To test a polling function I want to mock the calling of a sub function so that the first time it is called it will fail, and the second time it is called it will succeed. Here's a very simplified version of it: poll_function(var1): value = sub_function(var1) # First call will return None while not value: time.sleep(POLLING_INTERVAL) value = sub_function(var1) # A subsequent call will return a string, e.g "data" return value Is this possible to do with a Mock object from the mock framework? I know Mock objects have a call_count attribute I should be able to use somehow. Right now I've solved it by creating a custom mock object that I use to monkey patch sub_function(), but I feel there should be a better less verbose way of doing it: def test_poll(): class MyMock(object): def __init__(self, *args): self.call_count = 0 def sub_function(self, *args, **kwargs): if self.call_count > 1: return "data" else: self.call_count += 1 return None my_mock = MyMock() with patch('sub_function', my_mock.sub_function): ok_(poll_function())