`typing`: one `None`-valued member makes `isinstance()` on a `runtime_checkable` Protocol O(N) per call

A class satisfying a @runtime_checkable Protocol with any class attribute set to None still passes isinstance(), but is orders of magnitude slower, scaling linearly with member count. Best of 5 runs of 20,000 calls, at 5/20/50/110 members: 25x/84x/316x/735x slower.

import timeit
from typing import Protocol, runtime_checkable

@runtime_checkable
class P(Protocol):
    @property
    def a(self) -> bool: ...
    @property
    def b(self) -> bool: ...

class Good:
    a = b = True

class Bad:
    a, b = True, None

for x in (Good(), Bad()):
    print(isinstance(x, P), timeit.timeit(lambda: isinstance(x, P), number=100_000))

On 3.13.13: True 0.0121 then True 0.1207. Only the timing differs.

Linked PRs

• gh-156451

Start with the minimal Protocol/runtime_checkable reproduction in the issue and compare the timing for Good and Bad across member counts. Review the linked PR gh-156451 and the typing implementation it changes; done means the None-valued case no longer causes linear-time isinstance() checks while preserving the reported True results.