How do I mock a django signal handler?

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

@receiver(post_save, sender=User, 
          dispatch_uid='myfile.signal_handler_post_save_user')
def signal_handler_post_save_user(sender, *args, **kwargs):
   # do stuff

What I want to do is to mock it with the mock library http://www.voidspace.org.uk/python/mock/ in a test, to check how many times django calls it. My code at the moment is something like:

def test_cache():
    with mock.patch('myapp.myfile.signal_handler_post_save_user') as mocked_handler:
        # do stuff that will call the post_save of User
    self.assert_equal(mocked_handler.call_count, 1)

The problem here is that the original signal handler is called even if mocked, most likely because the @receiver decorator is storing a copy of the signal handler somewhere, so I'm mocking the wrong code.

So the question: how do I mock my signal handler to make my test work?

Note that if I change my signal handler to:

def _support_function(*args, **kwargs):
    # do stuff

@receiver(post_save, sender=User, 
          dispatch_uid='myfile.signal_handler_post_save_user')
def signal_handler_post_save_user(sender, *args, **kwargs):
   _support_function(*args, **kwargs)

and I mock _support_function instead, everything works as expected.

Solution 1:

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)

Solution 2:

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.

Solution 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 :)

Solution 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

Solution 5:

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.