[match-case]: `TypedDict` interaction with mapping pattern

It seems match-cases mapping pattern is predestined to work well with TypedDict, but currently the spec is silent regarding TypedDict subtyping with respect to match case: https://typing.python.org/en/latest/spec/typeddict.html#subtyping-with-mapping

At the time of writing, not a single type checker (tested with mypy=1.19.1, pyright=1.1.408, ty=0.0.17, pyrefly=0.53.0 and zuban=0.6.0) can solve the following tests[^1], suggesting that some standardization / typing spec guidance may be useful.

from typing import TypedDict, assert_type, assert_never

class D(TypedDict):
    name: str
    value: int

def test_match_key(x: D | int) -> None:
    match x:
        case {"name": _}:
            # runtime equivalent to (isinstance(x, Mapping) and "name" in x)
            assert_type(x, D)
        case _:
            assert_type(x, int)

def test_match_key_and_value(x: D | int) -> None:
    match x:
        case {"name": str()}:
            # runtime equivalent to (isinstance(x, Mapping)
            # and "name" in x and isinstance(x["name"], str))
            assert_type(x, D)
        case _:
            assert_type(x, int)

def test_non_match(x: D | int) -> None:
    match x:
        case {"value": str()}:
            # runtime equivalent to (isinstance(x, Mapping)
            # and "value" in x and isinstance(x["value"], str))
            assert_never(x)
        case _:
            assert_type(x, D | int)

[^1]: technically, it is possible to make a shared subclass of int and collections.abc.Mapping, so the assert_type in the case {...} are debatable, though the inference in the case _ branches should not be affected.

Commencez par la section sur TypedDict subtyping-with-mapping de la typing specification et par les règles des mapping patterns de PEP 634. Exécutez les trois exemples avec les vérificateurs de types indiqués afin de comparer le comportement actuel. Le travail est considéré comme terminé lorsqu’un accord est trouvé sur le narrowing attendu et qu’une documentation standardisée est rédigée, avec les cas de validation correspondants.