8.1 Aside: The sys Module

The sys module contains several functions and attributes internal to Python; sys in this case means Python system, not operating system. Some of the most useful attributes are:

sys. path: A list containing the directories Python looks into when doing imports.
sys.modules: A dictionary of the modules that have been loaded in the current session.
sys.platform: A string referring to the current platform. Its possible values include ' win32' , ' mac' , ' osf1' , ' linux-i386' , ' sunos4' , etc. It's sometimes useful to check the value of sys.platform when doing platform-specific things (such as starting a window manager).
sys.ps1 and sys.ps2: Two printable objects, used by Python in the interactive interpreter as the primary and secondary prompts. Their default values are ... and >>> . You can set them to strings or to instances of classes that define a __ repr _ _ method.

One of the most frequently used attributes of the sys module is sys.argv , a list of the words input on the command line, excluding the reference to Python itself if it exists. In other words, if you type at the shell:

 csh> python run.py a x=3 foo

then when run.py starts, the value of the sys.argv attribute is ['run.py', 'a', 'x=3', 'foo'] . The sys.argv attribute is mutable (after all, it's just a list). Common usage involves iterating over the arguments of the Python program, that is, sys.argv[1:] ; slicing from index 1 till the end gives all of the arguments to the program itself, but doesn't include the name of the program (module) stored in sys.argv[0] .

Finally, there are three file attributes in the sys module: sys.stdin , sys. stdout , and sys.stderr . They are references to the standard input, output, and error streams respectively. Standard input is generally associated by the operating system with the user 's keyboard; standard output and standard error are usually associated with the console. The print statement in Python outputs to standard output ( sys.stdout ), while error messages such as exceptions are output on the standard error stream ( sys.stderr ). Finally, as we'll see in an example, these are mutable attributes: you can redirect output of a Python program to a file simply by assigning to sys.stdout :

 sys.stdout = open('log.out', 'w')