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The problem only affects autospecced functions.
Apparently, the problem lies here (all code excerpts from Lib/unittest/mock.py):
- When autospeccing functions, the Mock._mock_delegate field is populated at the end of _setup_func (and its the only place I found where the delegate is set):
197 def _setup_func(funcopy, mock):
[...]
237 mock._mock_delegate = funcopy
- Mock.side_effect is a property, and proxies the get/set to _mock_delegate when it exists, and on the way out does not detect the exception and tries to make an _IterMock out of it and everything explodes.
 504 def __get_side_effect(self):
 505 delegated = self._mock_delegate
 506 if delegated is None:
 507 return self._mock_side_effect
 508 sf = delegated.side_effect
 509 if sf is not None and not callable(sf) and not isinstance(sf, _MockIter):
 510 sf = _MockIter(sf)
I've attached a patch which adds a test for this use case, and a proposed fix. Hope it helps :)