I'm starting out with wxPython and have been working my way through every tutorial and example I can get my hands on.  I've run into a slight problem, however, and it has to do with the wx.App versus the wx.Frame and which should contain specific methods.  Just about every example I've seen don't go much beyond layouts/sizers and event handling, none really tackle project organization of a wxPython project.
For example, I have a method that gets a list of folders.  The way most examples would deal with this would be to stick the method right in the frame class.  This method has the potential to be used in several other parts of the application, so it would make more sense to store it at the application class level.
How should I organize and call "universal" methods like these so that I don't clutter up my frame classes.
UPDATE:
To clarify, the "list of folders" was just an example, my actual method does a lot more work.  What I'm saying is I have code that isn't Frame-specific.  If I had this in the application class, what is the best way to call it from and event method in my frame.
I'm looking for actual project organization techniques, not programming fundamentals.
In a proper OOP design, this would be independent or part of a filesystem class - it wouldn't be part of the app or the frame.
Calling Application Methods from a wx Frame Class