python patch with side_effect on Object's method is not called with self

Question

I encounter a surprising behaviour of the side_effect parameter in patch.object where the function replacing the original does not receive self

class Animal():
    def __init__(self):
        self.noise = 'Woof'

    def make_noise(self):
        return self.noise

def loud(self):
    return self.noise.upper() + '!!'


from unittest.mock import patch

dog = Animal()
dog.make_noise()
with patch.object(Animal, 'make_noise', side_effect=loud):
    dog.make_noise()

This give:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/lustre/home/production/Applications/python/python-3.4.4/lib/python3.4/unittest/mock.py", line 902, in __call__
    return _mock_self._mock_call(*args, **kwargs)
  File "/lustre/home/production/Applications/python/python-3.4.4/lib/python3.4/unittest/mock.py", line 968, in _mock_call
    ret_val = effect(*args, **kwargs)
TypeError: loud() missing 1 required positional argument: 'self'

If I change the loud function to

def loud(*args, **kwargs):
    print(args)
    print(kwargs)

I get the following output:

()
{}

Is there a way to replace a function from an object and still receive self?

Answer 1

self is only supplied for bound methods (because functions are descriptors). A Mock object is not such a method, and the side_effect function is not bound, so self is indeed not going to be passed in.

If you must have access the instance in a side_effect object, you'll have to patch the function on the class with an actual function:

with patch.object(Animal, 'make_noise', new=loud):

Now make_noise is replaced by the loud function on the Animal class, so it'll be bound:

>>> with patch.object(Animal, 'make_noise', new=loud):
...     dog.make_noise()
...
'WOOF!!'

The alternative is to set autospec=True, at which point mock will use a real function to mock out make_noise():

>>> with patch.object(Animal, 'make_noise', autospec=True, side_effect=loud):
...     dog.make_noise()
...
'WOOF!!'

Also see the Mocking Unbound Methods section in the mock getting started section.