Ideal way to separate GUI and logic?

[fronagzen at gmail.com]

Thanks for all the responses!

So as a general idea, I should at the very least separate the GUI from the program logic by defining the logic as a function, correct? And the next level of separation is to define the logic as a class in one or more separate files, and then import it to the file with the GUI, correct?

My next question is, to what degree should I 'slice' my logic into functions? How small or how large should one function be, as a rule of thumb?