gh-135368: Fix mocks on dataclass specs with instance=True

Lib/test/test_unittest/testmock/testhelpers.py

@@ -1050,6 +1050,7 @@ def __post_init__(self):
             create_autospec(WithPostInit()),
         ]:
             with self.subTest(mock=mock):
+                self.assertIsInstance(mock, WithPostInit)
                 self.assertIsInstance(mock.a, int)
                 self.assertIsInstance(mock.b, int)
 
@@ -1072,6 +1073,7 @@ class WithDefault:
             create_autospec(WithDefault(1)),
         ]:
             with self.subTest(mock=mock):
+                self.assertIsInstance(mock, WithDefault)
                 self.assertIsInstance(mock.a, int)
                 self.assertIsInstance(mock.b, int)
 
@@ -1087,6 +1089,7 @@ def b(self) -> int:
             create_autospec(WithMethod(1)),
         ]:
             with self.subTest(mock=mock):
+                self.assertIsInstance(mock, WithMethod)
                 self.assertIsInstance(mock.a, int)
                 mock.b.assert_not_called()
 
@@ -1102,11 +1105,27 @@ class WithNonFields:
             create_autospec(WithNonFields(1)),
         ]:
             with self.subTest(mock=mock):
+                self.assertIsInstance(mock, WithNonFields)
                 with self.assertRaisesRegex(AttributeError, msg):
                     mock.a
                 with self.assertRaisesRegex(AttributeError, msg):
                     mock.b
 
+    def test_dataclass_special_attrs(self):
+        @dataclass
+        class Description:
+            name: str
+
+        for mock in [
+            create_autospec(Description, instance=True),
+            create_autospec(Description(1)),
+        ]:
+            with self.subTest(mock=mock):
+                self.assertIsInstance(mock, Description)
+                self.assertIs(mock.__class__, Description)
+                self.assertIsInstance(mock.__dataclass_fields__, MagicMock)
+                self.assertIsInstance(mock.__dataclass_params__, MagicMock)
+
 class TestCallList(unittest.TestCase):
 
     def test_args_list_contains_call_list(self):

Lib/unittest/mock.py

@@ -2771,9 +2771,13 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
     if is_type and instance and is_dataclass(spec):
         dataclass_fields = fields(spec)
         entries.extend((f.name, f.type) for f in dataclass_fields)
-        _kwargs = {'spec': [f.name for f in dataclass_fields]}
+        spec_list = [f.name for f in dataclass_fields]
+        spec_list.extend(['__dataclass_fields__', '__dataclass_params__'])
+        _kwargs = {'spec': spec_list}  # we set `__class__` further
+        is_dataclass_spec = True
     else:
         _kwargs = {'spec': spec}
+        is_dataclass_spec = False
 
     if spec_set:
         _kwargs = {'spec_set': spec}
@@ -2810,6 +2814,8 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
 
     mock = Klass(parent=_parent, _new_parent=_parent, _new_name=_new_name,
                  name=_name, **_kwargs)
+    if is_dataclass_spec:
+        mock.__class__ = spec  # we need this for `isinstance` to work
 
     if isinstance(spec, FunctionTypes):
         # should only happen at the top level because we don't

Misc/NEWS.d/next/Library/2025-06-12-10-45-02.gh-issue-135368.OjWVHL.rst

@@ -0,0 +1,2 @@
+Fix :class:`unittest.mock.Mock` generation on :func:`dataclasses.dataclass`
+objects. Now all special attributes are set as it was before :gh:`124429`.