gh-105726: Add __slots__ to AbstractContextManager and AbstractAsyncContextManager (#106771)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/contextlib.py

@@ -20,6 +20,8 @@ class AbstractContextManager(abc.ABC):
 
     __class_getitem__ = classmethod(GenericAlias)
 
+    __slots__ = ()
+
     def __enter__(self):
         """Return `self` upon entering the runtime context."""
         return self
@@ -42,6 +44,8 @@ class AbstractAsyncContextManager(abc.ABC):
 
     __class_getitem__ = classmethod(GenericAlias)
 
+    __slots__ = ()
+
     async def __aenter__(self):
         """Return `self` upon entering the runtime context."""
         return self

Lib/test/test_contextlib.py

@@ -24,6 +24,16 @@ class TestAbstractContextManager(unittest.TestCase):
         manager = DefaultEnter()
         self.assertIs(manager.__enter__(), manager)
 
+    def test_slots(self):
+        class DefaultContextManager(AbstractContextManager):
+            __slots__ = ()
+
+            def __exit__(self, *args):
+                super().__exit__(*args)
+
+        with self.assertRaises(AttributeError):
+            DefaultContextManager().var = 42
+
     def test_exit_is_abstract(self):
         class MissingExit(AbstractContextManager):
             pass

Lib/test/test_contextlib_async.py

@@ -37,6 +37,18 @@ class TestAbstractAsyncContextManager(unittest.TestCase):
         async with manager as context:
             self.assertIs(manager, context)
 
+    @_async_test
+    async def test_slots(self):
+        class DefaultAsyncContextManager(AbstractAsyncContextManager):
+            __slots__ = ()
+
+            async def __aexit__(self, *args):
+                await super().__aexit__(*args)
+
+        with self.assertRaises(AttributeError):
+            manager = DefaultAsyncContextManager()
+            manager.var = 42
+
     @_async_test
     async def test_async_gen_propagates_generator_exit(self):
         # A regression test for https://bugs.python.org/issue33786.

Misc/NEWS.d/next/Library/2023-07-15-12-52-50.gh-issue-105726.NGthO8.rst

@@ -0,0 +1,3 @@
+Added ``__slots__`` to :class:`contextlib.AbstractContextManager` and :class:`contextlib.AbstractAsyncContextManager`
+so that child classes can use ``__slots__``.
+