bpo-29403: Fix mock's broken autospec behavior on method-bound builtin functions (GH-3)

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. Patch by Aaron Gallagher.

Author: habnabit

Lib/unittest/mock.py

@@ -155,7 +155,7 @@ def _set_signature(mock, original, instance=False):
  skipfirst = isinstance(original, type)
  result = _get_signature_object(original, instance, skipfirst)
  if result is None:
- return
+ return mock
  func, sig = result
  def checksig(*args, **kwargs):
  sig.bind(*args, **kwargs)

Lib/unittest/test/testmock/testhelpers.py

@@ -1,3 +1,5 @@
+import time
+import types
 import unittest
 
 from unittest.mock import (
@@ -856,6 +858,19 @@ def check_data_descriptor(mock_attr):
  check_data_descriptor(foo.desc)
 
 
+ def test_autospec_on_bound_builtin_function(self):
+ meth = types.MethodType(time.ctime, time.time())
+ self.assertIsInstance(meth(), str)
+ mocked = create_autospec(meth)
+
+ # no signature, so no spec to check against
+ mocked()
+ mocked.assert_called_once_with()
+ mocked.reset_mock()
+ mocked(4, 5, 6)
+ mocked.assert_called_once_with(4, 5, 6)
+
+
 class TestCallList(unittest.TestCase):
 
  def test_args_list_contains_call_list(self):

Misc/NEWS.d/next/Library/2017-07-20-02-29-49.bpo-29403.3RinCV.rst

@@ -0,0 +1,2 @@
+Fix ``unittest.mock``'s autospec to not fail on method-bound builtin
+functions. Patch by Aaron Gallagher.