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.

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 @@ class SpecSignatureTest(unittest.TestCase):
         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.