Spying on a cached method through its class drops self #657

Spying on an instance method decorated with functools.lru_cache or functools.cache through its class breaks calls through an instance: the spy does not bind self, so a previously valid call raises TypeError. Spying on the instance directly works.

from functools import lru_cache


def test_cached_method(mocker):
    class Worker:
        @lru_cache(maxsize=128)
        def calculate(self, value):
            return value * 2

    worker = Worker()
    assert worker.calculate(3) == 6
    spy = mocker.spy(Worker, "calculate")
    assert worker.calculate(3) == 6
    spy.assert_called_once_with(worker, 3)

The second call raises TypeError: ...calculate() missing 1 required positional argument: 'value'. Expected: it returns 6, preserves caching, and records the instance and argument like a spy on an ordinary instance method.

Reproduced on Linux with Python 3.12.16 and pytest-mock main 163f3221165929d64ac6549f693ea34edeb2b95d, using both unittest.mock and the standalone mock backend.