fix: Narrowing with "tags" on unions (of TypedDicts or normal classes) doesn't work with the match statement #18791
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
…) doesn't work with the match statement Signed-off-by: Gene Parmesan Thomas <201852096+gopoto@users.noreply.github.com>
This comment has been minimized.
This comment has been minimized.
CHANGELOG.md
Outdated
|
|
||
| Contributed by Marc Mueller (PR [18641](https://github.com/python/mypy/pull/18641)) | ||
|
|
||
| ### Other Notable Fixes and Improvements |
There was a problem hiding this comment.
we typically will do changelog entries later, can you remove this?
There was a problem hiding this comment.
Removed the changelog entry for now; will add it later alongside other notes.
mypy/checker.py
Outdated
| 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. |
There was a problem hiding this comment.
thanks for docs, but i think this is fairly clear from the propagate_up_typemap_info docstring, so could you remove the comment?
There was a problem hiding this comment.
Dropped the extra comment here since the helper’s docstring is enough
hauntsaninja
approved these changes
Mar 14, 2025
Signed-off-by: gopoto <201852096+gopoto@users.noreply.github.com>
codex-maintainers
force-pushed
the
gopoto/16286
branch
from
6d03c52 to
0a6333a
Compare
|
According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
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