Allow Protocols to inherit from typing_extensions.Buffer or collections.abc.Buffer (#220)

Author: AlexWaygood

CHANGELOG.md

@@ -4,6 +4,9 @@
 - Fix tests on Python 3.13, which removes support for creating
   `TypedDict` classes through the keyword-argument syntax. Patch by
   Jelle Zijlstra.
+- Allow `Protocol` classes to inherit from `typing_extensions.Buffer` or
+  `collections.abc.Buffer`. Patch by Alex Waygood (backporting
+  https://github.com/python/cpython/pull/104827, by Jelle Zijlstra).
 
 # Release 4.6.3 (June 1, 2023)
 

src/test_typing_extensions.py

@@ -2737,6 +2737,28 @@ def close(self):
         self.assertIsSubclass(B, Custom)
         self.assertNotIsSubclass(A, Custom)
 
+    @skipUnless(
+        hasattr(collections.abc, "Buffer"),
+        "needs collections.abc.Buffer to exist"
+    )
+    @skip_if_py312b1
+    def test_collections_abc_buffer_protocol_allowed(self):
+        @runtime_checkable
+        class ReleasableBuffer(collections.abc.Buffer, Protocol):
+            def __release_buffer__(self, mv: memoryview) -> None: ...
+
+        class C: pass
+        class D:
+            def __buffer__(self, flags: int) -> memoryview:
+                return memoryview(b'')
+            def __release_buffer__(self, mv: memoryview) -> None:
+                pass
+
+        self.assertIsSubclass(D, ReleasableBuffer)
+        self.assertIsInstance(D(), ReleasableBuffer)
+        self.assertNotIsSubclass(C, ReleasableBuffer)
+        self.assertNotIsInstance(C(), ReleasableBuffer)
+
     def test_builtin_protocol_allowlist(self):
         with self.assertRaises(TypeError):
             class CustomProtocol(TestCase, Protocol):
@@ -2745,6 +2767,24 @@ class CustomProtocol(TestCase, Protocol):
         class CustomContextManager(typing.ContextManager, Protocol):
             pass
 
+    @skip_if_py312b1
+    def test_typing_extensions_protocol_allowlist(self):
+        @runtime_checkable
+        class ReleasableBuffer(Buffer, Protocol):
+            def __release_buffer__(self, mv: memoryview) -> None: ...
+
+        class C: pass
+        class D:
+            def __buffer__(self, flags: int) -> memoryview:
+                return memoryview(b'')
+            def __release_buffer__(self, mv: memoryview) -> None:
+                pass
+
+        self.assertIsSubclass(D, ReleasableBuffer)
+        self.assertIsInstance(D(), ReleasableBuffer)
+        self.assertNotIsSubclass(C, ReleasableBuffer)
+        self.assertNotIsInstance(C(), ReleasableBuffer)
+
     def test_non_runtime_protocol_isinstance_check(self):
         class P(Protocol):
             x: int

src/typing_extensions.py

@@ -453,9 +453,10 @@ def clear_overloads():
 _PROTO_ALLOWLIST = {
     'collections.abc': [
         'Callable', 'Awaitable', 'Iterable', 'Iterator', 'AsyncIterable',
-        'Hashable', 'Sized', 'Container', 'Collection', 'Reversible',
+        'Hashable', 'Sized', 'Container', 'Collection', 'Reversible', 'Buffer',
     ],
     'contextlib': ['AbstractContextManager', 'AbstractAsyncContextManager'],
+    'typing_extensions': ['Buffer'],
 }