Up rsync, Metadata: Data About Data, Metadata: Data About Data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

archiving, compressing, imaging, and restoring, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

comparing data, Comparing Data, Using os.list

copying, moving, renaming, and deleting, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data

merging data, Merging Data, MD5 Checksum Comparisons

metadata, Metadata: Data About Data, Metadata: Data About Data

os module, Using the OS Module to Interact with Data, Using the OS Module to Interact with Data

paths, directories, and files, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files

pattern matching, Pattern Matching Files and Directories, Pattern Matching Files and Directories

rsync utility for, Wrapping Up rsync, Wrapping Up rsync

database application, building with Django (example), Simple Database Application, Simple Database Application

.deb packages, Installing IPython

definition headers, printing, pdef

deleting (removing), bookmark, reset, Built-in methods for str data extraction, Built-in methods for str data extraction, Copying, Moving, Renaming, and Deleting Data, MD5 Checksum Comparisons

bookmarks, bookmark

content from strings, Built-in methods for str data extraction, Built-in methods for str data extraction

data tree (shutil example), Copying, Moving, Renaming, and Deleting Data

files, MD5 Checksum Comparisons

variables from interactive namespace, reset

delimiter, splitting strings at, Built-in methods for str data extraction, Built-in methods for str data extraction

device control with SNMP, SNMP Device Control, SNMP Device Control

dhist command, dhist

dircmp() function (filecmp module), Using the filecmp Module

directories, cd, cd, pwd, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files, Using the filecmp Module, Using os.list, Merging Data, MD5 Checksum Comparisons, Pattern Matching Files and Directories, Pattern Matching Files and Directories, Wrapping Up rsync, Wrapping Up rsync, Using tarfile Module to Create TAR Archives, Install an Unpacked Source Distribution in Current Working Directory

archiving with tar, Using tarfile Module to Create TAR Archives

bookmarked, navigating to, cd

changing between, cd (see cd command)

comparing with filecmp, Using the filecmp Module, Using os.list

current, identifying with pwd, pwd

installing unpacked sources into, Install an Unpacked Source Distribution in Current Working Directory

merging directory trees, Merging Data, MD5 Checksum Comparisons

pattern-matching, Pattern Matching Files and Directories, Pattern Matching Files and Directories

synchronizing with rsync, Wrapping Up rsync, Wrapping Up rsync

walking, with os module, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files

directory history, cd, dhist

directory trees, MD5 Checksum Comparisons, MD5 Checksum Comparisons, Pattern Matching Files and Directories, Wrapping Up rsync, Wrapping Up rsync

(see also directories)

finding duplicates in, MD5 Checksum Comparisons

renaming files in, Pattern Matching Files and Directories

synchronizing with rsync, Wrapping Up rsync, Wrapping Up rsync

discovering data centers (SNMP), Discovering a Data Center, Discovering a Data Center, Retrieving Multiple-Values with Net-SNMP, Finding Memory

retrieving multiple values, Retrieving Multiple-Values with Net-SNMP, Finding Memory

dispatcher, SSH-based, Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

distributing information, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

sending email, Sending Email, Sending attachments with Python

sending email attachments, Sending attachments with Python

disutils, Setuptools and Python Eggs, Distutils, Distutils

Django templating system, Django, Simple Database Application, Apache Log Viewer Application, Apache Log Viewer Application, Simple Database Application, Simple Database Application

Apache Log Viewer, building (example), Apache Log Viewer Application, Apache Log Viewer Application

simple database application, building (example), Simple Database Application, Simple Database Application

DNS, managing with Python, Managing DNS with Python, Managing DNS with Python

dnspython module, Managing DNS with Python

documentation and reporting, Documentation and Reporting, Summary, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email, Manual Information Gathering, Manual Information Gathering, Information Formatting, PDFs, PDFs, PDFs, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

automated information gathering, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email

receiving email, Receiving Email, Receiving Email

information distribution, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

sending email, Sending Email, Sending attachments with Python

sending email attachments, Sending attachments with Python

information formatting, Information Formatting, PDFs, PDFs, PDFs

saving as PDF files, PDFs, PDFs

manual information gathering, Manual Information Gathering, Manual Information Gathering

dollar sign ($), Shell Execute

for shell execute variables, Shell Execute

DOM (Document Object Model), for XML, ElementTree

double quotation marks ("), Creating strings

creating strings with, Creating strings

downloading IPython, The Basics, Installing IPython

drawString() method (ReportLab), PDFs

DSCL (Directory Services Command Line), Scripting DSCL or Directory Services Utility

duplicates in merged directories, finding, MD5 Checksum Comparisons

E

easy install module, easy_install Advanced Features, Easy Install Advanced Features Summary

advanced features, easy_install Advanced Features, Easy Install Advanced Features Summary

easy to learn, Python as, Why Python?, Motivation, Motivation

easy_install utility, Installing IPython

edit function (“magic”), Magic Edit

.egg files (eggs), Installing IPython

eggs, Setuptools and Python Eggs, Install Egg Located on Local or Network Filesystem, Changing Standalone .py File into egg, Creating Eggs, Creating Eggs, Creating Eggs

changing standalone .py file into, Changing Standalone .py File into egg

defined, Creating Eggs

for package management, Setuptools and Python Eggs, Creating Eggs, Creating Eggs

installing on filesystems, Install Egg Located on Local or Network Filesystem

ElementTree library, ElementTree, ElementTree

email (incoming), processing, Receiving Email, Receiving Email

email (outgoing), writing, Sending Email, Sending attachments with Python

email attachments, sending, Sending attachments with Python

email package, Sending basic messages

end() method, re

endswith() method, Built-in methods for str data extraction

__enter__() method, Creating files

entry points, Entry Points and Console Scripts, Entry Points and Console Scripts

EPM Package Manager, EPM Package Manager, EPM Summary: It Really Is That Easy

escape sequences, Creating strings

event handler, threaded, Threaded Event Handler

event handlers, GUI Building Theory

event-driven networks, Twisted

exclamation point (!), Shell Execute, Shell Execute

for shell execute, Shell Execute

!! for shell execute, Shell Execute

executing files, at shell, run

executing statements, Executing Statements in Python, Summary

exec_command() method, SSH

__exit__() method, Creating files

extracting data from strings, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction

looking within strings, Built-in methods for str data extraction, Built-in methods for str data extraction

F

fields() method, String Processing

file object, creating, Creating files

filecmp module, Using the filecmp Module, Using os.list

files, run, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files, Using the filecmp Module, Using os.list, Merging Data, MD5 Checksum Comparisons, MD5 Checksum Comparisons, Pattern Matching Files and Directories, Pattern Matching Files and Directories, Pattern Matching Files and Directories, Metadata: Data About Data, Metadata: Data About Data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files, Using tarfile Module to Create TAR Archives

archiving with tar, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

comparing with filecmp, Using the filecmp Module, Using os.list

compression with bzip2, Using tarfile Module to Create TAR Archives

deleting, MD5 Checksum Comparisons

executing (IPython shell), run

merging directories together, Merging Data, MD5 Checksum Comparisons

metadata about, Metadata: Data About Data, Metadata: Data About Data

pattern-matching, Pattern Matching Files and Directories, Pattern Matching Files and Directories

renaming, within directory tree, Pattern Matching Files and Directories

walking, with os module, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files

files, working with, Working with Files, Additional resources, Creating files, Creating files, Reading files, Reading files, Writing files, Writing files, Standard Input and Output, Standard Input and Output, Log Parsing, Log Parsing, ElementTree, ElementTree

(see also input)

(see also output)

creating files, Creating files, Creating files

log parsing (example), Log Parsing, Log Parsing

parsing XML with ElementTree, ElementTree, ElementTree

reading files, Reading files, Reading files

writing files, Writing files, Writing files

find() method, Built-in methods for str data extraction

find() method (ElementTree), ElementTree

findall() method, re, re, re

findall() method (ElementTree), ElementTree

finding, MD5 Checksum Comparisons (see searching)

finditer() method, re

fingerprinting operating system type, Cross-Platform Unix Programming in Python, Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

fnmatch module, Pattern Matching Files and Directories

fork() method, daemonizer

formatting information, Information Formatting, PDFs, PDFs, PDFs

as PDF files, PDFs, PDFs

FTP mirror, building, FTP Mirror, FTP Mirror

ftplib module, ftplib, ftplib

Fulton, Jim, Buildout

functions, Using Functions in Python, Using Functions in Python, Help with Magic Functions

“magic”, Help with Magic Functions (see “magic” functions)

G

gathering information automatically, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email

receiving email, Receiving Email, Receiving Email

gathering information manually, Manual Information Gathering, Manual Information Gathering

gdchart module, Graphical Images

generator objects, Working with Paths, Directories, and Files

get() method (Element object), ElementTree

getresponse() method (httplib module), httplib

Gibbs, Kevin, Google App Engine

glob module, Pattern Matching Files and Directories

GNU/Linux, PyInotify with, PyInotify, PyInotify

Google App Engine, Google App Engine, Building a sample Google App Engine application

grep() method, String Processing, String Processing

groupdict() method, re

groups() method, re

GUIs, building, Building GUIs, Conclusion, GUI Building Theory, GUI Building Theory, Building a Simple PyGTK App, Building a Simple PyGTK App, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses, Web Applications, Web Applications, Django, Simple Database Application, Apache Log Viewer Application, Apache Log Viewer Application, Simple Database Application, Simple Database Application

Apache Log Viewer (example), Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses, Apache Log Viewer Application, Apache Log Viewer Application

with curses library, Building an Apache Log Viewer Using Curses, Building an Apache Log Viewer Using Curses

with Django, Apache Log Viewer Application, Apache Log Viewer Application

with PyGTK, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK

database application (example), Simple Database Application, Simple Database Application

Django templating system, Django, Simple Database Application

example of (simple PyGTK application), Building a Simple PyGTK App, Building a Simple PyGTK App

theory of, GUI Building Theory, GUI Building Theory

Web applications, Web Applications, Web Applications

gzip archiving, Using tarfile Module to Create TAR Archives

H

HardwareComponent class, Simple Database Application

help documentation, Executing Statements in Python, Help with Magic Functions, Help with Magic Functions, Help with Magic Functions

on “magic” functions, Help with Magic Functions

question mark (?) for, Executing Statements in Python, Help with Magic Functions

%quickref command, Help with Magic Functions

Hillegass, Aaron, Manual Information Gathering

hist (history) function, hist command

history, command, History

history, directory, cd, dhist

history, results, History results, History results

HTML, converting ReST to, Manual Information Gathering

httplib module, httplib, httplib

hybrid Kudzu design pattern, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

to change the behavior, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior

to spawn processes, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

hybrid SNMP tools, creating, Creating Hybrid SNMP Tools, Creating Hybrid SNMP Tools

I

imaging data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

IMAP protocol, Receiving Email

imaplib module, Receiving Email

import statement, Reusing Code with the Import Statement, Reusing Code with the Import Statement

importing LDIF files, Importing an LDIF File

importing modules, Executing Statements in Python, Executing Statements in Python

In built-in variable, Interacting with IPython

in test operator, Built-in methods for str data extraction

indenting Python code, Why Python?, Using Functions in Python

index() method, Built-in methods for str data extraction

information distribution, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

sending email, Sending Email, Sending attachments with Python

sending email attachments, Sending attachments with Python

information formatting, Information Formatting, PDFs, PDFs, PDFs

saving as PDF files, PDFs, PDFs

information gathering, automated, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email

receiving email, Receiving Email, Receiving Email

information gathering, manual, Manual Information Gathering, Manual Information Gathering

input, Standard Input and Output, Standard Input and Output

standard input and output, Standard Input and Output, Standard Input and Output

input prompts, Python versus IPython, Interacting with IPython

installing eggs on filesystems, Install Egg Located on Local or Network Filesystem

installing IPython, The Basics, Installing IPython

interacting with IPython, Interacting with IPython, Interacting with IPython

Internet content, processing as input, urllib

interpreter, IPython (see IPython shell)

interprocess communication (IPC), Remote Procedure Call Facilities

inventorying several machines, Retrieving Multiple-Values with Net-SNMP, Finding Memory

__IP variable, rehash

IPAddress class (Django), Simple Database Application

IPC (interprocess communication), Remote Procedure Call Facilities

IPython community, IPython

.ipython directory, Configuring IPython

IPython shell, The Basics, IPython, Installing IPython, Basic Concepts, Configuring IPython, Interacting with IPython, Interacting with IPython, Tab Completion, Magic Edit, Configuring IPython, Unix Shell, sh Profile, alias, Shell Execute, rehash, rehashx, cd, cd, bookmark, dhist, pwd, Variable Expansion, String Processing, String Processing, sh Profile, sh Profile, Information Gathering, History results, History, Automation and Shortcuts, rep

automation and shortcuts, Automation and Shortcuts, rep

basic concepts, Basic Concepts, Configuring IPython, Interacting with IPython, Interacting with IPython, Tab Completion, Magic Edit, Configuring IPython

configuring IPython, Configuring IPython

interacting with IPython, Interacting with IPython, Interacting with IPython

“magic” edit function, Magic Edit

tab completion, Tab Completion

downloading and installing, The Basics

information-gathering techniques, Information Gathering, History results, History

command history, History

installing, Installing IPython

working with Unix shell, Unix Shell, sh Profile, alias, Shell Execute, rehash, rehashx, cd, cd, bookmark, dhist, pwd, Variable Expansion, String Processing, String Processing, sh Profile, sh Profile

alias function, alias

bookmark command, bookmark

cd command, cd, cd

dhist command, dhist

pwd command, pwd

rehash command, rehash

rehashx command, rehashx

shell execute, Shell Execute

string processing, String Processing, String Processing

variable expansion, Variable Expansion

ipy_user_conf.py file, Configuring IPython

issue tracking system (Trac), Trac

J

join() method, Built-in methods for str data extraction

K

Kudzu design pattern, Kudzu Usage Pattern: Wrapping a Tool in Python, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

and changing the behavior, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior

and spawning processes, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

L

LDAP, using with Python, Using LDAP with OpenLDAP, Active Directory, and More with Python, Importing an LDIF File

LDIF files, importing, Importing an LDIF File

leading whitespace, stripping from strings, Built-in methods for str data extraction

Linux operating systems, PyInotify, PyInotify, Red Hat Linux Systems Administration, Ubuntu Administration, Using Zenoss to Manage Windows Servers from Linux, Using Zenoss to Manage Windows Servers from Linux

managing Windows servers from Linux, Using Zenoss to Manage Windows Servers from Linux, Using Zenoss to Manage Windows Servers from Linux

PyInotify with GNU/Linux, PyInotify, PyInotify

Red Hat systems administration, Red Hat Linux Systems Administration

Ubuntu administration, Ubuntu Administration

listdir() function (os module), Using os.list

log parsing (how to), Log Parsing, Log Parsing

loops, Why Python?

in Perl and Bash, Why Python?

lower() method, Built-in methods for str data extraction

lowercase, converting entire string to, Built-in methods for str data extraction

ls() function (Scapy), Scapy

lsmagic function, Help with Magic Functions

lstrip() method, Built-in methods for str data extraction

M

macro function, macro

magic function, Help with Magic Functions

“magic” functions, Magic Edit, Help with Magic Functions, Help with Magic Functions

edit function, Magic Edit

mail (incoming), processing, Receiving Email, Receiving Email

mail (outgoing), writing, Sending Email, Sending attachments with Python

mail attachments, sending, Sending attachments with Python

maintainability of Python, Why Python?

managed objects (in MIBs), SNMP Overview

manual information gathering, Manual Information Gathering, Manual Information Gathering

match() method, re

McGreggor, Duncan, Twisted

MD5 checksum comparisons, MD5 Checksum Comparisons, MD5 Checksum Comparisons

memory inventory of several machines (examples), Retrieving Multiple-Values with Net-SNMP, Finding Memory

menu-complete functionality, Tab Completion

merging data, Merging Data, MD5 Checksum Comparisons

metadata, Metadata: Data About Data, Metadata: Data About Data

mglob command, sh Profile

MIBs (management information bases), SNMP Overview, IPython and Net-SNMP

walking MIB tree, IPython and Net-SNMP

Model-View-Template (MVT) framework, Django

modules, psource

obtaining source code for, psource

modules (scripts), Executing Statements in Python, Executing Statements in Python

importing, Executing Statements in Python, Executing Statements in Python

motivation for using Python, Motivation, Motivation

moving files, Copying, Moving, Renaming, and Deleting Data, Wrapping Up rsync, Wrapping Up rsync

shutil to move data trees, Copying, Moving, Renaming, and Deleting Data

using rsync utility, Wrapping Up rsync, Wrapping Up rsync

multiline strings, Creating strings, Built-in methods for str data extraction, Writing files

splitting into individual lines, Built-in methods for str data extraction

when writing files, Writing files

multithreading, Threads in Python (see threads)

MVT (Model-View-Template) framework, Django

N

\n (newline character), Creating strings, Writing files

when writing files, Writing files

name binding, Callbacks

__name__ object, psearch

namespaces, psearch

skipping during object searches, psearch

naming conventions, rehash

__ (double underscore), rehash

Net-SNMP, SNMP Installation and Configuration, SNMP Installation and Configuration, IPython and Net-SNMP, IPython and Net-SNMP, Retrieving Multiple-Values with Net-SNMP, Finding Memory, Extending Net-SNMP, Extending Net-SNMP

extending, Extending Net-SNMP, Extending Net-SNMP

installing and configuring, SNMP Installation and Configuration, SNMP Installation and Configuration

retrieving multiple values with, Retrieving Multiple-Values with Net-SNMP, Finding Memory

Net-SNMP library, SNMP Installation and Configuration

networking, Networking, Creating Scripts with Scapy, Network Clients, urllib2, socket, socket, httplib, httplib, ftplib, ftplib, urllib, urllib, urllib2, urllib2, Remote Procedure Call Facilities, Pyro, XML-RPC, XML-RPC, Pyro, Pyro, SSH, SSH, Twisted, Twisted, Scapy, Scapy, Creating Scripts with Scapy, Creating Scripts with Scapy

network clients, Network Clients, urllib2, socket, socket, httplib, httplib, ftplib, ftplib, urllib, urllib, urllib2, urllib2

ftplib, ftplib, ftplib

httplib module, httplib, httplib

socket module, socket, socket

urllib module, urllib, urllib

urllib2 module, urllib2, urllib2

remote procedure call facilities, Remote Procedure Call Facilities, Pyro, XML-RPC, XML-RPC, Pyro, Pyro

Pyro framework, Pyro, Pyro

XML-RPC, XML-RPC, XML-RPC

Scapy program, Scapy, Scapy, Creating Scripts with Scapy, Creating Scripts with Scapy

creating scripts with, Creating Scripts with Scapy, Creating Scripts with Scapy

SSH protocol, SSH, SSH

Twisted framework, Twisted, Twisted

newline character (\n), Creating strings, Writing files

when writing files, Writing files

NFS src directory, mounting, Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

NFS-mounted directories, installing eggs on, Install Egg Located on Local or Network Filesystem

no options usage pattern (optparse), No Options Usage Pattern

not in test operator, Built-in methods for str data extraction

numbered prompt, Interacting with IPython

O

object identifiers (OIDs), SNMP Overview

object-oriented programming (OOP), Why Python?

Object-Relationship Mapping (ORM), Storm ORM, Storm ORM

(see also SQLAlchemy ORM; Storm ORM)

objects, pinfo, psearch, psearch, who, whos

listing, functions for, who, whos

obtaining information on, with pinfo, pinfo

searching for, with psearch, psearch, psearch

OIDs (object identifiers), SNMP Overview

open() method, Creating files

open() method (shelve), shelve

OpenLDAP using with Python, Using LDAP with OpenLDAP, Active Directory, and More with Python, Importing an LDIF File

operating systems, Introduction, Using Zenoss to Manage Windows Servers from Linux, Cross-Platform Unix Programming in Python, Creating a Cross-Platform Build Network, PyInotify, PyInotify, OS X, Managing Plist Files from Python, Red Hat Linux Systems Administration, Ubuntu Administration, Solaris Systems Administration, Virtualization, Cloud Computing, Building a sample Google App Engine application

cloud computing, Cloud Computing, Building a sample Google App Engine application

GNU/Linux, PyInotify with, PyInotify, PyInotify

OS X, OS X, Managing Plist Files from Python

Red Hat systems administration, Red Hat Linux Systems Administration

Solaris systems administration, Solaris Systems Administration

Ubuntu administration, Ubuntu Administration

Unix programming, cross-platform, Cross-Platform Unix Programming in Python, Creating a Cross-Platform Build Network

Virtualization, Virtualization

OperatingSystem class (Django), Simple Database Application

option with multiple arguments usage pattern (optparse), Option with Multiple Arguments Usage Pattern

optparse, Introduction to Optparse, Option with Multiple Arguments Usage Pattern

ORM (Object-Relationship Mapping), Storm ORM, Storm ORM

(see also SQLAlchemy ORM; Storm ORM)

os module, Using the OS Module to Interact with Data, Using the OS Module to Interact with Data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files, Using os.list

copying, moving, renaming, and deleting data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data

listdir() function, Using os.list

paths, directories, and files, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files

OS X programming, OS X, Managing Plist Files from Python, OS X, Managing Plist Files from Python

OSA (Open Scripting Architecture), OS X Scripting APIs

Out built-in variable, Interacting with IPython

output, Standard Input and Output, Standard Input and Output

standard input and output, Standard Input and Output, Standard Input and Output

output history, History results, History results

output paging, with page function, page

output prompts, Python versus IPython, Interacting with IPython

P

package management, Introduction, EPM Summary: It Really Is That Easy, Registering a Package with the Python Package Index, Registering a Package with the Python Package Index, Distutils, Distutils, Distutils, Buildout, Developing with Buildout, Developing with Buildout, virtualenv, Creating a Custom Bootstrapped Virtual Environment, EPM Package Manager, EPM Summary: It Really Is That Easy

building pages with setuptools, Distutils (see setuptools)

Buildout tool, Buildout, Developing with Buildout, Developing with Buildout

developing with, Developing with Buildout

creating packages with disutils, Distutils, Distutils

EPM package manager, EPM Package Manager, EPM Summary: It Really Is That Easy

registering packages with Python Package Index, Registering a Package with the Python Package Index, Registering a Package with the Python Package Index

virtualenv tool, virtualenv, Creating a Custom Bootstrapped Virtual Environment

package version, changing active, Change Active Version of Package

packet manipulation program, Scapy (see Scapy program)

page function, page

paramkio library, SSH, SSH

parse() method (ElementTree), ElementTree

parsing logfiles (example), Log Parsing, Log Parsing

parsing XML files with ElementTree, ElementTree, ElementTree

partition re-imaging, Automatically Re-Imaging Machines

password-protected sites, installing eggs on, Authenticating to a Password Protected Site

paths, walking with os module, Working with Paths, Directories, and Files, Working with Paths, Directories, and Files

pattern matching, re (see regular expressions)

pattern matching with files and directories, Pattern Matching Files and Directories, Pattern Matching Files and Directories

pdef function, pdef

PDF files, saving data as, PDFs, PDFs

pdoc function, pdoc

Perez, Fernando, IPython

Perl, Python versus, Why Python?

persistent data, Data Persistence (see data persistence)

Perspective Broker mechanism, Twisted

pfile function, pfile

pickle module, Pickle, Pickle, Pickle

(see also cPickle library)

pie charts, creating, Graphical Images

pinfo function, pinfo, pinfo

platform module, Cross-Platform Unix Programming in Python

Plist files, managing, Managing Plist Files from Python

Plone content management system, Using Buildout

POP3 protocol, Receiving Email

Popen() method (Subprocess), Subprocess, Threads in Python

poplib module, Receiving Email

port checker (example), socket, socket, Twisted

using socket module, socket, socket

using Twisted, Twisted

print statement, Interacting with IPython

processes, Introduction, Summary, Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess, Using Supervisor to Manage Processes, Using Supervisor to Manage Processes, Using Screen to Manage Processes, Using Screen to Manage Processes, Threads in Python, Threaded Event Handler, Processes, Processes, Scheduling Python Processes, Scheduling Python Processes, daemonizer, daemonizer

currency and, Processes, Processes

Daemonizer, daemonizer, daemonizer

managing with screen application, Using Screen to Manage Processes, Using Screen to Manage Processes

managing with Supervisor, Using Supervisor to Manage Processes, Using Supervisor to Manage Processes

scheduling, Scheduling Python Processes, Scheduling Python Processes

Subprocess module, Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess

using return codes, Using Return Codes with Subprocess, Using Return Codes with Subprocess

threads, Threads in Python, Threaded Event Handler

processing module, Processing Module

profiles, sh Profile

prompt (IPython), numbering of, Interacting with IPython

psearch function, psearch, psearch

psource function, psource

public ssh-key, creating, Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

pwd command, pwd

.py file, changing to egg, Changing Standalone .py File into egg

py-appscript project, OS X Scripting APIs

PyDNS module, Managing DNS with Python

Pyexpect tool, SNMP Device Control

PyGTK applications, Building a Simple PyGTK App, Building a Simple PyGTK App, Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK

Apache Log Viewer, building (example), Building an Apache Log Viewer Using PyGTK, Building an Apache Log Viewer Using PyGTK

simple application (example), Building a Simple PyGTK App, Building a Simple PyGTK App

PyInotify module, PyInotify, PyInotify

pypi (Python Package Index), Why Python?

Pyro framework, Pyro, Pyro

PySNMP library, SNMP Installation and Configuration

pysysinfo module, Reusing Code with the Import Statement

Python, motivation for using, Motivation, Motivation

Python, reasons to use, Why Python?, Why Python?

Python basics, The Basics, Executing Statements in Python, Summary, Using Functions in Python, Using Functions in Python, Using Functions in Python, Reusing Code with the Import Statement, Reusing Code with the Import Statement

executing statements, Executing Statements in Python, Summary

functions, Using Functions in Python, Using Functions in Python, Using Functions in Python

(see also functions)

reusing code, Reusing Code with the Import Statement, Reusing Code with the Import Statement

Python community, Why Python?, IPython

Python eggs, Setuptools and Python Eggs, Install Egg Located on Local or Network Filesystem, Changing Standalone .py File into egg, Creating Eggs, Creating Eggs, Creating Eggs

changing standalone .py file into, Changing Standalone .py File into egg

defined, Creating Eggs

for package management, Setuptools and Python Eggs, Creating Eggs, Creating Eggs

installing on filesystems, Install Egg Located on Local or Network Filesystem

Python Package Index, Registering a Package with the Python Package Index, Registering a Package with the Python Package Index

Python packages, Installing IPython

Python prompts, versus IPython prompts, Interacting with IPython

Python Remote Objects, Pyro (see Pyro framework)

Python Standard Library, Why Python?

Python wrappers, Executing Statements in Python (see wrappers)

python-ldap module, Using LDAP with OpenLDAP, Active Directory, and More with Python

python-reportlab package, PDFs

python-textile package, Manual Information Gathering

Q

question mark (?), pinfo, pinfo, psearch

to obtain object information, pinfo

to search for objects, psearch

?? to obtain object information, pinfo

question mark (?) for help, Executing Statements in Python, Help with Magic Functions

queues, threading, Threads in Python

%quickref command, Help with Magic Functions

quotation marks, Creating strings

creating strings with, Creating strings

R

raw strings, Creating strings, re

in regular expressions, re

re module, re, re

(see also regular expressions)

re-imaging partitions, Automatically Re-Imaging Machines

read() method, Reading files

readability of Python, Why Python?, Why Python?

(see also whitespace)

reading files, Reading files, Reading files

reading input, Standard Input and Output (see input)

readline support, Readline support

readline() method, Reading files

readlines() method, Reading files

rec directive, sh Profile

recv( ) function (socket module), socket

Red Hat systems administration, Red Hat Linux Systems Administration

registering packages with Python Index, Registering a Package with the Python Package Index, Registering a Package with the Python Package Index

regular expressions, re, re, re

raw strings in, re

rehash command, rehash, rehash

rehashx command, rehashx

relational serialization, Relational Serialization, SQLAlchemy ORM

remote procedure calls (RPCs), Remote Procedure Call Facilities, Pyro, XML-RPC, XML-RPC, Pyro, Pyro

Pyro framework, Pyro, Pyro

XML-RPC, XML-RPC, XML-RPC

remove() method (os module), MD5 Checksum Comparisons

removing, bookmark (see deleting)

renaming files in directory tree, Pattern Matching Files and Directories

rep function, rep, rep

replace() method, Built-in methods for str data extraction

reporting and documentation, Documentation and Reporting, Summary, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email, Manual Information Gathering, Manual Information Gathering, Information Formatting, PDFs, PDFs, PDFs, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

automated information gathering, Automated Information Gathering, Receiving Email, Receiving Email, Receiving Email

receiving email, Receiving Email, Receiving Email

information distribution, Information Distribution, Trac, Sending Email, Sending attachments with Python, Sending attachments with Python

sending email, Sending Email, Sending attachments with Python

sending email attachments, Sending attachments with Python

information formatting, Information Formatting, PDFs, PDFs, PDFs

saving as PDF files, PDFs, PDFs

manual information gathering, Manual Information Gathering, Manual Information Gathering

ReportLab library, PDFs

__repr__ representation of strings, Creating strings

request() method (httplib module), httplib

reset function, reset

ReST (reStructuredText) format, Manual Information Gathering, Registering a Package with the Python Package Index

ReSTless utility, Registering a Package with the Python Package Index

restoring data, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files

results history, History results, History results

retrbinary() method (ftpmodule), ftplib

return codes, with Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess

reusing code, Reusing Code with the Import Statement, Reusing Code with the Import Statement

rstrip() method, Built-in methods for str data extraction

rsync, FTP Mirror

rsync utility, Wrapping Up rsync, Wrapping Up rsync

run function, run

running shell commands, Executing Statements in Python

S

s attribute, String Processing

save function, save

SAX (simple API for XML), ElementTree

Scapy, Creating Scripts with Scapy, Creating Scripts with Scapy

creating scripts with, Creating Scripts with Scapy, Creating Scripts with Scapy

Scapy program, Scapy, Scapy

scheduling process, Scheduling Python Processes, Scheduling Python Processes

screen application, Using Screen to Manage Processes, Using Screen to Manage Processes

script wrappers, Executing Statements in Python (see wrappers)

scripts, Reusing Code with the Import Statement (see modules)

search() method, re

search() method (imaplib), Receiving Email

searching, psearch, psearch, hist command, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, re, re

command history, hist command

for objects by name, psearch, psearch

within strings, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, re, re

(see also substrings)

using regular expressions, re, re

searching (finding), MD5 Checksum Comparisons, Pattern Matching Files and Directories, Pattern Matching Files and Directories, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

for duplicates in directory trees, MD5 Checksum Comparisons

TAR file contents, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

using pattern matching, Pattern Matching Files and Directories, Pattern Matching Files and Directories

searching strings, Built-in methods for str data extraction

for substrings, Built-in methods for str data extraction

and replacing them, Built-in methods for str data extraction

serialization, Simple Serialization, ZODB, Pickle, Pickle, cPickle, cPickle, shelve, shelve, YAML, ZODB, ZODB, Relational Serialization, SQLAlchemy ORM, SQLite, SQLite, Storm ORM, Storm ORM, SQLAlchemy ORM, SQLAlchemy ORM

relational, Relational Serialization, SQLAlchemy ORM, SQLite, SQLite, Storm ORM, Storm ORM, SQLAlchemy ORM, SQLAlchemy ORM

SQLAlchemy ORM, SQLAlchemy ORM, SQLAlchemy ORM

SQLite library, SQLite, SQLite

Storm ORM, Storm ORM, Storm ORM

simple, Simple Serialization, ZODB, Pickle, Pickle, cPickle, cPickle, shelve, shelve, YAML, ZODB, ZODB

cPickle library, cPickle, cPickle

pickle module, Pickle, Pickle

shelve module, shelve, shelve

YAML data format, YAML

ZODB module, ZODB, ZODB

Server class (Django), Simple Database Application, Simple Database Application

Service class (Django), Simple Database Application

setuptools, Setuptools and Python Eggs, easy_install Advanced Features, Easy Install Advanced Features Summary, Entry Points and Console Scripts, Entry Points and Console Scripts

Easy Install module, easy_install Advanced Features, Easy Install Advanced Features Summary

advanced features, easy_install Advanced Features, Easy Install Advanced Features Summary

entry points and console scripts, Entry Points and Console Scripts, Entry Points and Console Scripts

SFTP (Secure FTP), SSH

sh profile, sh Profile, sh Profile

shadow history, hist command

shell, The Basics (see IPython shell)

shell commands, integrating with command-line tools, Unix Mashups: Integrating Shell Commands into Python Command-Line Tools, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

shell execute, Shell Execute

shell prompt, numbering of, Interacting with IPython

shelve module, Automated Information Gathering, shelve, shelve

shortcuts, with IPython shell, Automation and Shortcuts, rep

showPage() method (ReportLab), PDFs

shutil module, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data, Copying, Moving, Renaming, and Deleting Data

copying data tree (example), Copying, Moving, Renaming, and Deleting Data

deleting data tree (example), Copying, Moving, Renaming, and Deleting Data

moving data tree (example), Copying, Moving, Renaming, and Deleting Data

simple serialization, Simple Serialization, ZODB, Pickle, Pickle, cPickle, cPickle, shelve, shelve, YAML, ZODB, ZODB

cPickle library, cPickle, cPickle

pickle module, Pickle, Pickle

shelve module, shelve, shelve

YAML data format, YAML

ZODB module, ZODB, ZODB

simplicity of Python, Why Python?, Motivation, Easy Install Advanced Features Summary

single quotation mark ('), Creating strings

creating strings with, Creating strings

slicing strings, Built-in methods for str data extraction, Built-in methods for str data extraction

(see also strings; substrings)

SMTP authentication, Using SMTP authentication

smtplib package, Sending basic messages

SNMP, Introduction, Device API, SNMP Overview, SNMP Installation and Configuration, SNMP Installation and Configuration, SNMP Installation and Configuration, SNMP Installation and Configuration, IPython and Net-SNMP, IPython and Net-SNMP, Discovering a Data Center, Discovering a Data Center, Retrieving Multiple-Values with Net-SNMP, Finding Memory, Creating Hybrid SNMP Tools, Creating Hybrid SNMP Tools, Extending Net-SNMP, Extending Net-SNMP, SNMP Device Control, SNMP Device Control, Enterprise SNMP Integration with Zenoss, Device API

device control, SNMP Device Control, SNMP Device Control

discovering data centers, Discovering a Data Center, Discovering a Data Center

enterprise integration with Zenoss, SNMP Installation and Configuration, Enterprise SNMP Integration with Zenoss, Device API

extending Net-SNMP, Extending Net-SNMP, Extending Net-SNMP

hybrid SNMP tools, creating, Creating Hybrid SNMP Tools, Creating Hybrid SNMP Tools

Net-SNMP, about, IPython and Net-SNMP, IPython and Net-SNMP

overview of, SNMP Overview, SNMP Installation and Configuration, SNMP Installation and Configuration, SNMP Installation and Configuration

installation and configuration, SNMP Installation and Configuration, SNMP Installation and Configuration

retrieving multiple values, Retrieving Multiple-Values with Net-SNMP, Finding Memory

snmpstatus tool, Creating Hybrid SNMP Tools

socket( ) function (socket module), socket

socket module, socket, socket

Solaris systems administration, Solaris Systems Administration

source code, obtaining, psource

span() method, re

spawning processes from command-line tool, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

split() method, Built-in methods for str data extraction, Built-in methods for str data extraction

splitlines() method, Built-in methods for str data extraction

SQLAlchemy, Metadata: Data About Data

SQLAlchemy ORM, SQLAlchemy ORM, SQLAlchemy ORM, SQLAlchemy ORM, SQLAlchemy ORM

SQLite library, SQLite, SQLite, SQLite, SQLite

ssh keys, Using SSH Keys, NFS-Mounted Source Directory, and Cross-Platform Python to Manage Systems

SSH protocol, SSH, SSH

SSL (Secure Sockets Layer), Using SMTP authentication

standard input and output, Standard Input and Output, Standard Input and Output

standard library, Why Python?

standard out, suppressing, Subprocess

start() method, re

startswith() method, Built-in methods for str data extraction

starttls() method, Using SMTP authentication

statement execution, Executing Statements in Python, Summary

store function, store

Storm ORM, Storm ORM, Storm ORM, Storm ORM, Storm ORM

__str__ representation of strings, Creating strings

str type, str, str

(see also strings)

string input, Standard Input and Output (see input)

string output, Standard Input and Output (see output)

string processing with Unix shell, String Processing, String Processing

string representations, Interacting with IPython

stringIO module, StringIO

strings, str, Creating strings, Creating strings, Creating strings, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Unicode strings, Unicode strings, Unicode strings, Unicode strings, re, re, re, Apache Config File Hacking, Apache Config File Hacking, StringIO

converting case of, Built-in methods for str data extraction

creating (str type), Creating strings, Creating strings

creating (Unicode), Unicode strings, Unicode strings

extracting data from, Built-in methods for str data extraction, Built-in methods for str data extraction

looking within strings, Built-in methods for str data extraction, Built-in methods for str data extraction

hacking Apache config file (example), Apache Config File Hacking, Apache Config File Hacking

joining together (concatenating), Built-in methods for str data extraction

processing as input, StringIO

raw strings, Creating strings, re

removing content from, Built-in methods for str data extraction, Built-in methods for str data extraction

replacing substrings within, Built-in methods for str data extraction

searching within, Built-in methods for str data extraction, Built-in methods for str data extraction, re, re

using regular expressions, re, re

splitting at delimiter, Built-in methods for str data extraction, Built-in methods for str data extraction

unicode strings, Unicode strings, Unicode strings

strip() method, Built-in methods for str data extraction, Built-in methods for str data extraction

subprocess module, Executing Statements in Python, Executing Statements in Python

subprocess.call, Executing Statements in Python

Subprocess module, Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess, Using Return Codes with Subprocess

using return codes, Using Return Codes with Subprocess, Using Return Codes with Subprocess

substrings, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction, Built-in methods for str data extraction

(see also strings)

joining (concatenating), Built-in methods for str data extraction

replacing within strings, Built-in methods for str data extraction

searching strings for, Built-in methods for str data extraction, Built-in methods for str data extraction

Sun Solaris systems administration, Solaris Systems Administration

Supervisor utility, Using Supervisor to Manage Processes, Using Supervisor to Manage Processes

synchronizing directories with rsync, Wrapping Up rsync, Wrapping Up rsync

sys module, Standard Input and Output, Standard Input and Output

sys.argv, Basic Standard Input Usage

sysDescr OID, SNMP Overview, IPython and Net-SNMP

system_profiler utility, ElementTree

T

%-TAB, Help with Magic Functions

tab complete, Executing Statements in Python

tab completion, Tab Completion

tabbing code, Using Functions in Python (see indenting Python code)

tag attribute (Element object), ElementTree

tar package, Archiving, Compressing, Imaging, and Restoring, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

searching TAR files, Using a tarfile Module to Examine the Contents of TAR Files, Using a tarfile Module to Examine the Contents of TAR Files

text attribute (Element object), ElementTree

text files, working with, Working with Files, Additional resources, Creating files, Creating files, Reading files, Reading files, Writing files, Writing files, Standard Input and Output, Standard Input and Output, Log Parsing, Log Parsing, ElementTree, ElementTree

(see also input)

(see also output)

creating files, Creating files, Creating files

log parsing (example), Log Parsing, Log Parsing

parsing XML with ElementTree, ElementTree, ElementTree

reading files, Reading files, Reading files

writing files, Writing files, Writing files

text strings, str, Built-in methods for str data extraction (see strings)

(see also substrings)

textile, Manual Information Gathering, Manual Information Gathering

third-party packages, Why Python?

threading timer, Timed Delay of Threads with threading.Timer

threads, Threads in Python, Threaded Event Handler

ticket tracking system (Trac), Trac

time utility (Unix), re

timeit utility, re

timer for threads, Timed Delay of Threads with threading.Timer

Trac wiki, Trac

tracking system (Trac), Trac

trailing whitespace, stripping from strings, Built-in methods for str data extraction

triple quotes for string creation, Creating strings

true/false usage pattern (optparse), True/False Usage Pattern

Twisted framework, Twisted, Twisted

TwistedSNMP library, SNMP Installation and Configuration

U

Ubuntu administration, Ubuntu Administration

UDP ports for SNMP, SNMP Overview

underscore (_), rehash, psearch, psearch, History results, History results

for results history, History results, History results

__ (in variable names), rehash

__ object, psearch

___ object, psearch

unicode strings, Unicode strings, Unicode strings

Unix mashups, Unix Mashups: Integrating Shell Commands into Python Command-Line Tools, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

Unix programming, cross-platform, Cross-Platform Unix Programming in Python, Creating a Cross-Platform Build Network

Unix shell, working with, Unix Shell, sh Profile, alias, Shell Execute, rehash, rehashx, cd, cd, bookmark, dhist, pwd, Variable Expansion, String Processing, String Processing, sh Profile, sh Profile

alias function, alias

bookmark command, bookmark

dhist command, dhist

pwd command, pwd

rehash command, rehash

rehashx command, rehashx

sh profile, sh Profile, sh Profile

shell execute, Shell Execute

string processing, String Processing, String Processing

variable expansion, Variable Expansion

upgrading packages, Upgrading Packages

upper() method, Built-in methods for str data extraction

uppercase, converting entire string to, Built-in methods for str data extraction

URL-based content, processing as input, urllib

urllib module, urllib, urllib, urllib

urllib2 module, urllib2, urllib2

V

Vainio, Ville, IPython

variable expansion, Variable Expansion

variables, rehash, reset

deleting from interactive namespace, reset

starting with __ (double underscore), rehash

VBox (PyGTK), Building an Apache Log Viewer Using PyGTK

vertical box (PyGTX), Building an Apache Log Viewer Using PyGTK

Vim, IPython and Net-SNMP

virtual machine, creating, Creating a Cross-Platform Build Network

virtualenv tool, virtualenv, Creating a Custom Bootstrapped Virtual Environment

Virtualization, Virtualization

W

walking MIB trees, IPython and Net-SNMP

web applications, building, Web Applications, Web Applications

web server port monitoring (example), socket, socket

Web-based content, processing as input, urllib

web2ldap tool, Importing an LDIF File

whitespace, Built-in methods for str data extraction, Built-in methods for str data extraction

as string delimiter, when splitting, Built-in methods for str data extraction

stripping from strings, Built-in methods for str data extraction

whitespace in Python code, Why Python?, Using Functions in Python

who function, who

whos function, whos

who_ls function, who_ls

Windows servers, managing from Linux, Using Zenoss to Manage Windows Servers from Linux, Using Zenoss to Manage Windows Servers from Linux

with statement, Creating files

working directory, installing source in, Install an Unpacked Source Distribution in Current Working Directory

wrapper() function (curses library), Building an Apache Log Viewer Using Curses

wrappers, Executing Statements in Python, Reusing Code with the Import Statement

importing scripts, Reusing Code with the Import Statement (see import statement)

wrapping command-line tools in Python, Kudzu Usage Pattern: Wrapping a Tool in Python, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

and changing the behavior, Hybrid Kudzu Design Pattern: Wrapping a Tool in Python, and Then Changing the Behavior

and spawning processes, Hybrid Kudzu Design Pattern: Wrapping a Unix Tool in Python to Spawn Processes

write() method, Creating files, Writing files

write() method (ftplib module), ftplib

writelines() method, Writing files

writing files, Writing files, Writing files

writing output, Standard Input and Output (see output)

X

XHTML, converting Textile to, Manual Information Gathering

XML file parsing with ElementTree, ElementTree, ElementTree

XML-RPC, XML-RPC, XML-RPC

XML-RPC, communicating with Zenoss via, Device API

Y

YAML data format, YAML, YAML

Z

zendmd API, Using Zendmd

Zenoss API, SNMP Installation and Configuration, Enterprise SNMP Integration with Zenoss, Device API, Using Zenoss to Manage Windows Servers from Linux, Using Zenoss to Manage Windows Servers from Linux

managing Windows servers from Linux, Using Zenoss to Manage Windows Servers from Linux, Using Zenoss to Manage Windows Servers from Linux

ZODB module, ZODB, ZODB

About the Authors

Noah Gift is the co-author of Python For Unix and Linux by O'Reilly. He is an author, speaker, consultant, and community leader, writing for publications such as IBM Developerworks, Red Hat Magazine, O'Reilly, and MacTech, and Manning.

His consulting company is Giftcs, LLC and it provides solutions for Python Development and Systems Engineering. His personal website is www.noahgift.com. Noah is also the former organizer for PyAtl, which is the Python User Group for Atlanta, GA. He has given presentations at PyCon and PyAtl.

He has a Master's degree in CIS from Cal State Los Angeles, B.S. in Nutritional Science from Cal Poly San Luis Obispo, is an Apple ACSA and LPI certified SysAdmin, as well as a Avid Certified Support Representative. He has worked at companies such as, Caltech, Disney Feature Animation, Sony Imageworks, and Turner Studios, and Weta Digital. You can see all of his film credits at IMBD.

As a teenager he was a freelance television editor for ABC Network News. While at Caltech he worked for the Nobel Prize Winning President as a Mac Expert, and at Disney and Sony worked on the first feature animated films for both companies: Chicken Little, and Surf's Up, respectively. Recently he has worked on Python development projects as diverse as writing an SNMP auto-discovery system, writing a Content Management System from scratch, creating a large scale Web 2.0/Social Networking Application in Django for Turner Studios, to writing IPhone applications that talk to Google App Engine. He is also involved in a new media journalism project, Spotlight on FOSS, that had a kickoff interview of Mark Shuttleworth.

He is currently co-authoring a book on Google App Engine and writing a large Google App Engine Exercise and Nutrition Tracking Application. Most recently, he works as a Python programmer for Weta Digital in New Zealand, which has one of the world's largest render farms/super computer sites.

In his free time he enjoys spending time with his wife Leah, and their son Liam, and playing and composing piano music. He is also into exercising religiously, including running in and training for marathons, and blogging about it. When he gets a chance, he likes to write open source software. He is also interested in Artificial Intelligence research and software development.

Jeremy Jones is a software engineer/system administrator who works for Predictix. His weapon of choice is Python but he has done plenty of shell and Perl and a touch of Java.

He is the author of the open source projects Munkware, a multiproducer/multiconsumer, transactional, and persistent queuing mechanism, ediplex, an EDI (Electronic Data Interchange) parsing engine, and podgrabber a podcast downloader. All three projects were written in the Python language.

Jeremy spends his spare time enjoying his family and doing a little writing. He lives in Conyers, Georgia, just east of Atlanta, with his wife, Debra and his two children, Zane and Justus.

Colophon

The image on the cover of Python for Unix and Linux System Administration is a boa constrictor (boa constrictor). Found throughout South and Central America and some islands in the Caribbean, boa constrictors are non-venomous snakes that can thrive in a wide array of environments, from deserts to open savannas and wet tropical forests, but they prefer arid terrain over wet surroundings. They are both terrestrial and arboreal, but as they get older, they tend to spend more time on the ground.

Boa constrictors have very unique markings that include diamond- and oval-like patterns. Their scales change colors depending on their habitat, allowing them to hide from the forest-dwelling animals that hunt them.

In the wild, boa constrictors thrive on small- to medium-size rodents, lizards, bats, birds, mongooses, squirrels, and have even been known to feast on other mammals as large as ocelots. Being cold-blooded and slow moving, boas can go up to a week without eating after capturing large prey. They are solitary and nocturnal hunters, with heat-sensitive pads on their heads to help them hunt. Particularly fond of bats, boas will hang in trees and from the mouths of caves waiting for them to fly by, then they can grab the bats with their mouths. Not surprisingly, boa constrictors kill by constriction. The snake wraps its body around its prey in coils, tightening its grip each time the victim breathes out, eventually suffocating it to death.

Boas are a common attraction in zoos, and they are even relatively common pets. In fact, thousands of dollars are made every year importing them into the U.S. In South America, they are revered as “destroyers of rodents” and are often domesticated for that reason. Boa constrictors grow quite tame in captivity and can live there as such for 20–30 years. Hunted for the exotic pet trade and their decorative markings, some boa constrictors are endangered and have protected status.

Boa constrictors are seasonal breeders. To attract males, females emit a scent from their cloacas, which is the chamber into which the intestinal and urogenital tracts discharge. Fertilization happens internally, and females can give birth to up to 60 live babies at one time. Significantly smaller than their anaconda cousins, newborn boas average 2 feet in length and can grow up to 13 feet long and weigh more than 100 pounds. Found in South America, the largest boa constrictor on record was 18 feet!

The cover image is a 19th-century engraving from the Dover Pictorial Archive. The cover font is Adobe ITC Garamond. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont’s TheSansMonoCondensed.