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

[Alex Waygood]

Alex Waygood <Alex.Waygood at Gmail.com> added the comment:

As a fix, I propose that two changes be made to `dict_keys`, `dict_values` and `dict_items`:

* They should be officially exposed in the `types` module.
* `__class_getitem__` should be added to the classes.

These two changes would mean that users would be able to do the following:

```
from types import dict_keys
from typing import TypeVar

K = TypeVar("K")
V = TypeVar("V")

class DictSubclass(dict[K, V]):
    def keys(self) -> dict_keys[K, V]:
        return super().keys()
```

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