Spying on `__call__` doesn't work

[hsorsky]

Copied from the docs but updated to use __call__ instead of bar

def test_spy_method(mocker):
    class Foo(object):
        def __call__(self, v):
            return v * 2

    foo = Foo()
    spy = mocker.spy(foo, '__call__')
    assert foo(21) == 42

    spy.assert_called_once_with(21)

raises an assertion error

E       AssertionError: Expected '__call__' to be called once. Called 0 times.

when I'd expect it to pass.

I assume this has something to do with the special nature of __call__?

[nicoddemus]

What happens (IIRC) is that special methods are called via descriptors and bypass instance lookup; if you install the spy into Foo (the class, instead of the instance) then it works.

Closing for now because there's nothing that can be done on pytest-mock's side, feel free to follow up with further questions. 👍