The relevant code in CallTips.py is
  argspec = ""
  if hasattr(ob, '__call__'):
...
    if isinstance(fob, (types.FunctionType, types.MethodType)):
      argspec = inspect.formatargspec(*inspect.getfullargspec(fob))

So I want to broaden the second condition (or remove it), but if it returns something .formatargspec cannot digest, the outer call will have to be conditioned. My impression is that str(signature object) returns pretty much what .formatargspec does, but I have not experimented yet.

My #20122 patch moves currently commented out tests in CallTips.py that use the above to test_calltips.py. I will commit as soon as I test on 3.4. Until then, getfullargspec is not being used in the Idle test suite.