[issue46399] Addition of `mapping` attribute to dict views classes has inadvertently broken type-checkers

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

In other words,

a. If `.keys()` in all dict subclasses must return subclass of `dict_keys`: `dict.keys() -> dict_keys`.
b. If `.keys().mapping` must be accessible for all dict subclasses: Add `.mapping` to `KeysView`.
c. If `.keys().mapping` is optional for dict subclasses: typeshed can't add `.mapping` to anywhere, AFAIK.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue46399>
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