Due to stdlib/_collections_abc.pyi importing most directly from stdlib/typing.pyi, MappingView and its subclasses are missing the self._mapping attribute. This is an issue if inheriting from them for implementing custom views. This causes pyright and mypy (with --check-untyped-defs) to fail.

Minimal Example:

import collections.abc

class MyView(collections.abc.KeysView):
    def __iter__(self):
        for key in self._mapping:
            yield key + 1

mypy --check-untyped-defs test.py

test.py:5: error: "MyView" has no attribute "_mapping"

pyright test.py

test.py
  test.py:5:25 - error: Cannot access member "_mapping" for type "MyView"
    Member "_mapping" is unknown (reportGeneralTypeIssues)
1 error, 0 warnings, 0 informations

I'm open to trying to PR this myself, though I have little experience writing stub files.