Fix type narrowing with not isinstance(x, cls) in classmethods#21312
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sanyamk23 wants to merge 3 commits intopython:masterfrom
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Fix type narrowing with not isinstance(x, cls) in classmethods#21312sanyamk23 wants to merge 3 commits intopython:masterfrom
sanyamk23 wants to merge 3 commits intopython:masterfrom
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Adjust get_type_range_of_type to set is_upper_bound=False for Self types, allowing precise narrowing in negative checks. Fixes python#21271.
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Diff from mypy_primer, showing the effect of this PR on open source code: comtypes (https://github.com/enthought/comtypes)
+ comtypes/_post_coinit/unknwn.py:347: error: Unused "type: ignore" comment [unused-ignore]
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Adjust get_type_range_of_type to set is_upper_bound=False for Self types, allowing precise narrowing in negative checks. Fixes #21271.
Description
This PR addresses an issue where
not isinstance(x, cls)inside a@classmethodfailed to correctly narrow the type ofxwhen it involvedSelf.In
mypy/checker.py,get_type_range_of_typewas conservatively settingis_upper_bound=Truefor allTypeTypeobjects (likecls: Type[Self]). This prevented the narrowing logic from removingSelffrom a union in theelsebranch, as it assumed the check wasn't precise.This change sets
is_upper_bound=Falsespecifically forSelftypes (and also handlesNoneTypeand final classes similarly). This allowsmypyto correctly narrowxto the remaining types in the union when theisinstance(x, cls)check fails.Checklist