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Narrow individual items when matching a tuple to a sequence pattern #16905

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loic-simon
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Fixes #12364

When matching a tuple to a sequence pattern, this change narrows the type of tuple items inside the matched case:

def test(a: bool, b: bool) -> None:
    match a, b:
        case True, True:
            reveal_type(a)  # before: "builtins.bool", after: "Literal[True]"

This also works with nested tuples, recursively:

def test(a: bool, b: bool, c: bool) -> None:
    match a, (b, c):
        case _, [True, False]:
            reveal_type(c)  # before: "builtins.bool", after: "Literal[False]"

This only partially fixes issue #12364; see my comment there for more context.


This is my first contribution to mypy, so I may miss some context or conventions; I'm eager for any feedback!

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typ_ = get_proper_type(typ)
if isinstance(expr, TupleExpr) and isinstance(typ_, TupleType):
# When matching a tuple expression with a sequence pattern, narrow individual tuple items
assert len(expr.items) == len(typ_.items)
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Can you add some test cases where the length doesn't match, to make sure we don't fail this assertion? Especially in nested tuples.

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Done! I did not check anything in non-matching cases, just that the assertion didn't fail, eg.

match m, n, o:
    case [3, "foo"]:
        pass
    case [3, "foo", True, True]:
        pass

Interestingly, these paths are not considered unreachable by mypy. They probably should, but that's unrelated to this issue!

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github-actions bot commented Apr 2, 2024

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

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mypy unable to narrow type of tuple elements in case clause in pattern matching
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