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If you remove the widget.config calls in GUI.ready you will notice the
problem "goes away", but note that this method is called from another
thread while you have a non-thread-safe tcl/tk lib (I'm assuming you
didn't compile it with --enable-threads). So.. using multiple threads in
Python while Tcl is compiled without --enable-threads isn't supported at
all.
To reproduce the problem try this (change Tkinter to tkinter if py3k):
import threading
import Tkinter
lbl = Tkinter.Label(text="hi")
threading.Thread(target=lambda: lbl.configure(text="hi there")).start()
lbl.mainloop()
If your tcl/tk libs weren't compiled with --enable-threads this should
get you an "TclError: out of stack space (infinite loop?)". A
documentation note may be added somewhere, but nothing much else is
going to happen (that is what I believe at least).