Python and text files

I'm trying to work this out, but all I can find out is how to edit specific lines and such.

1 - Let the user select the file (done)
2 - Let the user open the file for editing (not done)
3 - Let the user save the file (not done)

>>2
Let me correct that
os.system('nano') for simple editing. Anything else should be :
os.system('emacs').