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.

Patch adapted from https://github.com/testing-cabal/mock/pull/389

This PR appears to have been approved, perhaps pending input from michael.foord?

New changeset 856cbcc12f2e4cca93af5dc7ed6bcea4dd942f10 by Berker Peksag (Aaron Gallagher) in branch 'master':
bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
https://github.com/python/cpython/commit/856cbcc12f2e4cca93af5dc7ed6bcea4dd942f10

New changeset 64b9a15886840df422c5203fad25c9801b4cf61e by Berker Peksag in branch '3.6':
[3.6] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
https://github.com/python/cpython/commit/64b9a15886840df422c5203fad25c9801b4cf61e

New changeset e78dc0aaf03cd98373910150c2d35418cf938254 by Berker Peksag in branch '3.5':
[3.5] bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)
https://github.com/python/cpython/commit/e78dc0aaf03cd98373910150c2d35418cf938254

Thank you for the PR, Aaron!