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

Prometheus3375 · 2026-01-19T05:45:30Z

Specifying frozen=True for a dataclass adds methods __setattr__ and __detattr__ to it. These methods reference the class via closure. If slots=True is also present, then the class is recreated with slots with all methods being copied. Notably, every method referencing the old class via closure in variable __class__ is properly updated, but __setattr__ and __detattr__ reference the old class via cls; hence, they are still referencing the old class instead of the new one. This PR fixes this issue.

Issue: dataclasses: frozen dataclasses with slots' __setattr__ and __delattr__ does not work. #105936

Additional details:

The issue is present in Python as early as in 3.10. This PR must be backported to 3.14 and possibly 3.13.

Originally this PR was referencing gh-135228, but soon I realized that the issue is not connected to regression in 3.14. There is an older PR #105937 that fixes the issue, but it is heavily outdated.

python-cla-bot · 2026-01-19T05:45:34Z

All commit authors signed the Contributor License Agreement.

JelleZijlstra · 2026-01-19T05:52:05Z

Thanks! For the test you can do something like that in the original fix, https://github.com/python/cpython/pull/137047/files#diff-212e368b34eb9b134f87e765787d6d26b747235a358e13da8922f83861c0d676 .

Prometheus3375 · 2026-01-24T06:45:02Z

While this PR is not merged, should AKCS be updated?

JelleZijlstra · 2026-01-25T00:53:24Z

If I remember correctly the latest policy on Misc/ACKS is that people can add themselves if they want but it's not required. The file is a pain to keep fully correct and sort of redundant with git log, so there's been talk of getting rid of it but not sure anything concrete has changed yet.

vstinner

LGTM as well.

cc @ericvsmith @carljm

gpshead

Thanks!

miss-islington-app · 2026-04-12T21:12:23Z

Thanks @Prometheus3375 for the PR, and @gpshead for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

…`__delattr__` in frozen dataclasses with slots (pythonGH-144021) (cherry picked from commit 8a398bf) Co-authored-by: Sviataslau <35541026+Prometheus3375@users.noreply.github.com>

miss-islington-app · 2026-04-12T21:12:39Z

Sorry, @Prometheus3375 and @gpshead, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 8a398bfbbc6769f6cabb3177702e7a506e203d61 3.13

bedevere-app · 2026-04-12T21:12:43Z

GH-148469 is a backport of this pull request to the 3.14 branch.

…`__delattr__` in frozen dataclasses with slots (pythonGH-144021) (cherry picked from commit 8a398bf) Co-authored-by: Sviataslau <35541026+Prometheus3375@users.noreply.github.com>

… `__delattr__` in frozen dataclasses with slots (GH-144021) (#148469) gh-105936: Properly update closure cells for `__setattr__` and `__delattr__` in frozen dataclasses with slots (GH-144021) (cherry picked from commit 8a398bf) Co-authored-by: Prometheus3375 <prometheus3375@gmail.com> Co-authored-by: Sviataslau <35541026+Prometheus3375@users.noreply.github.com>

…_` and `__delattr__` in frozen dataclasses with slots (pythonGH-144021) pythongh-105936: Properly update closure cells for `__setattr__` and `__delattr__` in frozen dataclasses with slots (pythonGH-144021) (cherry picked from commit 8a398bf) The cherry-pick required additional changes beyond the original commit because 3.13 lacks the `__class__` closure cell fixup machinery that was added in 3.14 by pythonGH-124455 (pythongh-90562). Specifically: - Backported `_update_func_cell_for__class__()` helper function and the closure fixup loop in `_add_slots()` from pythonGH-124455. Without these, renaming the closure variable from `cls` to `__class__` has no effect because nothing updates the cell when the class is recreated with slots. - Changed `_add_slots()` to use `newcls` instead of reusing `cls` for the recreated class, so both old and new class references are available for the fixup loop. - Replaced `assertNotHasAttr` with `assertFalse(hasattr(...))` in tests (assertNotHasAttr was added in 3.14). - Dropped `test_original_class_is_gced` additions (that test does not exist on 3.13; it was added by pythonGH-137047 for pythongh-135228 which was not backported to 3.13). Co-authored-by: Prometheus3375 <prometheus3375@gmail.com> Co-authored-by: Sviataslau <35541026+Prometheus3375@users.noreply.github.com> Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bedevere-app · 2026-04-12T23:16:29Z

GH-148476 is a backport of this pull request to the 3.13 branch.

…__` and `__delattr__` in frozen dataclasses with slots (pythonGH-144021)" This reverts commit 8a398bf.

…`__delattr__` in frozen dataclasses with slots (pythonGH-144021)

…_delattr__ in frozen dataclasses with slots (GH-144021) (GH-148476) gh-105936: Properly update closure cells for `__setattr__` and `__delattr__` in frozen dataclasses with slots (GH-144021) (cherry picked from commit 8a398bf) The cherry-pick required additional changes beyond the original commit because 3.13 lacks the `__class__` closure cell fixup machinery that was added in 3.14 by GH-124455 (gh-90562). Specifically: - Backported `_update_func_cell_for__class__()` helper function and the closure fixup loop in `_add_slots()` from GH-124455. Without these, renaming the closure variable from `cls` to `__class__` has no effect because nothing updates the cell when the class is recreated with slots. - Changed `_add_slots()` to use `newcls` instead of reusing `cls` for the recreated class, so both old and new class references are available for the fixup loop. - Replaced `assertNotHasAttr` with `assertFalse(hasattr(...))` in tests (assertNotHasAttr was added in 3.14). - Dropped `test_original_class_is_gced` additions (that test does not exist on 3.13; it was added by GH-137047 for gh-135228 which was not backported to 3.13). gh-148947: dataclasses: fix error on empty __class__ cell (GH-148948) Also add a test demonstrating the need for the existing "is oldcls" check. (cherry picked from commit 6d7bbee) --------- Co-authored-by: Prometheus3375 <prometheus3375@gmail.com> Co-authored-by: Sviataslau <35541026+Prometheus3375@users.noreply.github.com> Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com> Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>

Prometheus3375 requested a review from ericvsmith as a code owner January 19, 2026 05:45

bedevere-appBot mentioned this pull request Jan 19, 2026
3.14 regression: slot dataclasses classes leak original class #135228
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bedevere-appBot added the awaiting review label Jan 19, 2026

Prometheus3375 added 2 commits January 19, 2026 05:59

Use __class__ in setattr and delattr
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Rename _frozen_get_del_attr
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Prometheus3375 force-pushed the fix-dataclass-frozen-slots-leak branch from fdc81bb to a6de29bCompare January 19, 2026 06:19

Add tests
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Prometheus3375and others added 6 commits January 19, 2026 20:50

Rename test
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📜🤖 Added by blurb_it.
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Reformat news entry
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Move tests to other case
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Add another test
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Prometheus3375 changed the title ~~gh-135228: Properly update closure cells for __setattr__ and __detattr__ in frozen dataclasses with slots~~gh-105936: Properly update closure cells for __setattr__ and __detattr__ in frozen dataclasses with slotsJan 22, 2026

bedevere-appBot mentioned this pull request Jan 22, 2026
dataclasses: frozen dataclasses with slots' __setattr__ and __delattr__ does not work. #105936
Closed

Prometheus3375 added 2 commits January 22, 2026 04:13

Update news entry name
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Merge some tests with existing ones
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JelleZijlstra reviewed Jan 22, 2026
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Comment threadLib/test/test_dataclasses/__init__.py Outdated
Comment threadMisc/NEWS.d/next/Library/2026-01-19-21-23-18.gh-issue-105936.dGrzjM.rst Outdated

Prometheus3375 added 3 commits January 22, 2026 16:57

Reword news entry
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Remove redundant tests
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Improve Testfrozen.test_frozen
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Prometheus3375 requested a review from JelleZijlstra January 22, 2026 17:57

JelleZijlstra approved these changes Jan 24, 2026
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bedevere-appBot added awaiting merge and removed awaiting review labels Jan 24, 2026

vstinner approved these changes Mar 10, 2026
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gpshead changed the title ~~gh-105936: Properly update closure cells for __setattr__ and __detattr__ in frozen dataclasses with slots~~gh-105936: Properly update closure cells for __setattr__ and __delattr__ in frozen dataclasses with slotsApr 12, 2026

gpshead added needs backport to 3.13 bugs and security fixes needs backport to 3.14 bugs and security fixes labels Apr 12, 2026

gpshead approved these changes Apr 12, 2026
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gpshead self-assigned this Apr 12, 2026

gpshead merged commit 8a398bf into python:mainApr 12, 2026
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bedevere-appBot removed the awaiting merge label Apr 12, 2026

bedevere-appBot removed the needs backport to 3.14 bugs and security fixes label Apr 12, 2026

gpshead mentioned this pull request Apr 12, 2026
[3.13] gh-105936: Properly update closure cells for __setattr__ and __delattr__ in frozen dataclasses with slots (GH-144021) #148476
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bedevere-appBot removed the needs backport to 3.13 bugs and security fixes label Apr 12, 2026

gpshead mentioned this pull request Apr 13, 2026
gh-105936: Fix frozen dataclasses with slots' __setattr__ and __delattr_#105937
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Prometheus3375 deleted the fix-dataclass-frozen-slots-leak branch April 13, 2026 10:29

ljfp pushed a commit to ljfp/cpython that referenced this pull request Apr 25, 2026
pythongh-105936: Properly update closure cells for__setattr__and …
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…`__delattr__` in frozen dataclasses with slots (pythonGH-144021)

sobolevn mentioned this pull request May 19, 2026
gh-150069: Test frozen dataclass and properties interaction #150070
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