Opening a file, and taking input from the file opened in a Python program

Opening a file, and taking input from the file opened in a Python program

I have written a program in python that is another front end for
un-tarring files on OSX. I want to be able to double click on the file and
open with the program that I wrote. How do I make the program
automatically know that it is being opened with that file, and to work on
that file?