python · JelleZijlstra · Jun 5, 2023 · Jun 2, 2023 · Jun 2, 2023 · Jun 2, 2023
@@ -2758,6 +2758,65 @@ def x(self): ...
         with self.assertRaisesRegex(TypeError, only_classes_allowed):
             issubclass(1, BadPG)
 
+    def test_issubclass_and_isinstance_on_Protocol_itself(self):
+        class C:
+            def x(self): pass
+
+        self.assertNotIsSubclass(object, Protocol)
+        self.assertNotIsInstance(object(), Protocol)
+
+        self.assertNotIsSubclass(str, Protocol)
+        self.assertNotIsInstance('foo', Protocol)
+
+        self.assertNotIsSubclass(C, Protocol)
+        self.assertNotIsInstance(C(), Protocol)
+
+        only_classes_allowed = r"issubclass\(\) arg 1 must be a class"
+
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass(1, Protocol)
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass('foo', Protocol)
+        with self.assertRaisesRegex(TypeError, only_classes_allowed):
+            issubclass(C(), Protocol)
+
+        T = TypeVar('T')
+
+        @runtime_checkable
+        class EmptyProtocol(Protocol): pass
+
+        @runtime_checkable
+        class SupportsStartsWith(Protocol):
+            def startswith(self, x: str) -> bool: ...
+
+        @runtime_checkable
+        class SupportsX(Protocol[T]):
+            def x(self): ...
+
+        for proto in EmptyProtocol, SupportsStartsWith, SupportsX:
+            with self.subTest(proto=proto.__name__):
+                self.assertIsSubclass(proto, Protocol)
+
+        # gh-105237 / PR #105239:
+        # check that the presence of Protocol subclasses
+        # where `issubclass(X, <subclass>)` evaluates to True
+        # doesn't influence the result of `issubclass(X, Protocol)`
+
+        self.assertIsSubclass(object, EmptyProtocol)
+        self.assertIsInstance(object(), EmptyProtocol)
+        self.assertNotIsSubclass(object, Protocol)
+        self.assertNotIsInstance(object(), Protocol)
+
+        self.assertIsSubclass(str, SupportsStartsWith)
+        self.assertIsInstance('foo', SupportsStartsWith)
+        self.assertNotIsSubclass(str, Protocol)
+        self.assertNotIsInstance('foo', Protocol)
+
+        self.assertIsSubclass(C, SupportsX)
+        self.assertIsInstance(C(), SupportsX)
+        self.assertNotIsSubclass(C, Protocol)
+        self.assertNotIsInstance(C(), Protocol)
+
     def test_protocols_issubclass_non_callable(self):
         class C:
             x = 1

@@ -1782,6 +1782,8 @@ def __init__(cls, *args, **kwargs):
             )
 
     def __subclasscheck__(cls, other):
+        if cls is Protocol:
+            return type.__subclasscheck__(cls, other)
         if not isinstance(other, type):
             # Same error message as for issubclass(1, int).
             raise TypeError('issubclass() arg 1 must be a class')
@@ -1862,7 +1864,7 @@ def meth(self) -> T:
                 ...
     """
     __slots__ = ()
-    _is_protocol = True
+    _is_protocol = False
     _is_runtime_protocol = False
 
     def __init_subclass__(cls, *args, **kwargs):
@@ -1904,7 +1906,7 @@ def _proto_hook(other):
 
         # ... otherwise check consistency of bases, and prohibit instantiation.
         for base in cls.__bases__:
-            if not (base in (object, Generic) or
+            if not (base in {object, Generic, Protocol} or
                     base.__module__ in _PROTO_ALLOWLIST and
                     base.__name__ in _PROTO_ALLOWLIST[base.__module__] or
                     issubclass(base, Generic) and getattr(base, '_is_protocol', False)):

diff --git a/Misc/NEWS.d/next/Library/2023-06-02-14-57-11.gh-issue-105239.SAmuuj.rst b/Misc/NEWS.d/next/Library/2023-06-02-14-57-11.gh-issue-105239.SAmuuj.rst
@@ -0,0 +1,2 @@
+Fix longstanding bug where ``issubclass(object, typing.Protocol)`` would
+evaluate to ``True`` in some edge cases. Patch by Alex Waygood.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix longstanding bug where ``issubclass(object, typing.Protocol)`` would
		evaluate to ``True`` in some edge cases. Patch by Alex Waygood.