Strange interaction between typing.Protocol and unrelated isinstance() checks

[AlexWaygood]

On main, an isinstance() check against Sized works just the same as it does on Python 3.11:

>>> from typing import Sized
>>> isinstance(1, Sized)
False

However! If you first subclass Sized like this, TypeError is raised on that isinstance() check!

>>> from typing import Sized, Protocol
>>> class Foo(Sized, Protocol): pass
...
>>> isinstance(1, Sized)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1153, in __instancecheck__
    return self.__subclasscheck__(type(obj))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1428, in __subclasscheck__
    return issubclass(cls, self.__origin__)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1793, in __subclasscheck__
    return super().__subclasscheck__(other)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen abc>", line 123, in __subclasscheck__
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1876, in _proto_hook
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

This was originally reported by @vnmabus in python/typing_extensions#207.

Note that (because of the abc-module cache), this doesn't reproduce if you do an isinstance() check before subclassing typing.Sized:

>>> import typing
>>> isinstance(1, typing.Sized)
False
>>> class Foo(typing.Sized, typing.Protocol): pass
...
>>> isinstance(1, typing.Sized)
False

Linked PRs

• gh-105144: Runtime-checkable protocols: move all 'sanity checks' to _ProtocolMeta.__subclasscheck__ #105152
• gh-105144: abc: Suppress errors raised by unrelated other subclasses #105159
• [3.12] gh-105144: Runtime-checkable protocols: move all 'sanity checks' to _ProtocolMeta.__subclasscheck__ (GH-105152) #105160

[AlexWaygood]

This bisects to b27fe67

[AlexWaygood]

Also reproduces if you use collections.abc.Sized rather than typing.Sized:

>>> import collections.abc
>>> import typing
>>> class Foo(collections.abc.Sized, typing.Protocol): pass
...
>>> isinstance(1, collections.abc.Sized)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 119, in __instancecheck__
    return _abc_instancecheck(cls, instance)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1796, in __subclasscheck__
    return super().__subclasscheck__(other)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\abc.py", line 123, in __subclasscheck__
    return _abc_subclasscheck(cls, subclass)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\alexw\coding\cpython\Lib\typing.py", line 1879, in _proto_hook
    raise TypeError("Instance and class checks can only be used with"
TypeError: Instance and class checks can only be used with @runtime_checkable protocols

[JelleZijlstra]

With some debugging we found that this is due to code in _abc.c that iterates over all the __subclasses__ of the ABC:

cpython/Modules/_abc.c

Line 768 in c05c31d

subclasses = PyObject_CallMethod(self, "__subclasses__", NULL);

Alex's PR #105152 works around this problem by re-arranging the code in typing.py. In #105159 I propose a fix on the ABC side instead. We should consider merging that PR too to prevent similar problems with other ABC subclasses.

[AlexWaygood]

This issue is fixed, and Jelle's decided not to pursue #105159, so I'll close this