How do I mock a django signal handler?

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.

Possibly a better idea is to mock out the functionality inside the signal handler rather than the handler itself. Using the OP's code:

@receiver(post_save, sender=User, dispatch_uid='myfile.signal_handler_post_save_user')
def signal_handler_post_save_user(sender, *args, **kwargs):
  do_stuff()  # <-- mock this

def do_stuff():
   ... do stuff in here

Then mock do_stuff:

with mock.patch('myapp.myfile.do_stuff') as mocked_handler:
    self.assert_equal(mocked_handler.call_count, 1)

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 :)