GH-93521: For dataclasses, filter out `__weakref__` slot if present in bases (GH-93535)

1 files changed, 9 insertions, 4 deletions

diff --git a/Lib/dataclasses.py b/Lib/dataclasses.py
index 4645ebf..18ab690 100644
--- a/Lib/dataclasses.py
+++ b/Lib/dataclasses.py
@@ -1156,11 +1156,16 @@ def _add_slots(cls, is_frozen, weakref_slot):
         itertools.chain.from_iterable(map(_get_slots, cls.__mro__[1:-1]))
     )
     # The slots for our class.  Remove slots from our base classes.  Add
-    # '__weakref__' if weakref_slot was given.
+    # '__weakref__' if weakref_slot was given, unless it is already present.
     cls_dict["__slots__"] = tuple(
-        itertools.chain(
-            itertools.filterfalse(inherited_slots.__contains__, field_names),
-            ("__weakref__",) if weakref_slot else ())
+        itertools.filterfalse(
+            inherited_slots.__contains__,
+            itertools.chain(
+                # gh-93521: '__weakref__' also needs to be filtered out if
+                # already present in inherited_slots
+                field_names, ('__weakref__',) if weakref_slot else ()
+            )
+        ),
     )
 
     for field_name in field_names: