fix: Narrowing with "tags" on unions (of TypedDicts or normal classes) doesn't work with the match statement #18791
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…) doesn't work with the match statement Signed-off-by: Gene Parmesan Thomas <[email protected]>
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CHANGELOG.md
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| @@ -45,6 +45,14 @@ class A: | |||
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| Contributed by Marc Mueller (PR [18641](https://github.com/python/mypy/pull/18641)) | |||
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| ### Other Notable Fixes and Improvements | |||
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we typically will do changelog entries later, can you remove this?
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Removed the changelog entry for now; will add it later alongside other notes.
mypy/checker.py
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| @@ -5527,6 +5527,14 @@ def visit_match_stmt(self, s: MatchStmt) -> None: | |||
| pattern_map, else_map = conditional_types_to_typemaps( | |||
| named_subject, pattern_type.type, pattern_type.rest_type | |||
| ) | |||
| # Also refine the parent expression of the subject. | |||
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thanks for docs, but i think this is fairly clear from the propagate_up_typemap_info docstring, so could you remove the comment?
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Dropped the extra comment here since the helper’s docstring is enough
hauntsaninja
approved these changes
Mar 14, 2025
Signed-off-by: gopoto <[email protected]>
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
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Summary
Addressed issue #16286, which reported that
matchstatements failed to narrow unions when matching on discriminant values (e.g.tagfields inTypedDicts or classes). This led to mypy complaining about missing keys/attributes insidecaseblocks even though plainif ... ==branches worked.What changed?
mypy/checker.py:visit_match_stmt, after callingconditional_types_to_typemapsfor the subject, the resulting type map is passed throughpropagate_up_typemap_info(just like in theif/isinstancecode paths). This means if the subject is an attribute or index expression (d.tag/d["tag"]), we try to refine the parentdbased on the pattern.pattern_mapwhen it's defined, to avoidUnboundLocalErrorin unreachable branches.test-data/unit/check-python310.test:testMatchNarrowingUnionTypedDictViaIndexchecks thatmatch d["tag"]refines aTypedDictunion and allows access to the appropriate key in each branch.testMatchNarrowingUnionClassViaAttributechecks the same for a class union usingmatch d.tag.Why?
Previously, the binder only narrowed the type of the expression being matched (e.g.
d["tag"]), not the parent expression. Without propagating that information upwards, mypy didn't realize it could rule out unrelatedTypedDict/class variants in eachcase, leading to erroneoustypeddict-itemandunion-attrerrors. The fix mirrors the existing behaviour forifand similar checks.How was it tested?
pytest -k testMatchNarrowingUnionTypedDictViaIndexandpytest -k testMatchNarrowingUnionClassViaAttributeto confirm the new tests passed.pytest -k check-python310to ensure the entire pattern-matching suite still passes after the change.python runtests.py self).pre-commit run --all-filesto satisfy formatting and lint hooks.Checklist
CHANGELOG.md.Fixes #16286.
This PR was generated by an AI system in collaboration with maintainers: @hauntsaninja