Beginning Python - From Novice to Professional

CHAPTER 10 ■ BATTERIES INCLUDED 215
The Standard Library: A Few Favorites
Chances are that you’re beginning to wonder what the title of this chapter means. The phrase
was originally coined by Frank Stajano and refers to Python’s copious standard library. When
you install Python, you get lots of useful modules (the batteries) for “free.” Because there are so
many ways of getting more information about these modules (as explained in the first part of
this chapter), I won’t include a full reference here (which would take up far too much space
anyway), but I’ll describe a few of my favorite standard modules to whet your appetite for
exploration. You’ll encounter more standard modules in the project chapters (Chapter 20 and
later). The module descriptions are not complete but highlight some of the interesting features
of each module.
sys
This module gives you access to variables and functions that are closely linked to the Python
interpreter. Some of these are shown in Table 10-2.
Table 10-2. Some Important Functions and Variables in the sys Module
Function/Variable
argv
exit([arg])
modules
path
platform
stdin
stdout
stderr
Description
The command-line arguments, including the script name
Exits the current program, optionally with a given return value or
error message
A dictionary mapping module names to loaded modules
A list of directory names where modules can be found
Contains a platform identifier such as sunos5 or win32
Standard input stream—a file-like object
Standard output stream—a file-like object
Standard error stream—a file-like object
The variable sys.argv contains the arguments passed to the Python interpreter, including
the script name.
The function sys.exit exits the current program. (If called within a try/finally block, the
finally clause is executed.) You can supply an integer to indicate whether the program succeeded
or not—a UNIX convention. You’ll probably be fine in most cases if you rely on the default
(which is zero, indicating success). Alternatively, you can supply a string, which is used as an
error message and can be very useful for a user trying to figure out why the program halted;
then, the program exits with that error message and a code indicating failure.
The mapping sys.modules maps module names to actual modules. It only applies to currently
imported modules.
The module variable sys.path was discussed earlier in this chapter. It’s a list of strings, in
which each string is the name of a directory where the interpreter will look for modules when
an import statement is executed.