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GeezerVison is showing github.com — gh-155752: Do not crash when GenericAlias parameters change during substitution by dariushoule · Pull Request #155761 · python/cpython

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@dariushoule

@dariushoule dariushoule commented Aug 13, 2026 •
edited by bedevere-app Bot
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Fixes #155752

_Py_subs_parameters() assumed that every argument with a __typing_subst__ attribute was present in the cached __parameters__ tuple. However, __typing_subst__ can be added to an argument after __parameters__ has been cached, causing the lookup to return -1, which is subsequently looked up.

Check the lookup result before indexing the substitution arguments and raise TypeError when the changed argument is not in __parameters__.

  • Issue: GenericAlias crashes when an argument gains __typing_subst__ after parameters are cached #155752

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@dariushoule
pythongh-155752: Do not crash when GenericAlias parameters change dur…
7a1932e 
…ing substitution

An alias argument can gain __typing_subst__ after __parameters__ has been cached, including during a preparation or substitution callback. Check that the argument is present before indexing the substitution arguments.
@dariushoule
dariushoule requested review from AlexWaygood and JelleZijlstra as code owners August 13, 2026 21:00
@python-cla-bot

python-cla-bot Bot commented Aug 13, 2026 •
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@bedevere-app bedevere-app Bot mentioned this pull request Aug 13, 2026
GenericAlias crashes when an argument gains __typing_subst__ after parameters are cached #155752
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@bedevere-app bedevere-app Bot added the awaiting review label Aug 13, 2026
@JelleZijlstra JelleZijlstra added needs backport to 3.10 needs backport to 3.11 only security fixes needs backport to 3.12 only security fixes needs backport to 3.13 only security fixes needs backport to 3.14 bugs and security fixes needs backport to 3.15 bugs and security fixes labels Aug 13, 2026
JelleZijlstra
JelleZijlstra approved these changes Aug 13, 2026
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@bedevere-app bedevere-app Bot added awaiting merge and removed awaiting review labels Aug 13, 2026
picnixz
picnixz reviewed Aug 13, 2026
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Comment thread Lib/test/test_typing.py Resolved

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picnixz
picnixz approved these changes Aug 13, 2026
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My comment is just a nitpick btw

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@dariushoule
Move issue comment
3c9c477
@JelleZijlstra
JelleZijlstra merged commit c0006fa into python:main Aug 14, 2026
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@miss-islington-app

miss-islington-app Bot commented Aug 14, 2026

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Thanks @dariushoule for the PR, and @JelleZijlstra for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10, 3.11, 3.12, 3.13, 3.14, 3.15.
🐍🍒⛏🤖
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@bedevere-app bedevere-app Bot removed the awaiting merge label Aug 14, 2026
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bedevere-app Bot commented Aug 14, 2026

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GH-155770 is a backport of this pull request to the 3.15 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.15 bugs and security fixes label Aug 14, 2026
@bedevere-app

bedevere-app Bot commented Aug 14, 2026

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GH-155771 is a backport of this pull request to the 3.14 branch.
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@bedevere-app bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Aug 14, 2026
@miss-islington-app

miss-islington-app Bot commented Aug 14, 2026

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Sorry, @dariushoule and @JelleZijlstra, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c0006faf4f48f01126a8a0752da8f159142e410d 3.12
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@miss-islington-app miss-islington-app Bot assigned JelleZijlstra Aug 14, 2026
@bedevere-app

bedevere-app Bot commented Aug 14, 2026

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GH-155772 is a backport of this pull request to the 3.13 branch.
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@miss-islington-app

miss-islington-app Bot commented Aug 14, 2026

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Sorry, @dariushoule and @JelleZijlstra, I could not cleanly backport this to 3.11 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c0006faf4f48f01126a8a0752da8f159142e410d 3.11
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@bedevere-app bedevere-app Bot removed the needs backport to 3.13 only security fixes label Aug 14, 2026
@miss-islington-app

miss-islington-app Bot commented Aug 14, 2026

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Sorry, @dariushoule and @JelleZijlstra, I could not cleanly backport this to 3.10 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker c0006faf4f48f01126a8a0752da8f159142e410d 3.10
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@BHUVANSH855

BHUVANSH855 commented Aug 14, 2026

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@JelleZijlstra @picnixz
can I work on the backport PRs ?
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hugovk pushed a commit that referenced this pull request Aug 14, 2026
@miss-islington @dariushoule
[3.15] gh-155752: Do not crash when GenericAlias parameters change du…
b8a9987 
…ring substitution (GH-155761) (#155770)

gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761)

An alias argument can gain __typing_subst__ after __parameters__ has been cached, including during a preparation or substitution callback. Check that the argument is present before indexing the substitution arguments.
(cherry picked from commit c0006fa)

Co-authored-by: Darius Houle <[email protected]>
mbeijen pushed a commit to mbeijen/cpython that referenced this pull request Aug 14, 2026
@dariushoule @mbeijen
pythongh-155752: Do not crash when GenericAlias parameters change dur…
1772486 
…ing substitution (python#155761)

An alias argument can gain __typing_subst__ after __parameters__ has been cached, including during a preparation or substitution callback. Check that the argument is present before indexing the substitution arguments.
@StanFromIreland StanFromIreland removed needs backport to 3.10 needs backport to 3.11 only security fixes needs backport to 3.12 only security fixes labels Aug 16, 2026
@StanFromIreland

StanFromIreland commented Aug 16, 2026

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I'm assuming they were added by accident so I've removed them, this seems like a regular bug to me.
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@AlexWaygood

AlexWaygood commented Aug 16, 2026

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I think we usually backport crashes as far as we can since they can be security vulnerabilities; segfaults can corrupt memory, etc
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@StanFromIreland

StanFromIreland commented Aug 16, 2026

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I think we usually backport crashes as far as we can since they can be security vulnerabilities; segfaults can corrupt memory, etc

Under our security policy I don't see how this qualifies, I don't think a bug is a vulnerability simply because it causes a segfault. Vulnerabilities can be exploited, whereas this does not seem like something exploitable in code that may exist in the real world.
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terryjreedy pushed a commit that referenced this pull request Sep 8, 2026
@miss-islington @dariushoule
[3.14] gh-155752: Do not crash when GenericAlias parameters change du…
96964c8 
…ring substitution (GH-155761) (#155771)

gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761)

An alias argument can gain __typing_subst__ after __parameters__ has been cached, including during a preparation or substitution callback. Check that the argument is present before indexing the substitution arguments.
(cherry picked from commit c0006fa)

Co-authored-by: Darius Houle <[email protected]>
terryjreedy pushed a commit that referenced this pull request Sep 8, 2026
@miss-islington @dariushoule
[3.13] gh-155752: Do not crash when GenericAlias parameters change du…
649e421 
…ring substitution (GH-155761) (#155772)

gh-155752: Do not crash when GenericAlias parameters change during substitution (GH-155761)

An alias argument can gain __typing_subst__ after __parameters__ has been cached, including during a preparation or substitution callback. Check that the argument is present before indexing the substitution arguments.
(cherry picked from commit c0006fa)

Co-authored-by: Darius Houle <[email protected]>
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@JelleZijlstra JelleZijlstra JelleZijlstra approved these changes
@picnixz picnixz picnixz approved these changes
@AlexWaygood AlexWaygood Awaiting requested review from AlexWaygood AlexWaygood is a code owner

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@dariushoule @BHUVANSH855 @StanFromIreland @AlexWaygood @JelleZijlstra @picnixz
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