How to pass a global variable to a module?

[Mars creature]

Dear Python users,
  I just start to use python and love this language. I met this
problem when I try to save my functions in a separate file.
The question is how I can pass a global variable to a function which
is saved in another file. If I save the function I defined in the same
file with the main program, there is no problem after I declare the
global variable. But problem comes out when I save all the function is
a separate file. Help is very much appreciated! Thanks!
Jinbo