[mod_python] Beginner question re: getting form data and key errors when var not defined

Why am I getting key errors when merely attempting to get <possible> 
form data from a mod_python request...ala:

author = req.form['author']

I get the following:

    raise KeyError, key

KeyError: subject

I do kind of expect certain form fields to possibly not have 
values...why the explicit error? Can't I just check for null or 
uninitialized in my code?

thanks,

mike