In mock.py, in method: def _mock_call(_mock_self, *args, **kwargs): There is a following piece of code: if not _callable(effect): result = next(effect) if _is_exception(result): raise result if result is DEFAULT: result = self.return_value return result ret_val = effect(*args, **kwargs) This works correctly for iterables (such as lists) that are not defined as generators. However, if one defined a generator as a function this would not work. It seems like the check should be not for callable, but for iterable: try: iter(effect) except TypeError: # If not iterable then callable or exception if _callable(effect): ret_val = effect(*args, **kwargs) else: raise effect else: # Iterable result = next(effect) if _is_exception(result): raise result if result is DEFAULT: result = self.return_value return result
I am not sure if the snippets you are referring to are from testing-cabal/mock repo which could be different from master branch. Current code is at [0] if effect is not None: if _is_exception(effect): raise effect elif not _callable(effect): result = next(effect) if _is_exception(result): raise result else: result = effect(*args, **kwargs) if result is not DEFAULT: return result > This works correctly for iterables (such as lists) that are not defined as generators. However, if one defined a generator as a function this would not work. This does seem to work for generator function as below. Sorry, maybe I am getting it wrong with respect to terminologies and understanding the issue. Can you add a short script around what you are expecting? $ cat ../backups/bpo36598.py from unittest.mock import patch def gen(i): while i < 5: yield i i += 1 def foo(): return 1 with patch('__main__.foo', side_effect=gen(0)): for _ in range(2): print(foo()) for _ in range(2): print(foo()) $ ./python.exe ../backups/bpo36598.py 0 1 2 3 [0] https://github.com/python/cpython/blob/a9bd8925c7fa50dd3cfab125b824ec192133ef49/Lib/unittest/mock.py#L1043
