Cython will, in the right circumstances, offer a MethodType instance where im_func is a builtin function. Any instance of MethodType is automatically assumed to be a python-defined function (more specifically, a function that has an inspectable signature), but _set_signature was still conservative in its assumptions. As a result _set_signature would return early with None instead of a mock since the im_func had no inspectable signature. This causes problems deeper inside mock, as _set_signature is assumed to always return a mock, and nothing checked its return value.

In similar corner cases, autospec will simply not check the spec of the function, so _set_signature is amended to now return early with the original, not-wrapped mock object.

There is another None early return in _set_signature, but it seems.. harder to trigger? It seems more appropriate to raise an exception there if this is being fixed.