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That's right: Tkinter silently discards all background Tcl
errors. 

All errors from Tcl are silenced by def _tkerror(err): pass
So Tkinter users will never see any errors that come from
any bugs in Tcl, Tk or its extensions, including of course
errors that may only show up under Python.

The proposed solution for this problem is simply to
remove the createcomand / def _tkerror. I see no possible
reason for it to be in there, and it violates the
fundamental principle that errors should be raised and dealt
with.

Although Tkinter behaved that way since the beginning, the
change takes backwards compatibility into account because
it serves no good purpose that I know of to begin with. At
worst buried bugs will become visible and can be dealt with.