typing: add _ProtocolMeta (#6394)

Co-authored-by: hauntsaninja <>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: hauntsaninja

stdlib/typing.pyi

@@ -122,6 +122,14 @@ if sys.version_info >= (3, 9):
 # Predefined type variables.
 AnyStr = TypeVar("AnyStr", str, bytes)
 
+if sys.version_info >= (3, 8):
+    # This class did actually exist in 3.7, but had a different base.
+    # We'll just pretend it didn't exist though: the main external use case for _ProtocolMeta is
+    # to inherit from for your own custom protocol metaclasses. If you're using 3.7, at runtime
+    # you'd use typing_extensions.Protocol, which would be unrelated to typing._ProtocolMeta and
+    # so you'd run into metaclass conflicts at runtime if you used typing._ProtocolMeta.
+    class _ProtocolMeta(ABCMeta): ...
+
 # Abstract base classes.
 
 def runtime_checkable(cls: _TC) -> _TC: ...

stdlib/typing_extensions.pyi

@@ -53,6 +53,11 @@ Literal: _SpecialForm = ...
 
 def IntVar(name: str) -> Any: ...  # returns a new TypeVar
 
+if sys.version_info < (3, 8):
+    # Technically in 3.6 this inherited from GenericMeta. But let's not reflect that, since
+    # type checkers tend to assume that Protocols all have the ABCMeta metaclass.
+    class _ProtocolMeta(abc.ABCMeta): ...
+
 # Internal mypy fallback type for all typed dicts (does not exist at runtime)
 class _TypedDict(Mapping[str, object], metaclass=abc.ABCMeta):
     __required_keys__: frozenset[str]