gh-135228: Properly update closure cells for __setattr__ and __detattr__ in frozen dataclasses with slots

Lib/dataclasses.py

@@ -725,25 +725,25 @@ def _init_fn(fields, std_fields, kw_only_fields, frozen, has_post_init,
                         annotation_fields=annotation_fields)
 
 
-def _frozen_get_del_attr(cls, fields, func_builder):
-    locals = {'cls': cls,
+def _frozen_set_del_attr(cls, fields, func_builder):
+    locals = {'__class__': cls,
               'FrozenInstanceError': FrozenInstanceError}
-    condition = 'type(self) is cls'
+    condition = 'type(self) is __class__'
     if fields:
         condition += ' or name in {' + ', '.join(repr(f.name) for f in fields) + '}'
 
     func_builder.add_fn('__setattr__',
                         ('self', 'name', 'value'),
                         (f'  if {condition}:',
                           '   raise FrozenInstanceError(f"cannot assign to field {name!r}")',
-                         f'  super(cls, self).__setattr__(name, value)'),
+                         f'  super(__class__, self).__setattr__(name, value)'),
                         locals=locals,
                         overwrite_error=True)
     func_builder.add_fn('__delattr__',
                         ('self', 'name'),
                         (f'  if {condition}:',
                           '   raise FrozenInstanceError(f"cannot delete field {name!r}")',
-                         f'  super(cls, self).__delattr__(name)'),
+                         f'  super(__class__, self).__delattr__(name)'),
                         locals=locals,
                         overwrite_error=True)
 
@@ -1199,7 +1199,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
                             overwrite_error='Consider using functools.total_ordering')
 
     if frozen:
-        _frozen_get_del_attr(cls, field_list, func_builder)
+        _frozen_set_del_attr(cls, field_list, func_builder)
 
     # Decide if/how we're going to create a hash function.
     hash_action = _hash_action[bool(unsafe_hash),

Lib/test/test_dataclasses/__init__.py

@@ -3076,6 +3076,20 @@ class C:
         with self.assertRaises(FrozenInstanceError):
             del c.i
 
+    def test_frozen_slotted(self):
+        # See https://github.com/python/cpython/pull/144021
+        @dataclass(frozen=True, slots=True)
+        class C:
+            pass
+
+        c = C()
+        # Mutating not defined fields must raise FrozenInstanceError.
+        with self.assertRaises(FrozenInstanceError):
+            c.any_field = 5
+
+        with self.assertRaises(FrozenInstanceError):
+            del c.any_field
+
     def test_inherit(self):
         @dataclass(frozen=True)
         class C:
@@ -3405,6 +3419,33 @@ class C:
             c = C('hello')
             self.assertEqual(deepcopy(c), c)
 
+    def test_slotted_set_del_attr_reference_new_class_via__class__(self):
+        # See https://github.com/python/cpython/pull/144021
+        @dataclass(frozen=True, slots=True)
+        class SetDelAttrTest:
+            pass
+
+        for method_name in ('__setattr__', '__delattr__'):
+            with self.subTest(method_name=method_name):
+                method = getattr(SetDelAttrTest, method_name)
+                cell_idx = method.__code__.co_freevars.index('__class__')
+                reference = method.__closure__[cell_idx].cell_contents
+                self.assertIs(reference, SetDelAttrTest)
+
+    def test_slotted_set_del_attr_do_not_reference_old_class(self):
+        # See https://github.com/python/cpython/pull/144021
+        class SetDelAttrTest:
+            pass
+
+        OriginalCls = SetDelAttrTest
+        SetDelAttrTest = dataclass(frozen=True, slots=True)(SetDelAttrTest)
+
+        for method_name in ('__setattr__', '__delattr__'):
+            with self.subTest(method_name=method_name):
+                method = getattr(SetDelAttrTest, method_name)
+                cell_contents = [cell.cell_contents for cell in method.__closure__]
+                self.assertNotIn(OriginalCls, cell_contents)
+
 
 class TestSlots(unittest.TestCase):
     def test_simple(self):
@@ -3971,6 +4012,14 @@ class SlotsTest:
 
             return SlotsTest
 
+        # See https://github.com/python/cpython/issues/135228#issuecomment-3755979059
+        def make_frozen():
+            @dataclass(frozen=True, slots=True)
+            class SlotsTest:
+                pass
+
+            return SlotsTest
+
         def make_with_annotations():
             @dataclass(slots=True)
             class SlotsTest:
@@ -3996,7 +4045,7 @@ class SlotsTest:
 
             return SlotsTest
 
-        for make in (make_simple, make_with_annotations, make_with_annotations_and_method, make_with_forwardref):
+        for make in (make_simple, make_frozen, make_with_annotations, make_with_annotations_and_method, make_with_forwardref):
             with self.subTest(make=make):
                 C = make()
                 support.gc_collect()

Misc/NEWS.d/next/Library/2026-01-19-21-23-18.gh-issue-135228.dGrzjM.rst

@@ -0,0 +1,4 @@
+When both *frozen* and *slots* are set to ``True``
+in :func:`~dataclasses.dataclass`, added methods
+``__setattr__`` and ``__delattr__`` were still referencing the original class,
+preventing it from being garbage collected and causing unexpected exceptions.