gh-129915: Add __qualname__ to class namespace when constructing slotted dataclass

Lib/dataclasses.py

@@ -1299,11 +1299,12 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
     # Clear existing `__weakref__` descriptor, it belongs to a previous type:
     cls_dict.pop('__weakref__', None)  # gh-102069
 
+    # Set the `__qualname__ accordingly
+    if (qualname := getattr(cls, '__qualname__', None) is not None):
+        cls_dict['__qualname__'] = qualname
+
     # And finally create the class.
-    qualname = getattr(cls, '__qualname__', None)
     newcls = type(cls)(cls.__name__, cls.__bases__, cls_dict)
-    if qualname is not None:
-        newcls.__qualname__ = qualname
 
     if is_frozen:
         # Need this for pickling frozen classes with slots.

Lib/test/test_dataclasses/__init__.py

@@ -3367,6 +3367,25 @@ class A:
         self.assertFalse(hasattr(A, "__slots__"))
         self.assertTrue(hasattr(B, "__slots__"))
 
+    def test_slots_qualname(self):
+        # Test that __qualname__ is set correctly when using slots
+        @dataclass(slots=True)
+        class C:
+            x: int
+
+            def __init_subclass__(cls):
+                expected = f'TestSlots.test_slots_qualname.<locals>.{cls.__name__}'
+                self.assertEqual(cls.__qualname__, expected)
+
+        self.assertTrue('__qualname__' in C.__dict__)
+        self.assertEqual(C.__qualname__, 'TestSlots.test_slots_qualname.<locals>.C')
+
+        @dataclass(slots=True)
+        class D(C):
+            pass
+
+        self.assertEqual(D.__qualname__, 'TestSlots.test_slots_qualname.<locals>.D')
+
     # Can't be local to test_frozen_pickle.
     @dataclass(frozen=True, slots=True)
     class FrozenSlotsClass: