gh-76273: Fix autospec mocking of instance and class methods

Currently, when patching instance / class methods with autospec, their
self / cls arguments are not consumed, causing call asserts to fail
(they expect an instance / class reference as the first argument).
Example:

  from unittest import mock

  class Something(object):
      def foo(self, a, b, c, d):
          pass

  with mock.patch.object(Something, 'foo', autospec=True):
      s = Something()
      s.foo()

Fix this by skipping the first argument when presented with a method.

Based on #4476

Signed-off-by: Stephen Finucane <stephen@that.guru>
Co-authored-by: Claudiu Belu <cbelu@cloudbasesolutions.com>

Author: stephenfin

Lib/test/test_unittest/testmock/testmock.py

@@ -2196,9 +2196,9 @@ def test_attach_mock_patch_autospec_signature(self):
             manager.attach_mock(mocked, 'attach_meth')
             obj = Something()
             obj.meth(1, 2, 3, d=4)
-            manager.assert_has_calls([call.attach_meth(mock.ANY, 1, 2, 3, d=4)])
-            obj.meth.assert_has_calls([call(mock.ANY, 1, 2, 3, d=4)])
-            mocked.assert_has_calls([call(mock.ANY, 1, 2, 3, d=4)])
+            manager.assert_has_calls([call.attach_meth(1, 2, 3, d=4)])
+            obj.meth.assert_has_calls([call(1, 2, 3, d=4)])
+            mocked.assert_has_calls([call(1, 2, 3, d=4)])
 
         with mock.patch(f'{__name__}.something', autospec=True) as mocked:
             manager = Mock()

Lib/test/test_unittest/testmock/testpatch.py

@@ -1046,6 +1046,21 @@ def test_autospec_classmethod(self):
             method.assert_called_once_with()
 
 
+    def test_autospec_method_signature(self):
+        # Patched methods should have the same signature
+        # https://github.com/python/cpython/issues/76273
+        class Foo:
+            def method(self, a, b=10, *, c): pass
+
+        with patch.object(Foo, 'method', autospec=True) as mock_method:
+            foo = Foo()
+            foo.method(1, 2, c=3)
+            mock_method.assert_called_once_with(1, 2, c=3)
+            self.assertRaises(TypeError, foo.method)
+            self.assertRaises(TypeError, foo.method, 1)
+            self.assertRaises(TypeError, foo.method, 1, 2, 3, c=4)
+
+
     def test_autospec_staticmethod_signature(self):
         # Patched methods which are decorated with @staticmethod should have the same signature
         class Foo:

Lib/unittest/mock.py

@@ -182,12 +182,12 @@ def _instance_callable(obj):
     return False
 
 
-def _set_signature(mock, original, instance=False):
+def _set_signature(mock, original, instance=False, skipfirst=False):
     # creates a function with signature (*args, **kwargs) that delegates to a
     # mock. It still does signature checking by calling a lambda with the same
     # signature as the original.
 
-    skipfirst = isinstance(original, type)
+    skipfirst = skipfirst or isinstance(original, type)
     result = _get_signature_object(original, instance, skipfirst)
     if result is None:
         return mock
@@ -200,28 +200,41 @@ def checksig(*args, **kwargs):
     if not name.isidentifier():
         name = 'funcopy'
     context = {'_checksig_': checksig, 'mock': mock}
-    src = """def %s(*args, **kwargs):
+    if skipfirst:
+        src = """def %s(_mock_self, /, *args, **kwargs):
+    _checksig_(*args, **kwargs)
+    return mock(*args, **kwargs)""" % name
+    else:
+        src = """def %s(*args, **kwargs):
     _checksig_(*args, **kwargs)
     return mock(*args, **kwargs)""" % name
     exec (src, context)
     funcopy = context[name]
     _setup_func(funcopy, mock, sig)
     return funcopy
 
-def _set_async_signature(mock, original, instance=False, is_async_mock=False):
+def _set_async_signature(mock, original, instance=False, is_async_mock=False, skipfirst=False):
     # creates an async function with signature (*args, **kwargs) that delegates to a
     # mock. It still does signature checking by calling a lambda with the same
     # signature as the original.
 
-    skipfirst = isinstance(original, type)
-    func, sig = _get_signature_object(original, instance, skipfirst)
+    skipfirst = skipfirst or isinstance(original, type)
+    result = _get_signature_object(original, instance, skipfirst)
+    if result is None:
+        return mock
+    func, sig = result
     def checksig(*args, **kwargs):
         sig.bind(*args, **kwargs)
     _copy_func_details(func, checksig)
 
     name = original.__name__
     context = {'_checksig_': checksig, 'mock': mock}
-    src = """async def %s(*args, **kwargs):
+    if skipfirst:
+        src = """async def %s(_mock_self, /, *args, **kwargs):
+    _checksig_(*args, **kwargs)
+    return await mock(*args, **kwargs)""" % name
+    else:
+        src = """async def %s(*args, **kwargs):
     _checksig_(*args, **kwargs)
     return await mock(*args, **kwargs)""" % name
     exec (src, context)
@@ -1593,7 +1606,10 @@ def __enter__(self):
                 raise TypeError("Can't use 'autospec' with create=True")
             spec_set = bool(spec_set)
             if autospec is True:
-                autospec = original
+                if isinstance(self.target, type):
+                    autospec = getattr(self.target, self.attribute, original)
+                else:
+                    autospec = original
 
             if _is_instance_mock(self.target):
                 raise InvalidSpecError(
@@ -1607,8 +1623,14 @@ def __enter__(self):
                     f'{target_name!r} as it has already been mocked out. '
                     f'[target={self.target!r}, attr={autospec!r}]')
 
+            # For regular methods on classes, self is passed by the descriptor
+            # protocol but should not be recorded in mock call args.
+            _eat_self = _must_skip(
+                self.target, self.attribute, isinstance(self.target, type)
+            )
             new = create_autospec(autospec, spec_set=spec_set,
-                                  _name=self.attribute, **kwargs)
+                                  _name=self.attribute, _eat_self=_eat_self,
+                                  **kwargs)
         elif kwargs:
             # can't set keyword args when we aren't creating the mock
             # XXXX If new is a Mock we could call new.configure_mock(**kwargs)
@@ -2746,7 +2768,7 @@ def call_list(self):
 
 
 def create_autospec(spec, spec_set=False, instance=False, _parent=None,
-                    _name=None, *, unsafe=False, **kwargs):
+                    _name=None, *, unsafe=False, _eat_self=False, **kwargs):
     """Create a mock object using another object as a spec. Attributes on the
     mock will use the corresponding attribute on the `spec` object as their
     spec.
@@ -2823,17 +2845,17 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
         Klass = NonCallableMagicMock
 
     mock = Klass(parent=_parent, _new_parent=_parent, _new_name=_new_name,
-                 name=_name, **_kwargs)
+                 name=_name, _eat_self=_eat_self or None, **_kwargs)
     if is_dataclass_spec:
         mock._mock_extend_spec_methods(dataclass_spec_list)
 
     if isinstance(spec, FunctionTypes):
         # should only happen at the top level because we don't
         # recurse for functions
         if is_async_func:
-            mock = _set_async_signature(mock, spec)
+            mock = _set_async_signature(mock, spec, skipfirst=_eat_self)
         else:
-            mock = _set_signature(mock, spec)
+            mock = _set_signature(mock, spec, skipfirst=_eat_self)
     else:
         _check_signature(spec, mock, is_type, instance)
 

Misc/NEWS.d/next/Library/2026-03-27-16-50-34.gh-issue-76273.BQjOvU.rst

@@ -0,0 +1,2 @@
+Calling ``unittest.mock.patch`` with ``autospec`` on an instance or class method
+will now correctly consume the ``self`` / ``cls`` argument.