[Tutor] calling user defined function

[roberto]

hello
i have a question which i didn't solved yet:
i can define a function using the text editor provided by IDLE 3.0;
then i'd like to call this function from the python prompt

but when i try to do it, python warns me that function doesn't exist
of course if i define the function directly using the >>> prompt,
after that everything is fine

may you tell me where i have to save the file that defines the
function is order to use it later ?
is it a problem of path ?

my version of python is 3.0, OS windows xp

thank you very much in advance
-- 
roberto
OS: GNU/Linux
       Debian
       Kubuntu, Edubuntu