A much easier way to solve this in Python, and get rid of all the related factory classes, is to use import statements:

In widget.py, you'd have something like this (I'm making it up):

if sys.platform == 'linux'
  from gtk import GtkTextBox as TextBox
elif sys.platform == 'win32'
  from winforms import TextBox as TextBox

Then in your GUI code you would just have this:

from widgets import TextBox
...

def buildForm():
   textbox = TextBox()

Of course, you might need an extra adapter layer to give them the same interface, but it's not hard.