Fix type narrowing for @final classes with identity checks #20627
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Fixes python#20590 When using 'is' operator or match/case patterns to check type objects of @Final classes, mypy now properly narrows the types since @Final classes cannot have subclasses at runtime. Changes: - typeops.py: Updated is_singleton_identity_type() to recognize CallableType and TypeType representing @Final class constructors as singleton types for identity-based narrowing - checker.py: Modified narrowing logic to: 1. Allow @Final class type objects in narrowable_operand check 2. Preserve else-branch narrowing for @Final class identity checks since no subclasses can exist This enables exhaustiveness checking with assert_never() when all @Final class cases are handled in if/elif or match/case branches.
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Author
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@p-sawicki , @JukkaL , @hauntsaninja , @ilevkivskyi can you guys please review this one ? |
Collaborator
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I'm in the middle of landing large refactoring to narrowing, so this will need to wait A correct change would be simpler (and also have tests) |
Contributor
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Diff from mypy_primer, showing the effect of this PR on open source code: pytest (https://github.com/pytest-dev/pytest)
+ src/_pytest/raises.py:1486: error: Unused "type: ignore" comment [unused-ignore]
trio (https://github.com/python-trio/trio)
+ src/trio/_core/_local.py:54: error: Unused "type: ignore" comment [unused-ignore]
+ src/trio/_core/_local.py:57: error: Unused "type: ignore" comment [unused-ignore]
isort (https://github.com/pycqa/isort)
+ isort/settings.py:893: error: Argument 2 to "isinstance" has incompatible type "Callable[[str], Any] | type[Any]"; expected "_ClassInfo" [arg-type]
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Fixes #20590
When using 'is' operator or match/case patterns to check type objects of @Final classes, mypy now properly narrows the types since @Final classes cannot have subclasses at runtime.
Changes:
This enables exhaustiveness checking with assert_never() when all @Final class cases are handled in if/elif or match/case branches.