bpo-46382 dataclass(slots=True) now takes inherited slots into account (GH-31980)

Do not include any members in __slots__ that are already in a base class's __slots__.

Lib/dataclasses.py

@@ -6,6 +6,7 @@ import inspect
 import keyword
 import builtins
 import functools
+import itertools
 import abc
 import _thread
 from types import FunctionType, GenericAlias
@@ -1122,6 +1123,20 @@ def _dataclass_setstate(self, state):
         object.__setattr__(self, field.name, value)
 
 
+def _get_slots(cls):
+    match cls.__dict__.get('__slots__'):
+        case None:
+            return
+        case str(slot):
+            yield slot
+        # Slots may be any iterable, but we cannot handle an iterator
+        # because it will already be (partially) consumed.
+        case iterable if not hasattr(iterable, '__next__'):
+            yield from iterable
+        case _:
+            raise TypeError(f"Slots of '{cls.__name__}' cannot be determined")
+
+
 def _add_slots(cls, is_frozen):
     # Need to create a new class, since we can't set __slots__
     #  after a class has been created.
@@ -1133,7 +1148,13 @@ def _add_slots(cls, is_frozen):
     # Create a new dict for our new class.
     cls_dict = dict(cls.__dict__)
     field_names = tuple(f.name for f in fields(cls))
-    cls_dict['__slots__'] = field_names
+    # Make sure slots don't overlap with those in base classes.
+    inherited_slots = set(
+        itertools.chain.from_iterable(map(_get_slots, cls.__mro__[1:-1]))
+    )
+    cls_dict["__slots__"] = tuple(
+        itertools.filterfalse(inherited_slots.__contains__, field_names)
+    )
     for field_name in field_names:
         # Remove our attributes, if present. They'll still be
         #  available in _MARKER.