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GeezerVison is showing github.com — gh-148442: eliminate race condition in list rich comparison by KowalskiThomas · Pull Request #148531 · python/cpython

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@KowalskiThomas

@KowalskiThomas KowalskiThomas commented Apr 13, 2026 •
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What is this PR?

This PR is a fix for #148442, where the list's rich comparison function could return an incorrect result due a race condition in non-trivial __eq__ implementations of list elements.

It also updates test_equal_operator_modifying_operand which would previously assert two different lists were equal; it now properly asserts they are different.

  • Issue: list's rich comparison has a race condition for non-trivial __eq__ and may return an incorrect result #148442

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@bedevere-app bedevere-app Bot mentioned this pull request Apr 13, 2026
list's rich comparison has a race condition for non-trivial __eq__ and may return an incorrect result #148442
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KowalskiThomas force-pushed the kowalski/fix-eliminate-race-condition-in-list-rich-comparison branch from cc583a3 to 3ddd03d Compare April 30, 2026 13:24
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KowalskiThomas marked this pull request as ready for review April 30, 2026 13:25
@bedevere-app bedevere-app Bot added the awaiting review label Apr 30, 2026
serhiy-storchaka
serhiy-storchaka reviewed May 30, 2026
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Could you please add a test specific to this issue, which shows why the new behavior can be better than the old one?

For example, let list1 = [x, 1] and list2 = [0, 0], where x > 0. During comparison list1 > list2, list1[0] is replaced with 0. Currently, the result will be False, even if both old and new values of list1 are larger than list2. Add also a test with replacing list2[0].

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KowalskiThomas added 5 commits May 30, 2026 17:34
@KowalskiThomas
fix: eliminate race condition in list rich comparison
be6338e
@KowalskiThomas
refactor: hoist the comparison logic
8d42bf6
@KowalskiThomas
test: update test_equal_operator_modifying_operand
21706fc
@KowalskiThomas
news: add entry
51ddb47
@KowalskiThomas
test: add regression test
b4ebf34
@KowalskiThomas
KowalskiThomas force-pushed the kowalski/fix-eliminate-race-condition-in-list-rich-comparison branch from 3ddd03d to b4ebf34 Compare May 30, 2026 15:45
@KowalskiThomas

KowalskiThomas commented May 30, 2026

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@serhiy-storchaka I added two tests (and rebased on main since the branch was pretty out of date).
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@serhiy-storchaka

serhiy-storchaka commented May 31, 2026

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I afraid this is too heavyweight. No need to use threads if we can trigger the issue with a single thread.
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@KowalskiThomas
test: do not use threads
9668111
@KowalskiThomas
KowalskiThomas force-pushed the kowalski/fix-eliminate-race-condition-in-list-rich-comparison branch from 41e55ba to 9668111 Compare May 31, 2026 14:18
@KowalskiThomas

KowalskiThomas commented May 31, 2026

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Ah yes, that's a very good point...

I just removed those tests and added two new ones that are single-threaded and do about the same thing with some... peculiar comparison operators.

Also, currently the release note says...

Fix a race condition in ``list.__lt__``, ``list.__le__``, ``list.__gt__``,
and ``list.__ge__``. Previously, under concurrent list mutation, the items
being compared could be replaced with other objects before the final comparison,
causing the final comparison result to be incorrect.

... should I also update it to frame it better and make it clear that it's not necessarily concurrent mutation? Even though that would be the most "usual" case I believe.
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@KowalskiThomas
KowalskiThomas requested a review from serhiy-storchaka May 31, 2026 14:21
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github-actions Bot commented Aug 30, 2026

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This PR is stale because it has been open for 90 days with no activity.
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@github-actions github-actions Bot added the stale Stale PR or inactive for long period of time. label Aug 30, 2026
serhiy-storchaka
serhiy-storchaka approved these changes Aug 30, 2026
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LGTM. 👍

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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting review labels Aug 30, 2026
@serhiy-storchaka
serhiy-storchaka merged commit ec8a420 into python:main Aug 30, 2026
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@bedevere-app bedevere-app Bot removed the awaiting merge label Aug 30, 2026
clin1234 pushed a commit to clin1234/cpython that referenced this pull request Sep 12, 2026
@KowalskiThomas @clin1234
pythongh-148442: Eliminate race condition in list rich comparison (py…
d2ef024 
…thonGH-148531)
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@serhiy-storchaka serhiy-storchaka serhiy-storchaka approved these changes

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@KowalskiThomas @serhiy-storchaka
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