`typing.Protocol` implementation means that `isinstance([], collections.abc.Mapping)` can sometimes evaluate to `True`

[AlexWaygood]

(In very specific circumstances)

Bug report

Due to some changes that have been made to the implementation of typing.Protocol in Python 3.12, whether or not isinstance([], collections.abc.Mapping) evaluates to True or False now depends on precisely when and if garbage collection happens.

Minimal repro:

>>> import gc, collections.abc, typing
>>> gc.disable()
>>> try:
...     class Foo(collections.abc.Mapping, typing.Protocol): pass
... except TypeError:
...     pass
...
>>> isinstance([], collections.abc.Mapping)
True

Why does this happen?! It's because Foo is an illegal class, so TypeError is raised during class initialisation here:

cpython/Lib/typing.py

Lines 1905 to 1912 in ce558e6

	# ... otherwise check consistency of bases, and prohibit instantiation.
	for base in cls.__bases__:
	if not (base in (object, Generic) or
	base.__module__ in _PROTO_ALLOWLIST and
	base.__name__ in _PROTO_ALLOWLIST[base.__module__] or
	issubclass(base, Generic) and getattr(base, '_is_protocol', False)):
	raise TypeError('Protocols can only inherit from other'
	' protocols, got %r' % base)

TypeError being raised here means that Foo never has __protocol_attrs__ set on it. But, the error is raised after class construction, meaning that if garbage collection happens at the wrong time, a reference to Foo is still retained in collections.abc.Mapping.__subclasses__(). This then creates problems in the isinstance() check, because of some behaviour in the abc module where it iterates through all of the subclasses of collections.abc.Mapping in order to determine whether an object is an instance of collections.abc.Mapping.

>>> import gc, collections.abc, typing
>>> gc.disable()
>>> try:
...     class Foo(collections.abc.Mapping, typing.Protocol): pass
... except TypeError:
...     pass
...
>>> x = collections.abc.Mapping.__subclasses__()
>>> x
[<class 'collections.abc.MutableMapping'>, <class '__main__.Foo'>]
>>> x[-1].__protocol_attrs__
set()

The fix is to raise TypeError before the class has actually been created, rather than during class initialisation.

Why does this matter?

This might seem like an absurdly specific bug report, but it would be good to see it fixed. It was causing bizarre test failures in the typing_extensions backport. The issue was that we did this in one test method:

    def test_protocols_bad_subscripts(self):
        T = TypeVar('T')
        S = TypeVar('S')
        with self.assertRaises(TypeError):
            class P(typing.Mapping[T, S], Protocol[T]): pass

...And that final P class wasn't being cleaned up by the garbage collector immediately after that test having been run. That then caused an unrelated test elsewhere to fail due to the fact that isinstance([], collections.abc.Mapping) was evaluating to True, and it was very hard to figure out why.

Linked PRs

• gh-105280: Ensure isinstance([], collections.abc.Mapping) always evaluates to False #105281
• [3.12] gh-105280: Ensure isinstance([], collections.abc.Mapping) always evaluates to False (GH-105281) #105318

[AlexWaygood]

Let us never again talk about the brief period in time when a list could sometimes be a mapping