Improve match inline list subject inference #19744
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cdce8p
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Aug 27, 2025
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| match [a, b]: | ||
| case [1, "Hello"]: | ||
| reveal_type(a) # N: Revealed type is "Literal[1]" | ||
| reveal_type(b) # N: Revealed type is "Literal['Hello']" |
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Without the change here, this would be inferred as int and str respectively. I.e. just the types from a and b without any narrowing applied.
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According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅ |
ilevkivskyi
reviewed
Aug 27, 2025
| def visit_List(self, n: ast3.List) -> ListExpr | TupleExpr: | ||
| expr_list: list[Expression] = [self.visit(e) for e in n.elts] | ||
| if isinstance(n.ctx, ast3.Store): | ||
| if isinstance(n.ctx, ast3.Store) or self.match_stmt_subject: |
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Wouldn't this cause problems in situations like this:
def foo(list_only: list[int]) -> None: ...
match foo([1, 2]): # type error because foo() got a tuple?
case whatever:
...
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Good catch, missed that one. Guess it's one reason to recommend inline tuples over lists for match subjects then. Going to close this one.
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The sequence pattern checker only knows how to deal with
TupleType. To match an inline sequence subject, users can use either a tuple or a list variant, both are basically equal. Adjust the node parsing to coerce the inline list in the match subject to a tuple type.The might be avoidable if the ast context for the sequence subject ever changes from
LoadtoStorethough I'm not sure if that's the right call even if it makes sense here. In any case this would only apply to future versions and we'd still need the fallback for quite some time.