calling external programs

[bougle gluce]

I was wanting to write a python program for linux that
can call upon external programs. for instance the newsreader
that I use is able to call my favorite text editor to edit
this post, when I quit and save the text it saves it to a
specific file then posts the text.

I think I understand how I might use system() to call upon
another program. what I don't understand is how is it possible
to take the user back to the calling program. for
instance how do I end up back in my newsreader after I'm finished
editing this post? and how did the editor know where to save 
the file?