Fix longstanding bug and add many tests

JelleZijlstra · Jun 5, 2023 · Jun 2, 2023 · Jun 2, 2023 · Jun 2, 2023 · Jun 2, 2023
commit 776bf2bdbed6c01cc48bdb34446a41cac78bacae
@@ -2758,15 +2758,55 @@ def x(self): ...
         with self.assertRaisesRegex(TypeError, only_classes_allowed):
             issubclass(1, BadPG)
 
-        for proto in P, PG, BadP, 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)
+
+        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)
 
-        # TODO: the behaviour of some of these is questionable!
-        # For now, just check that these don't raise any Exceptions
-        issubclass(object, Protocol)
-        issubclass(str, Protocol)
-        issubclass(C, 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:

@@ -1785,6 +1785,8 @@ def __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')
+        if cls is Protocol:
+            return type.__subclasscheck__(cls, other)
         if (
             getattr(cls, '_is_protocol', False)
             and not _allow_reckless_class_checks()

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.