Python's system interfaces span application domains, but for the next four chapters, most of our examples fall into the category of system tools -- programs sometimes called command-line utilities, shell scripts, or some permutation of such words. Regardless of their title, you are probably familiar with this sort of script already; they accomplish tasks like processing files in a directory, launching test scripts, and so on. Such programs historically have been written in nonportable and syntactically obscure shell languages such as DOS batch files, csh, and awk.
Even in this relatively simple domain, though, some of Python's better attributes shine brightly. For instance, Python's ease of use and extensive built-in library make it simple (and even fun) to use advanced system tools such as threads, signals, forks, sockets, and their kin; such tools are much less accessible under the obscure syntax of shell languages and the slow development cycles of compiled languages. Python's support for concepts like code clarity and object-oriented programming also help us write shell tools that can be read, maintained, and reused. When using Python, there is no need to start every new script from scratch.
Moreover, we'll find that Python not only includes all the interfaces we need to write system tools, it also fosters script portability. By employing Python's standard library, most system scripts written in Python are automatically portable to all major platforms. A Python directory-processing script written in Windows, for instance, can usually also be run in Linux without changing its source code at all -- simply copy over the source code. If used well, Python is the only system scripting tool you need to know.
Batteries Included
This chapter and those that follow deal with both the Python language and its standard library. Although Python itself provides an easy-to-use scripting language, much of the action in real Python development involves the vast library of programming tools (some 200 modules at last count) that ship with the Python package. In fact, the standard libraries are so powerful that it is not uncommon to hear Python described by the term "batteries included" -- a phrase generally credited to Frank Stajano, meaning that most of what you need for real day-to-day work is already there for the importing.
As we'll see, the standard libraries form much of the challenge in Python programming. Once you've mastered the core language, you'll find that most of your time is spent applying the built-in functions and modules that come with the system. On the other hand, libraries are where most of the fun happens. In practice, programs become most interesting when they start using services external to the language interpreter: networks, files, GUIs, databases, and so on. All of these are supported in the Python standard library, a collection of precoded modules written in Python and C that are installed with the Python interpreter.
Beyond the Python standard library, there is an additional collection of third-party packages for Python that must be fetched and installed separately. At this writing, most of these third-party extensions can be found via searches and links at http://www.python.org, and at the "Starship" and "Vaults of Parnassus" Python sites (also reachable from links at http://www.python.org). If you have to do something special with Python, chances are good that you can find a free and open source module that will help. Most of the tools we'll employ in this text are a standard part of Python, but I'll be careful to point out things that must be installed separately.