Programming Python, Second Edition

Command-Line Arguments

The sys module is also where Python makes available the words typed on the command used to start a Python script. These words are usually referred to as command-line arguments, and show up in sys.argv, a built-in list of strings. C programmers may notice its similarity to the C “argv” array (an array of C strings). It’s not much to look at interactively, because no command-line arguments are passed to start up Python in this mode:

>>> sys.argv
['']

To really see what arguments are about, we need to run a script from the shell command line. Example 2-2 shows an unreasonably simple one that just prints the argv list for inspection.

Example 2-2. PP2E\System\testargv.py

import sys
print sys.argv

Running this script prints the command-line arguments list; note that the first item is always the name of the executed Python script file itself, no matter how the script was started (see Executable Scripts on Unix later in this chapter):

C:\...\PP2E\System>python testargv.py
['testargv.py']

C:\...\PP2E\System>python testargv.py spam eggs cheese
['testargv.py', 'spam', 'eggs', 'cheese']

C:\...\PP2E\System>python testargv.py -i data.txt -o results.txt
['testargv.py', '-i', 'data.txt', '-o', 'results.txt']

The last command here illustrates a common convention. Much like function arguments, command-line options are sometimes passed by position, and sometimes by name using a “-name value” word pair. For instance, the pair -i data.txt means the -i option’s value is data.txt