Calling external program from within python

[Emmanouil Angelakis]

Hi,

I am tryiong to do something obviously trivial such as:
I have a c program called "tsys2list" that when it is ran it asks the user to give the value of "tcal" which is a variable. I want to call the  "tsys2list" from within a pyrthon script lets call it "gamma.py" >>>but<<< I want to pass the value of "tcal" to th eprogram automatically and not by interacting with the keyboard. 

How do I do that?

thanks i advance!
manolis