gh-155752: Do not crash when GenericAlias parameters change during substitution - #155761
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gh-155752: Do not crash when GenericAlias parameters change during substitution#155761dariushoule wants to merge 1 commit into
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…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
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August 13, 2026 21:00
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picnixz
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Aug 13, 2026
| with self.assertRaises(TypeError): | ||
| a[int] | ||
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| # gh-155752: GenericAlias parameters are cached before substitution, so |
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Can you put the comment inside the test as it is usually the case (but not as a docstring please)
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My comment is just a nitpick btw
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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
TypeErrorwhen the changed argument is not in__parameters__.