gh-124176: create_autospec must not change how dataclass defaults are mocked

Lib/test/test_unittest/testmock/testhelpers.py

@@ -1107,6 +1107,32 @@ class WithNonFields:
                 with self.assertRaisesRegex(AttributeError, msg):
                     mock.b
 
+    def test_dataclass_default_value_type_overrides_field_annotation(self):
+        # If field defines an actual default, we don't need to change
+        # the default type. Since this is how it used to work before #124176
+        @dataclass
+        class WithUnionAnnotation:
+            narrow_default: int | None = field(default=30)
+
+        for mock in [
+            create_autospec(WithUnionAnnotation, instance=True),
+            create_autospec(WithUnionAnnotation()),
+        ]:
+            with self.subTest(mock=mock):
+                self.assertIs(mock.narrow_default.__class__, int)
+
+    def test_dataclass_field_with_no_default_value(self):
+        @dataclass
+        class WithUnionAnnotation:
+            no_default: int | None
+
+        mock = create_autospec(WithUnionAnnotation, instance=True)
+        self.assertIs(mock.no_default.__class__, type(int | None))
+
+        mock = create_autospec(WithUnionAnnotation(1))
+        self.assertIs(mock.no_default.__class__, int)
+
+
 class TestCallList(unittest.TestCase):
 
     def test_args_list_contains_call_list(self):

Lib/unittest/mock.py

@@ -2758,13 +2758,19 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
                                f'[object={spec!r}]')
     is_async_func = _is_async_func(spec)
 
-    entries = [(entry, _missing) for entry in dir(spec)]
+    base_entries = {entry: _missing for entry in dir(spec)}
     if is_type and instance and is_dataclass(spec):
+        # Dataclass instance mocks created from a class may not have all of their fields
+        # prepopulated with default values. Create an initial set of attribute entries from
+        # the dataclass field annotations, but override them with the actual attribute types
+        # when fields have already been populated.
         dataclass_fields = fields(spec)
-        entries.extend((f.name, f.type) for f in dataclass_fields)
+        entries = {f.name: f.type for f in dataclass_fields}
+        entries.update(base_entries)
         _kwargs = {'spec': [f.name for f in dataclass_fields]}
     else:
         _kwargs = {'spec': spec}
+        entries = base_entries
 
     if spec_set:
         _kwargs = {'spec_set': spec}
@@ -2822,7 +2828,7 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
                                             _name='()', _parent=mock,
                                             wraps=wrapped)
 
-    for entry, original in entries:
+    for entry, original in entries.items():
         if _is_magic(entry):
             # MagicMock already does the useful magic methods for us
             continue