Apply suggestions from code review

Co-authored-by: Alyssa Coghlan <[email protected]>

Lib/test/test_unittest/testmock/testhelpers.py

@@ -1107,30 +1107,30 @@ class WithNonFields:
                 with self.assertRaisesRegex(AttributeError, msg):
                     mock.b
 
-    def test_dataclass_with_wider_default(self):
+    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 WithWiderDefault:
+        class WithUnionAnnotation:
             narrow_default: int | None = field(default=30)
 
         for mock in [
-            create_autospec(WithWiderDefault, instance=True),
-            create_autospec(WithWiderDefault()),
+            create_autospec(WithUnionAnnotation, instance=True),
+            create_autospec(WithUnionAnnotation()),
         ]:
             with self.subTest(mock=mock):
                 self.assertIs(mock.narrow_default.__class__, int)
 
-    def test_dataclass_with_no_default(self):
+    def test_dataclass_field_with_no_default_value(self):
         @dataclass
-        class WithWiderDefault:
-            narrow_default: int | None
+        class WithUnionAnnotation:
+            no_default: int | None
 
-        mock = create_autospec(WithWiderDefault, instance=True)
-        self.assertIs(mock.narrow_default.__class__, type(int | None))
+        mock = create_autospec(WithUnionAnnotation, instance=True)
+        self.assertIs(mock.no_default.__class__, type(int | None))
 
-        mock = create_autospec(WithWiderDefault(1))
-        self.assertIs(mock.narrow_default.__class__, int)
+        mock = create_autospec(WithUnionAnnotation(1))
+        self.assertIs(mock.no_default.__class__, int)
 
 
 class TestCallList(unittest.TestCase):

Lib/unittest/mock.py

@@ -2760,6 +2760,10 @@ def create_autospec(spec, spec_set=False, instance=False, _parent=None,
 
     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 = {f.name: f.type for f in dataclass_fields}
         entries.update(base_entries)