How do I mock a django signal handler?

Question

I have a signal_handler connected through a decorator, something like this very simple one:

@receiver(post_save, sender=User, 
          dispatch_uid=\'myfil         


        

Answer 1

As you mentioned, mock.patch('myapp.myfile._support_function') is correct but mock.patch('myapp.myfile.signal_handler_post_save_user') is wrong.

I think the reason is:

When init you test, some file import the signal's realization python file, then @receive decorator create a new signal connection.

In the test, mock.patch('myapp.myfile._support_function') will create another signal connection, so the original signal handler is called even if mocked.

Try to disconnect the signal connection before mock.patch('myapp.myfile._support_function'), like

post_save.disconnect(signal_handler_post_save_user)
with mock.patch("review.signals. signal_handler_post_save_user", autospec=True) as handler:
    #do stuff

Answer 2

There is a way to mock django signals with a small class.

You should keep in mind that this would only mock the function as a django signal handler and not the original function; for example, if a m2mchange trigers a call to a function that calls your handler directly, mock.call_count would not be incremented. You would need a separate mock to keep track of those calls.

Here is the class in question:

class LocalDjangoSignalsMock():
    def __init__(self, to_mock):
        """ 
        Replaces registered django signals with MagicMocks

        :param to_mock: list of signal handlers to mock
        """
        self.mocks = {handler:MagicMock() for handler in to_mock}
        self.reverse_mocks = {magicmock:mocked
                              for mocked,magicmock in self.mocks.items()}
        django_signals = [signals.post_save, signals.m2m_changed]
        self.registered_receivers = [signal.receivers
                                     for signal in django_signals]

    def _apply_mocks(self):
        for receivers in self.registered_receivers:
            for receiver_index in xrange(len(receivers)):
                handler = receivers[receiver_index]
                handler_function = handler[1]()
                if handler_function in self.mocks:
                    receivers[receiver_index] = (
                        handler[0], self.mocks[handler_function])

    def _reverse_mocks(self):
        for receivers in self.registered_receivers:
            for receiver_index in xrange(len(receivers)):
                handler = receivers[receiver_index]
                handler_function = handler[1]
                if not isinstance(handler_function, MagicMock):
                    continue
                receivers[receiver_index] = (
                    handler[0], weakref.ref(self.reverse_mocks[handler_function]))

    def __enter__(self):
        self._apply_mocks()
        return self.mocks

    def __exit__(self, *args):
        self._reverse_mocks()

Example usage

to_mock = [my_handler]
with LocalDjangoSignalsMock(to_mock) as mocks:
    my_trigger()
    for mocked in to_mock:
        assert(mocks[mocked].call_count)
        # 'function {0} was called {1}'.format(
        #      mocked, mocked.call_count)

Answer 3

You can mock a django signal by mocking the ModelSignal class at django.db.models.signals.py like this:

@patch("django.db.models.signals.ModelSignal.send")
def test_overwhelming(self, mocker_signal):
    obj = Object()

That should do the trick. Note that this will mock ALL signals no matter which object you are using.

If by any chance you use the mocker library instead, it can be done like this:

from mocker import Mocker, ARGS, KWARGS

def test_overwhelming(self):
    mocker = Mocker()
    # mock the post save signal
    msave = mocker.replace("django.db.models.signals")
    msave.post_save.send(KWARGS)
    mocker.count(0, None)

    with mocker:
        obj = Object()

It's more lines but it works pretty well too :)

Answer 4

take a look at mock_django . It has support for signals

https://github.com/dcramer/mock-django/blob/master/tests/mock_django/signals/tests.py

Answer 5

So, I ended up with a kind-of solution: mocking a signal handler simply means to connect the mock itself to the signal, so this exactly is what I did:

def test_cache():
    with mock.patch('myapp.myfile.signal_handler_post_save_user', autospec=True) as mocked_handler:
        post_save.connect(mocked_handler, sender=User, dispatch_uid='test_cache_mocked_handler')
        # do stuff that will call the post_save of User
    self.assertEquals(mocked_handler.call_count, 1)  # standard django
    # self.assert_equal(mocked_handler.call_count, 1)  # when using django-nose

Notice that autospec=True in mock.patch is required in order to make post_save.connect to correctly work on a MagicMock, otherwise django will raise some exceptions and the connection will fail.

Answer 6

In django 1.9 you can mock all receivers with something like this

# replace actual receivers with mocks
mocked_receivers = []
for i, receiver in enumerate(your_signal.receivers):
    mock_receiver = Mock()
    your_signal.receivers[i] = (receiver[0], mock_receiver)
    mocked_receivers.append(mock_receiver)

...  # whatever your test does

# ensure that mocked receivers have been called as expected
for mocked_receiver in mocked_receivers:
    assert mocked_receiver.call_count == 1
    mocked_receiver.assert_called_with(*your_args, sender="your_sender", signal=your_signal, **your_kwargs)

This replaces all receivers with mocks, eg ones you've registered, ones pluggable apps have registered and ones that django itself has registered. Don't be suprised if you use this on post_save and things start breaking.

You may want to inspect the receiver to determine if you actually want to mock it.