The re module is the core of text pattern recognition and processing. A regular expression is a formula that specifies how to recognize and parse strings. The re module is described in detail in Chapter 31, Complex Strings: the re Module .

The avowed purpose of the struct module is to allow a Python program to access C-language API's; it packs and unpacks C-language struct object. It turns out that this module can also help you deal with files in packed binary formats.

The difflib module contains the essential algorithms for comparing two sequences, usually sequences of lines of text. This has algorithms similar to those used by the Unix diff command (the Window COMP command).

There are two variations on StringIO which provide file-like objects that read from or write to a string buffer. The StringIO module defines the class StringIO, from which subclasses can be derived. The cStringIO module provides a high-speed C-language implementation that can't be subclassed.

Note that these modules have atypical mixed-case names.

This is a module to format plain text. While the word-wrapping task is sometimes handled by word processors, you may need this in other kinds of programs. Plain text files are still the most portable, standard way to provide a document.

This module has hundreds of text encodings. This includes the vast array of Windows code pages and the Macintosh code pages. The most commonly used are the various Unicode schemes (utf-16 and utf-8). However, there are also a number of codecs for translating between strings of text and arrays of bytes. These schemes include base-64, zip compression, bz2 compression, various quoting rules, and even the simple rot_13 substitution cipher.

The datetime handles details of the calendar, including dates and times. Additionally, the time module provides some more basic functions for time and date processing. We'll cover both modules in detail in Chapter 32, Dates and Times: the time and datetime Modules .

These modules mean that you never need to attempt your own calendrical calculations. One of the important lessons learned in the late 90's was that many programmers love to tackle calendrical calculations, but their efforts had to be tested and reworked prior to January 1, 2000, because of innumerable small problems.

This module contains routines for displaying and working with the calendar. This can help you determine the day of the week on which a month starts and ends; it can count leap days in an interval of years, etc.

This package contains two data types, and is likely to grow with future releases of Python. One tye is the deque -- a "double-ended queue" -- that can be used as stack (LIFO) or queue (FIFO). The other class is a specialized dictionary, defaultdict, which can return a default value instead of raising an exception for missing keys.

The bisect module contains the bisect function to search a sorted list for a specific value. It also contains the insort fucntion to insert an item into a list maintaining the sorted order. This module performs faster than simply appending values to a list and calling the sort method of a list. This module's source is instructive as a lesson in well-crafted algorithms.

The array module gives you a high-performance, highly compact collection of values. It isn't as flexible as a list or a tuple, but it is fast and takes up relatively little memory. This is helpful for processing media like image or sound files.

The sched module contains the definition for the scheduler class that builds a simple task scheduler. When a scheduler is contructed, it is given two user-supplied functions: one returns the “time” and the other executes a “delay” waiting for the time to arrive. For real-time scheduling, the time module time and sleep functions can be used. The scheduler has a main loop that calls the supplied time function and compares the current time with the time for scheduled tasks; it then calls the supplied a delay function for the difference in time. It runs the scheduled task, and calls the delay function with a duration of zero to release any resources.

Clearly, this simple algorithm is very versatile. By supplying custom time functions that work in minutes instead of seconds, and a delay function that does additional background processing while waiting for the scheduled time, a flexible task manager can be constructed.

The copy module contains functions for making copies of complex objects. This module contains a function to make a shallow copy of an object, where any objects contained within the parent are not copied, but references are inserted in the parent. It also contains a function to make a deep copy of an object, where all objects contained within the parent object are duplicated.

Note that Python's simple assignment only creates a variable which is a label (or reference) to an object, not a duplicate copy. This module is the easiest way to create an independent copy.

The pprint module contains some useful functions like pprint.pprint for printing easy-to-read representations of nested lists and dictionaries. It also has a PrettyPrinter class from which you can make subclasses to customize the way in which lists or dictionaries or other objects are printed.

The decimal module provides decimal-based arithmetic which correctly handles significant digits, rounding and other features common to currency amounts.

The math module was covered in the section called “The math Module”. It contains the math functions like sine, cosine and square root.

The random module was covered in the section called “The math Module”.

These modules all provide various kinds of conversions, ecapes or quoting so that binary data can be manipulated as safe, universal ASCII text. The number of these modules reflects the number of different clever solutions to the problem of packing binary data into ordinary email messages.

Ordinary HTML documents can be examined with the htmllib module. This module based on the sgmllib module. The basic HTMLParser class definition is a superclass; you will typically override the various functions to do the appropriate processing for your application.

One problem with parsing HTML is that browsers — in order to conform with the applicable standards — must accept incorrect HTML. This means that many web sites publish HTML which is tolerated by browsers, but can't easily be parsed by htmllib. When confronted with serious horrows, consider downloading the Beautiful Soup module. This handles erroneous HTML more gracefully than htmllib.

The xml.sax and xml.dom modules provide the classes necessary to conveniently read and process XML documents. A SAX parser separates the various types of content and passes a series of events the handler objects attached to the parser. A DOM parser decomposes the document into the Document Object Model (DOM).

The xml.dom module contains the classes which define an XML document's structure. The xml.dom.minidom module contains a parser which creates a DOM object.

The formatter module can be used in conjunction with the HTML and XML parsers. A formatter instance depends on a writer instance that produces the final (formatted) output. It can also be used on its own to format text in different ways.

The csv module helps you parse and create Comma-Separated Value (CSV) data files. This helps you exchange data with many desktop tools that produce or consume CSV files. We'll look at this in the section called “Comma-Separated Values: The csv Module”.

Configuration files can take a number of forms. The simplest approach is to use a Python module as the configuration for a large, complex program. Sometimes configurations are encoded in XML. Many Windows legacy programs use .INI files. The ConfigParser can gracefully parse these files. We'll look at this in the section called “Property Files and Configuration (or.INI) Files: The ConfigParser Module”.

Compute a secure hash or digest of a message to ensure that it was not tampered with. MD5, for example, is often used for validating that a downloaded file was recieved correctly and completely.

The os and os.path modules are critical for creating portable Python programs. The popular operating systems (Linux, Windows and MacOS) each have different approaches to the common services provided by an operating system. A Python program can depend on os and os.path modules behaving consistently in all environments.

One of the most obvious differences among operating systems is the way that files are named. In particular, the path separator can be either the POSIX standard /, or the windows \. Additionally, the Mac OS Classic mode can also use :. Rather than make each program aware of the operating system rules for path construction, Python provides the os.path module to make all of the common filename manipulations completely consistent.

The fileinput module helps your progam process a large number of files smoothly and simply.

The glob and fnmatch modules help a Windows program handle wild-card file names in a manner consistent with other operating systems.

The shutil module provides shell-like utilities for file copy, file rename, directory moves, etc. This module lets you write short, effective Pytthon programs that do things that are typically done by shell scripts.

Why use Python instead of the shell? Python is far easier to read, far more efficient, and far more capable of writing moderately sophisticated programs. Using Python saves you from having to write long, painful shell scripts.

These two modules create archive files, which contain a number of files that are bound together. The TAR format is not compressed, where the ZIP format is compressed. Often a TAR archive is compressed using GZIP to create a .tar.gz archive.

These modules are different compression algorithms. They all have similar features to compress or uncompress files.

The pickle and shelve modules are used to create persistent objects; objects that persist beyond the one-time execution of a Python program. The pickle module produces a serial text representation of any object, however complex; this can reconstitute an object from its text representation. The shelve module uses a dbm database to store and retrieve objects. The shelve module is not a complete object-oriented database, as it lacks any transaction management capabilities.

This module provides access to the SQLite relational database. This database provides a significant subset of SQL language features, allowing us to build a relational database that's compatible with products like MySQL or Postgres.

These modules provide access to a number of operating system features. The os module provides control over Processes, Files and Directories. We'll look at os and os.path in the section called “The os Module” and the section called “The os.path Module”.

The time module provides basic functions for time and date processing. Additionally datetime handles details of the calendar more gracefully than time does. We'll cover both modules in detail in Chapter 32, Dates and Times: the time and datetime Modules .

Having modules like datetime and time mean that you never need to attempt your own calendrical calculations. One of the important lessons learned in the late 90's was that many programmers love to tackle calendrical calculations, but their efforts had to be tested and reworked because of innumerable small problems.

A well-written program makes use of the command-line interface. It is configured through options and arguments, as well as properties files. We'll cover the getopt, optparse and glob modules in Chapter 35, Programs: Standing Alone .

Often, you want a simple, standardized log for errors as well as debugging information. We'll look at logging in detail in the section called “Log Files: The logging Module”.

The cgi module is used for web server applications invoked as CGI scripts. This allows you to put Python programming in the cgi-bin directory. When the web server invokes the CGI script, the Python interpreter is started and the Python script is executed.

These modules allow you to write relatively simple application programs which open a URL as if it were a standard Python file. The content can be read and perhaps parsed with the HTML or XML parser modules, described below. The urllib module depends on the httplib, ftplib and gopherlib modules. It will also open local files when the scheme of the URL is file:. The urlparse module includes the functions necessary to parse or assemble URL's. The urllib2 module handles more complex situations where there is authentication or cookies involved.

The httplib, ftplib and gopherlib modules include relatively complete support for building client applications that use these protocols. Between the html module and httplib module, a simple character-oriented web browser or web content crawler can be built.

The poplib and imaplib modules allow you to build mail reader client applications. The poplib module is for mail clients using the Post-Office Protocol, POP3 (RFC 1725), to extract mail from a mail server. The imaplib module is for mail servers using the Internet Message Access Protocol, IMAP4 (RFC 2060) to manage mail on an IMAP server.

The nntplib module allows you to build a network news reader. The newsgroups, like comp.lang.python, are processed by NNTP servers. You can build special-purpose news readers with this module.

The SocketServer module provides the relatively advanced programming required to create TCP/IP or UDP/IP server applications. This is typically the core of a stand-alone application server.

The SimpleHTTPServer and CGIHTTPServer modules rely on the basic BaseHTTPServer and SocketServer modules to create a web server. The SimpleHTTPServer module provides the programming to handle basic URL requests. The CGIHTTPServer module adds the capability for running CGI scripts; it does this with the fork and exec functions of the os module, which are not necessarily supported on all platforms.

The asyncore (and asynchat) modules help to build a time-sharing application server. When client requests can be handled quickly by the server, complex multi-threading and multi-processing aren't really necessary. Instead, this module simply dispatches each client communication to an appropriate handler function.

The cmd module contains a superclass useful for building the main command-reading loop of an interactive program. The standard features include printing a prompt, reading commands, providing help and providing a command history buffer. A subclass is expected to provide functions with names of the form do_command. When the user enters a line beginning with command, the appropriate do_command function is called.

The shlex module can be used to tokenize input in a simple language similar to the Linux shell languages. This module defines a basic shlex class with parsing methods that can separate words, quotes strings and comments, and return them to the requesting program.

The sys module contains execution context information. It has the command-line arguments (in sys.argv) used to start the Python interpreter. It has the standard input, output and error file definitions. It has functions for retrieving exception information. It defines the platform, byte order, module search path and other basic facts. This is typically used by a main program to get run-time environment information.