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gh-135228: When @dataclass(slots=True) replaces a dataclass, make the original class collectible #136893

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Jul 22, 2025
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15 changes: 15 additions & 0 deletions Lib/dataclasses.py
Original file line number Diff line number Diff line change
Expand Up @@ -1338,6 +1338,13 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
or _update_func_cell_for__class__(member.fdel, cls, newcls)):
break

# gh-135228: Make sure the original class can be garbage collected.
# Bypass mapping proxy to allow __dict__ to be removed
old_cls_dict = cls.__dict__ | _deproxier
old_cls_dict.pop('__dict__', None)
if "__weakref__" in cls.__dict__:
del cls.__weakref__

return newcls


Expand Down Expand Up @@ -1732,3 +1739,11 @@ def _replace(self, /, **changes):
# changes that aren't fields, this will correctly raise a
# TypeError.
return self.__class__(**changes)


# Hack to the get the underlying dict out of a mappingproxy
# Use it with: cls.__dict__ | _deproxier
class _Deproxier:
def __ror__(self, other):
return other
_deproxier = _Deproxier()
35 changes: 35 additions & 0 deletions Lib/test/test_dataclasses/__init__.py
Original file line number Diff line number Diff line change
Expand Up @@ -3804,6 +3804,41 @@ class WithCorrectSuper(CorrectSuper):
# that we create internally.
self.assertEqual(CorrectSuper.args, ["default", "default"])

def test_original_class_is_gced(self):
# gh-135228: Make sure when we replace the class with slots=True, the original class
# gets garbage collected.
def make_simple():
@dataclass(slots=True)
class SlotsTest:
pass

return SlotsTest

def make_with_annotations():
@dataclass(slots=True)
class SlotsTest:
x: int

return SlotsTest

def make_with_annotations_and_method():
@dataclass(slots=True)
class SlotsTest:
x: int

def method(self) -> int:
return self.x

return SlotsTest

for make in (make_simple, make_with_annotations, make_with_annotations_and_method):
with self.subTest(make=make):
C = make()
support.gc_collect()
candidates = [cls for cls in object.__subclasses__() if cls.__name__ == 'SlotsTest'
and cls.__firstlineno__ == make.__code__.co_firstlineno + 1]
self.assertEqual(candidates, [C])


class TestDescriptors(unittest.TestCase):
def test_set_name(self):
Expand Down
Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
When :mod:`dataclasses` replaces a class with a slotted dataclass, the
original class is now garbage collected again. Earlier changes in Python
3.14 caused this class to remain in existence together with the replacement
class synthesized by :mod:`dataclasses`.
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