Fix isinstance() for Protocols (#330)

Fixes #297

This allows to use isinstance() with classes that inherit from protocols in typing.py such as SupportsInt etc.

Author: ilevkivskyi

python2/test_typing.py

@@ -484,7 +484,13 @@ def test_reversible(self):
     def test_protocol_instance_type_error(self):
         with self.assertRaises(TypeError):
             isinstance(0, typing.SupportsAbs)
-
+        class C1(typing.SupportsInt):
+            def __int__(self):
+                return 42
+        class C2(C1):
+            pass
+        c = C2()
+        self.assertIsInstance(c, C1)
 
 class GenericTests(BaseTestCase):
 

python2/typing.py

@@ -1462,6 +1462,8 @@ class _ProtocolMeta(GenericMeta):
     """
 
     def __instancecheck__(self, obj):
+        if _Protocol not in self.__bases__:
+            return super(_ProtocolMeta, self).__instancecheck__(obj)
         raise TypeError("Protocols cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):

src/test_typing.py

@@ -513,6 +513,13 @@ def test_reversible(self):
     def test_protocol_instance_type_error(self):
         with self.assertRaises(TypeError):
             isinstance(0, typing.SupportsAbs)
+        class C1(typing.SupportsInt):
+            def __int__(self) -> int:
+                return 42
+        class C2(C1):
+            pass
+        c = C2()
+        self.assertIsInstance(c, C1)
 
 
 class GenericTests(BaseTestCase):

src/typing.py

@@ -1503,6 +1503,8 @@ class _ProtocolMeta(GenericMeta):
     """
 
     def __instancecheck__(self, obj):
+        if _Protocol not in self.__bases__:
+            return super().__instancecheck__(obj)
         raise TypeError("Protocols cannot be used with isinstance().")
 
     def __subclasscheck__(self, cls):