Explain how to properly mock user-defined `__call__`

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

Beginne mit dem bereitgestellten MagicMock-Reproducer unter Python 3.12 oder 3.13 und untersuche anschließend unittest/mock.py im Bereich von NonCallableMock.assert_called_with sowie die dabei verwendete Signaturbehandlung für spec=A. Vergleiche das Verhalten einer Klasse, die call definiert, mit dem aktuellen Traceback. Als abgeschlossen gilt die Aufgabe, wenn das dokumentierte oder implementierte Verhalten dafür sorgt, dass die gezeigte Assertion erfolgreich ist.