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Better handling of generics when narrowing - #20863
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hauntsaninja merged 2 commits intoFeb 26, 2026
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Notably we preserve behaviour on the testNarrowingCollections test I added
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hauntsaninja
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Feb 22, 2026
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mypy/checker.py
| if from_equality: | ||
| # We erase generic args because values with different generic types can compare equal | ||
| # For instance, cast(list[str], []) and cast(list[int], []) | ||
| proposed_type = shallow_erase_type_for_equality(proposed_type) |
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this is the one line difference (the rest is just refactoring for clarity)
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Diff from mypy_primer, showing the effect of this PR on open source code: graphql-core (https://github.com/graphql-python/graphql-core)
- tests/execution/test_resolve.py:322: error: Value of type "list[SourceLocation] | None" is not indexable [index]
rotki (https://github.com/rotki/rotki)
+ rotkehlchen/tests/integration/test_premium.py:820: error: Statement is unreachable [unreachable]
+ rotkehlchen/tests/integration/test_premium.py:834: error: Statement is unreachable [unreachable]
xarray (https://github.com/pydata/xarray)
+ xarray/tests/test_dataarray.py:291: error: Unused "type: ignore" comment [unused-ignore]
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Will merge this soon! |
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JukkaL
approved these changes
Feb 26, 2026
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test-data/unit/check-narrowing.test
| S = Callable[[B], "T"] | ||
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| def f(x: S, y: T): | ||
| if x == y: # E: Unsupported left operand type for == ("Callable[[B], T]") |
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Is this related to a recursive callable type?
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No, it's related to fixtures
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hauntsaninja
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Apr 2, 2026
Fixes python#21143 (on the same lines as python#20704) I also update a comment that is a little out of date since python#20863 and code in checkexpr.py that is out of date for modern typeshed
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Notably we preserve behaviour on the testNarrowingCollections test I added in a previous PR
Closes #20673