Python Standard Library

The sys Module

The sys module provides a number of functions and variables that can be used to manipulate different parts of the Python runtime environment.

Working with Command-line Arguments

The argv list contains the arguments that were passed to the script, when the interpreter was started, as shown in Example 1-66. The first item contains the name of the script itself.

Example 1-66. Using the sys Module to Get Script Arguments

File: sys-argv-example-1.py

import sys

print "script name is", sys.argv[0]

if len(sys.argv) > 1:
    print "there are", len(sys.argv)-1, "arguments:"
    for arg in sys.argv[1:]:
        print arg
else:
    print "there are no arguments!"

script name is sys-argv-example-1.py
there are no arguments!

If you read the script from standard input (like “python < sys-argv-example-1.py”), the script name is set to an empty string. If you pass in the program as a string (using the -c option), the script name is set to “-c.”

Working with Modules

The path list contains a list of directory names in which Python looks for extension modules (Python source modules, compiled modules, or binary extensions). When you start Python, this list is initialized from a mixture of built-in rules, the contents of the PYTHONPATH environment variable, and the registry contents (on Windows). But since it’s an ordinary list, you can also manipulate it from within the program, as Example 1-67 shows.

Example 1-67. Using the sys Module to Manipulate the Module Search Path

File: sys-path-example-1.py import ...