I have the actual association part down, but when I open the file that is associated with my Python program, how do I get the filepath of the file opened?
I think it is something like sys.argv? But that just returns the path to the python program, not the associated file.
The __file__ attribute of your module looks what you're looking for. E.g., save in foo.py:
$ cat foo.py
print 'Hello', __file__
$ python foo.py
Hello foo.py
os.path.abspath will help you if you want the absolute rather than relative path, etc, etc.
The contents of sys.argv are platform-dependent, as noted in sys.argv. I do know that sys.argv[0] is the full path on Windows only when you open .py files using the shell by double-clicking on it. Using the command line just results in the script name.
The os module provides platform-independent solutions. The full path to your script should always be available with the following code:
import os.path
import sys
print os.path.abspath(sys.argv[0])