Explain how to properly mock user-defined `__call__`

[clafoutis42]

Bug report

Bug description:

Reproduce

from unittest.mock import MagicMock

class A:
    def __init__(self, a):
        ...

    def __call__(self, b, c):
        ...

mocker = MagicMock(spec=A)
mocker(21, 42)
mocker.assert_called_once_with(21, 42)

Current Behavior

TypeError                                 Traceback (most recent call last)
TypeError: too many positional arguments

The above exception was the direct cause of the following exception:

AssertionError                            Traceback (most recent call last)
Cell In[6], line 10
      8 mocker = MagicMock(spec=A)
      9 mocker(21, 42)
---> 10 mocker.assert_called_once_with(21, 42)

File ~/.pyenv/versions/3.12.2/lib/python3.12/unittest/mock.py:956, in NonCallableMock.assert_called_once_with(self, *args, **kwargs)
    951     msg = ("Expected '%s' to be called once. Called %s times.%s"
    952            % (self._mock_name or 'mock',
    953               self.call_count,
    954               self._calls_repr()))
    955     raise AssertionError(msg)
--> 956 return self.assert_called_with(*args, **kwargs)

File ~/.pyenv/versions/3.12.2/lib/python3.12/unittest/mock.py:944, in NonCallableMock.assert_called_with(self, *args, **kwargs)
    942 if actual != expected:
    943     cause = expected if isinstance(expected, Exception) else None
--> 944     raise AssertionError(_error_message()) from cause

AssertionError: expected call not found.
Expected: mock(21, 42)
  Actual: mock(21, 42)

Expected behaviour

It succeeds

Investigation

When passing a class that implements the __call__ method as spec to a mock instance it takes the it takes the signature of the __init__ which if it is the intended behavior is quite confusing.

CPython versions tested on:

3.12, 3.13

Operating systems tested on:

macOS, Linux

[picnixz]

AFAICT, __call__ is not supported as a magic method (at least it's not documented in https://docs.python.org/3/library/unittest.mock.html#mocking-magic-methods). So I don't know if it's a bug or not.

[clafoutis42]

AFAICT, __call__ is not supported as a magic method (at least it's not documented in https://docs.python.org/3/library/unittest.mock.html#mocking-magic-methods). So I don't know if it's a bug or not.

Indeed, that's something I missed while browsing the doc. Considering this I would understand if this is not considered as a bug.

Either way, I also have a workaround:

from unittest.mock import MagicMock

class A:
    def __init__(self, a):
        ...

    def __call__(self, b, c):
        ...

mocker = MagicMock(spec=A, side_effect=MagicMock())
mocker(21, 42)
mocker.side_effect.assert_called_once_with(21, 42)

It's not that pretty but does the trick if you really have to mock a callable instance

[picnixz]

@cjw296 do you think it's a legitimate feature request or would it be impossible to make it work properly?^

[cjw296]

Evidently something is not right here :-)

I've only scanned this quickly, but:

• I can't think of a logical reason why __call__ shouldn't be supported, if there's a pragmatic one, I'm unaware of it
• The current behaviour if confusing, but, tbh, I've always found the spec'ing side of mock pretty confusing.

What happens if you do:

mocker = MagicMock(spec=A(1))`

?

[clafoutis42]

What happens if you do:

mocker = MagicMock(spec=A(1))`
?

Then the test passes and _spec_signature is correct.

[cjw296]

Okay, so if anything this feels like a documentation issue.
If you do mocker = MagicMock(spec=A), your spec is the type, so you're actually mocking out the class, and when you call a class, it's __init__ that gets executed, if you do mocker = MagicMock(spec=A(1)), then you're mocking the instance, and when you call an instance __call__ is the method used.

Maybe you could work up a documentation PR with changes that would have helped you here?

[cjw296]

Also:

TypeError                                 Traceback (most recent call last)
TypeError: too many positional arguments

This is the important exception. Why is your test going on to do mocker.assert_called_once_with(21, 42) if an exception has already occurred?