`get_original_bases` does not return what `cls.__orig_bases__` returns

Bug report

Bug description:

The types documentation states for the types.get_original_bases function:

For classes that have an __orig_bases__ attribute, this function returns the value of cls.__orig_bases__. For classes without the __orig_bases__ attribute, cls.__bases__ is returned.

I need the functionality of cls.__orig_bases__, but need to use types.get_original_bases to make the type checker happy.

A short MRE:

from types import get_original_bases
from typing import Generic, TypeVar

T = TypeVar("T")


class One(Generic[T]):
    pass


class Two(One[int]):
    pass


class Three(Two):
    pass


assert get_original_bases(One) == One.__orig_bases__
assert get_original_bases(Two) == Two.__orig_bases__
assert get_original_bases(Three) == Three.__orig_bases__

# Traceback (most recent call last):
#   File "c:\<cut>\test.py", line 34, in <module>
#     assert get_original_bases(Three) == Three.__orig_bases__
#            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# AssertionError

This is a easy solve, I would be happy to add a PR. Change here:

    try:
-       return cls.__dict__.get("__orig_bases__", cls.__bases__)
+       return getattr(cls, "__orig_bases__", cls.__bases__)
    except AttributeError:
        raise TypeError(
            f"Expected an instance of type, not {type(cls).__name__!r}"
        ) from None

To show this works:

from typing import Generic, TypeVar

T = TypeVar("T")


class One(Generic[T]):
    pass


class Two(One[int]):
    pass


class Three(Two):
    pass


def better_get_original_bases(cls):
    try:
        return getattr(cls, "__orig_bases__", cls.__bases__)
    except AttributeError:
        raise TypeError(
            f"Expected an instance of type, not {type(cls).__name__!r}"
        ) from None


assert better_get_original_bases(One) == One.__orig_bases__
assert better_get_original_bases(Two) == Two.__orig_bases__
assert better_get_original_bases(Three) == Three.__orig_bases__

# <No output>

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-156917

Lib/types.py の get_original_bases から始め、Three クラスの場合を含めて、issue の継承例を再現してください。関数が文書化された original-bases の動作に一致し、回帰テストのカバレッジで継承された属性のケースが検証されれば完了です。開始前にリンクされた PR gh-156917 を確認してください。