gh-143969: Add NEWS entry for frozen slotted dataclass fix

Author: dawdameet

Lib/dataclasses.py

@@ -732,29 +732,20 @@ def _frozen_get_del_attr(cls, fields, func_builder):
     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}")',
-            '  object.__setattr__(self, 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}")',
-            '  object.__delattr__(self, name)',
-        ),
-        locals=locals,
-        overwrite_error=True,
-    )
+    func_builder.add_fn('__setattr__',
+                        ('self', 'name', 'value'),
+                        (f'  if {condition}:',
+                         '   raise FrozenInstanceError(f"cannot assign to field {name!r}")',
+                         '  object.__setattr__(self, 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}")',
+                         '  object.__delattr__(self, name)'),
+                        locals=locals,
+                        overwrite_error=True)
 
 
 def _is_classvar(a_type, typing):

Lib/test/test_dataclasses/__init__.py

@@ -3404,6 +3404,16 @@ class C:
 
             c = C('hello')
             self.assertEqual(deepcopy(c), c)
+    def test_frozen_slots_setattr(self):
+        # gh-143969: Ensure frozen+slots uses object.__setattr__
+        @dataclass(frozen=True, slots=True)
+        class A:
+            x: int
+        a = A(1)
+        with self.assertRaisesRegex(FrozenInstanceError, 'cannot assign to field'):
+            a.x = 2
+        with self.assertRaisesRegex(FrozenInstanceError, 'cannot assign to field'):
+            object.__setattr__(a, 'x', 2)
 
 
 class TestSlots(unittest.TestCase):

Misc/NEWS.d/next/Library/2026-01-17-23-15-55.gh-issue-143969.Waq8zN.rst

@@ -0,0 +1,2 @@
+Fixed a crash in frozen slotted dataclasses where assigning to an attribute
+could raise an internal TypeError instead of failing cleanly.