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gh-105936: Properly update closure cells for `__setattr__` and `__delattr__` in frozen dataclasses with slots by Prometheus3375 · Pull Request #144021 · python/cpython · GitHub

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gh-105936: Properly update closure cells for __setattr__ and __delattr__ in frozen dataclasses with slots #144021

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Merged
gpshead merged 14 commits into from
Apr 12, 2026
Conversation 18 Commits 14 Checks 57 Files changed
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gh-105936: Properly update closure cells for __setattr__ and __delattr__ in frozen dataclasses with slots #144021

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2160254
Use __class__ in setattr and delattr
Prometheus3375 Jan 19, 2026
a6de29b
Rename _frozen_get_del_attr
Prometheus3375 Jan 19, 2026
63c4e19
Add tests
Prometheus3375 Jan 19, 2026
d357716
Rename test
Prometheus3375 Jan 19, 2026
9b33e89
📜🤖 Added by blurb_it.
blurb-it[bot] Jan 19, 2026
bff566a
Reformat news entry
Prometheus3375 Jan 19, 2026
883363f
Move tests to other case
Prometheus3375 Jan 19, 2026
1c8f7b9
Add another test
Prometheus3375 Jan 19, 2026
bb82978
Rephrase news entry
Prometheus3375 Jan 19, 2026
ef4c814
Update news entry name
Prometheus3375 Jan 22, 2026
d3ed33a
Merge some tests with existing ones
Prometheus3375 Jan 22, 2026
b8515f6
Reword news entry
Prometheus3375 Jan 22, 2026
1099d33
Remove redundant tests
Prometheus3375 Jan 22, 2026
a8f80e9
Improve Testfrozen.test_frozen
Prometheus3375 Jan 22, 2026
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12 changes: 6 additions & 6 deletions Lib/dataclasses.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)

Expand Down Expand Up @@ -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),
Expand Down
124 changes: 73 additions & 51 deletions Lib/test/test_dataclasses/__init__.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3052,29 +3052,41 @@ class C(base):


class TestFrozen(unittest.TestCase):
# Some tests have a subtest with a slotted dataclass.
# See https://github.com/python/cpython/issues/105936 for the reasons.

def test_frozen(self):
@dataclass(frozen=True)
class C:
i: int
for slots in (False, True):
with self.subTest(slots=slots):

c = C(10)
self.assertEqual(c.i, 10)
with self.assertRaises(FrozenInstanceError):
c.i = 5
self.assertEqual(c.i, 10)
@dataclass(frozen=True, slots=slots)
class C:
i: int

c = C(10)
self.assertEqual(c.i, 10)
with self.assertRaises(FrozenInstanceError):
c.i = 5
self.assertEqual(c.i, 10)
with self.assertRaises(FrozenInstanceError):
del c.i
self.assertEqual(c.i, 10)

def test_frozen_empty(self):
@dataclass(frozen=True)
class C:
pass
for slots in (False, True):
with self.subTest(slots=slots):

c = C()
self.assertNotHasAttr(c, 'i')
with self.assertRaises(FrozenInstanceError):
c.i = 5
self.assertNotHasAttr(c, 'i')
with self.assertRaises(FrozenInstanceError):
del c.i
@dataclass(frozen=True, slots=slots)
class C:
pass

c = C()
self.assertNotHasAttr(c, 'i')
with self.assertRaises(FrozenInstanceError):
c.i = 5
self.assertNotHasAttr(c, 'i')
with self.assertRaises(FrozenInstanceError):
del c.i

def test_inherit(self):
@dataclass(frozen=True)
Expand Down Expand Up @@ -3270,41 +3282,43 @@ class D(I):
d.i = 5

def test_non_frozen_normal_derived(self):
# See bpo-32953.

@dataclass(frozen=True)
class D:
x: int
y: int = 10

class S(D):
pass
# See bpo-32953 and https://github.com/python/cpython/issues/105936
for slots in (False, True):
with self.subTest(slots=slots):

s = S(3)
self.assertEqual(s.x, 3)
self.assertEqual(s.y, 10)
s.cached = True
@dataclass(frozen=True, slots=slots)
class D:
x: int
y: int = 10

# But can't change the frozen attributes.
with self.assertRaises(FrozenInstanceError):
s.x = 5
with self.assertRaises(FrozenInstanceError):
s.y = 5
self.assertEqual(s.x, 3)
self.assertEqual(s.y, 10)
self.assertEqual(s.cached, True)
class S(D):
pass

with self.assertRaises(FrozenInstanceError):
del s.x
self.assertEqual(s.x, 3)
with self.assertRaises(FrozenInstanceError):
del s.y
self.assertEqual(s.y, 10)
del s.cached
self.assertNotHasAttr(s, 'cached')
with self.assertRaises(AttributeError) as cm:
del s.cached
self.assertNotIsInstance(cm.exception, FrozenInstanceError)
s = S(3)
self.assertEqual(s.x, 3)
self.assertEqual(s.y, 10)
s.cached = True

# But can't change the frozen attributes.
with self.assertRaises(FrozenInstanceError):
s.x = 5
with self.assertRaises(FrozenInstanceError):
s.y = 5
self.assertEqual(s.x, 3)
self.assertEqual(s.y, 10)
self.assertEqual(s.cached, True)

with self.assertRaises(FrozenInstanceError):
del s.x
self.assertEqual(s.x, 3)
with self.assertRaises(FrozenInstanceError):
del s.y
self.assertEqual(s.y, 10)
del s.cached
self.assertNotHasAttr(s, 'cached')
with self.assertRaises(AttributeError) as cm:
del s.cached
self.assertNotIsInstance(cm.exception, FrozenInstanceError)

def test_non_frozen_normal_derived_from_empty_frozen(self):
@dataclass(frozen=True)
Expand Down Expand Up @@ -3971,6 +3985,14 @@ 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:
Expand All @@ -3996,7 +4018,7 @@ 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()
Expand Down
5 changes: 5 additions & 0 deletions Misc/NEWS.d/next/Library/2026-01-19-21-23-18.gh-issue-105936.dGrzjM.rst
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@@ -0,0 +1,5 @@
Attempting to mutate non-field attributes of :mod:`dataclasses`
with both *frozen* and *slots* being ``True`` now raises
:class:`~dataclasses.FrozenInstanceError` instead of :class:`TypeError`.
Their non-dataclass subclasses can now freely mutate non-field attributes,
and the original non-slotted class can be garbage collected.
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