gh-102978: Fix mock.patch function signatures for class and staticmethod decorators (#103228)

Fixes unittest.mock.patch not enforcing function signatures for methods
decorated with @classmethod or @staticmethod when patch is called with
autospec=True.

Backports: 59e0de4903c02e72b329e505fddf1ad9794928bc
Signed-off-by: Chris Withers <chris@simplistix.co.uk>

Author: tomasr8

NEWS.d/2023-04-03-23-44-34.gh-issue-102978.gy9eVk.rst

@@ -0,0 +1,3 @@
+Fixes :func:`unittest.mock.patch` not enforcing function signatures for methods
+decorated with ``@classmethod`` or ``@staticmethod`` when patch is called with
+``autospec=True``.

mock/mock.py

@@ -101,6 +101,12 @@ def _get_signature_object(func, as_instance, eat_self):
         func = func.__init__
         # Skip the `self` argument in __init__
         eat_self = True
+    elif isinstance(func, (classmethod, staticmethod)):
+        if isinstance(func, classmethod):
+            # Skip the `cls` argument of a class method
+            eat_self = True
+        # Use the original decorated method to extract the correct function signature
+        func = func.__func__
     elif not isinstance(func, FunctionTypes):
         # If we really want to model an instance of the passed type,
         # __call__ should be looked up, not __init__.

mock/tests/testhelpers.py

@@ -961,6 +961,24 @@ def __getattr__(self, attribute):
         self.assertFalse(hasattr(autospec, '__name__'))
 
 
+    def test_autospec_signature_staticmethod(self):
+        class Foo:
+            @staticmethod
+            def static_method(a, b=10, *, c): pass
+
+        mock = create_autospec(Foo.__dict__['static_method'])
+        self.assertEqual(inspect.signature(Foo.static_method), inspect.signature(mock))
+
+
+    def test_autospec_signature_classmethod(self):
+        class Foo:
+            @classmethod
+            def class_method(cls, a, b=10, *, c): pass
+
+        mock = create_autospec(Foo.__dict__['class_method'])
+        self.assertEqual(inspect.signature(Foo.class_method), inspect.signature(mock))
+
+
     def test_spec_inspect_signature(self):
 
         def myfunc(x, y): pass

mock/tests/testpatch.py

@@ -996,6 +996,36 @@ def test_autospec_classmethod(self):
             method.assert_called_once_with()
 
 
+    def test_autospec_staticmethod_signature(self):
+        # Patched methods which are decorated with @staticmethod should have the same signature
+        class Foo:
+            @staticmethod
+            def static_method(a, b=10, *, c): pass
+
+        Foo.static_method(1, 2, c=3)
+
+        with patch.object(Foo, 'static_method', autospec=True) as method:
+            method(1, 2, c=3)
+            self.assertRaises(TypeError, method)
+            self.assertRaises(TypeError, method, 1)
+            self.assertRaises(TypeError, method, 1, 2, 3, c=4)
+
+
+    def test_autospec_classmethod_signature(self):
+        # Patched methods which are decorated with @classmethod should have the same signature
+        class Foo:
+            @classmethod
+            def class_method(cls, a, b=10, *, c): pass
+
+        Foo.class_method(1, 2, c=3)
+
+        with patch.object(Foo, 'class_method', autospec=True) as method:
+            method(1, 2, c=3)
+            self.assertRaises(TypeError, method)
+            self.assertRaises(TypeError, method, 1)
+            self.assertRaises(TypeError, method, 1, 2, 3, c=4)
+
+
     def test_autospec_with_new(self):
         patcher = patch('%s.function' % __name__, new=3, autospec=True)
         self.assertRaises(TypeError, patcher.start)