It is pretty easy to pass a filename as a command-line argument to a Python program. However, this can be cumbersome if the users of your program are naive or you need to pass a path that is long or difficult. In such cases, a good solution is to display a File Open dialog and let the user pick the file she wants.
This can be done easily using the Tk library that ships with Python:
Tried with: Python 2.7.3 and Ubuntu 12.04 LTS