This is a Tkinter feature: if your callback function hits an
exception, you are dropped into pdb. Type 'w' to see a stack trace,
'l' to list the code around the error, 'q' to quit pwd.
Maybe someone can write a Tk-based interface to Pdb? (There is
already a stdwin-based interface, wdb.)
--Guido van Rossum, CWI, Amsterdam <Guido.van.Rossum@cwi.nl>
URL: <http://www.cwi.nl/cwi/people/Guido.van.Rossum.html>