Don't reimplement runtime_checkable on py38+ (python#194)

Author: AlexWaygood

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Unreleased
 
+- Due to changes in the implementation of `typing_extensions.Protocol`,
+  `typing.runtime_checkable` can now be used on `typing_extensions.Protocol`
+  (previously, users had to use `typing_extensions.runtime_checkable` if they
+  were using `typing_extensions.Protocol`).
 - Align the implementation of `TypedDict` with the implementation in the
   standard library on Python 3.9 and higher.
   `typing_extensions.TypedDict` is now a function instead of a class. The

src/test_typing_extensions.py

@@ -1783,6 +1783,22 @@ def __init__(self):
         self.assertIsInstance(Bar(), Foo)
         self.assertNotIsInstance(object(), Foo)
 
+    @skipUnless(
+        hasattr(typing, "runtime_checkable"),
+        "Test is only relevant if typing.runtime_checkable exists"
+    )
+    def test_typing_dot_runtimecheckable_on_Protocol(self):
+        @typing.runtime_checkable
+        class Foo(Protocol):
+            x: int
+
+        class Bar:
+            def __init__(self):
+                self.x = 42
+
+        self.assertIsInstance(Bar(), Foo)
+        self.assertNotIsInstance(object(), Foo)
+
     def test_no_instantiation(self):
         class P(Protocol): pass
         with self.assertRaises(TypeError):
@@ -4743,7 +4759,7 @@ def test_typing_extensions_defers_when_possible(self):
             exclude |= {'final', 'Any', 'NewType'}
         if sys.version_info < (3, 12):
             exclude |= {
-                'Protocol', 'runtime_checkable', 'SupportsAbs', 'SupportsBytes',
+                'Protocol', 'SupportsAbs', 'SupportsBytes',
                 'SupportsComplex', 'SupportsFloat', 'SupportsIndex', 'SupportsInt',
                 'SupportsRound', 'TypedDict', 'is_typeddict', 'NamedTuple', 'Unpack',
             }

src/typing_extensions.py

@@ -545,7 +545,6 @@ def _caller(depth=2):
 # so we backport the 3.12 version of Protocol to Python <=3.11
 if sys.version_info >= (3, 12):
     Protocol = typing.Protocol
-    runtime_checkable = typing.runtime_checkable
 else:
     def _allow_reckless_class_checks(depth=3):
         """Allow instance and class checks for special stdlib modules.
@@ -805,6 +804,10 @@ def __init_subclass__(cls, *args, **kwargs):
                 if cls.__init__ is Protocol.__init__:
                     cls.__init__ = _no_init
 
+
+if sys.version_info >= (3, 8):
+    runtime_checkable = typing.runtime_checkable
+else:
     def runtime_checkable(cls):
         """Mark a protocol class as a runtime protocol, so that it
         can be used with isinstance() and issubclass(). Raise TypeError