https://stackoverflow.com/questions/25750477/python-2-7-8-idle-will-not-open-on-mavericks -- because one of the two files has 'shift' instead of Shift.  When I make a change in either file (with custom keys being used), the traceback ends slightly differently until the final message.

  File "C:\Programs\Python34\lib\idlelib\EditorWindow.py", line 1100, in apply_bindings
    text.event_add(event, *keylist)
  File "C:\Programs\Python34\lib\idlelib\MultiCall.py", line 379, in event_add
    widget.event_add(self, virtual, seq)
  File "C:\Programs\Python34\lib\tkinter\__init__.py", line 1441, in event_add
    self.tk.call(args)

#21519 is about checking bindings entered in the dialog with either the basic or advanced methods.  Bindings also need to be checked when read so we know which file is bad.  Exception tracebacks should be replaced by more helpful messages that include fix-up instructions and which do not disappear into the void when running on pythonw.