bpo-40816 Add AsyncContextDecorator class (GH-20516)

Co-authored-by: Yury Selivanov <yury@edgedb.com>

2 files changed, 51 insertions, 1 deletions

diff --git a/Lib/contextlib.py b/Lib/contextlib.py
index 82ddc14..56b4968 100644
--- a/Lib/contextlib.py
+++ b/Lib/contextlib.py
@@ -80,6 +80,22 @@ class ContextDecorator(object):
         return inner
 
 
+class AsyncContextDecorator(object):
+    "A base class or mixin that enables async context managers to work as decorators."
+
+    def _recreate_cm(self):
+        """Return a recreated instance of self.
+        """
+        return self
+
+    def __call__(self, func):
+        @wraps(func)
+        async def inner(*args, **kwds):
+            async with self._recreate_cm():
+                return await func(*args, **kwds)
+        return inner
+
+
 class _GeneratorContextManagerBase:
     """Shared functionality for @contextmanager and @asynccontextmanager."""
 
@@ -167,9 +183,16 @@ class _GeneratorContextManager(_GeneratorContextManagerBase,
 
 
 class _AsyncGeneratorContextManager(_GeneratorContextManagerBase,
-                                    AbstractAsyncContextManager):
+                                    AbstractAsyncContextManager,
+                                    AsyncContextDecorator):
     """Helper for @asynccontextmanager."""
 
+    def _recreate_cm(self):
+        # _AGCM instances are one-shot context managers, so the
+        # ACM must be recreated each time a decorated function is
+        # called
+        return self.__class__(self.func, self.args, self.kwds)
+
     async def __aenter__(self):
         try:
             return await self.gen.__anext__()
diff --git a/Lib/test/test_contextlib_async.py b/Lib/test/test_contextlib_async.py
index 3765f6c..109807d 100644
--- a/Lib/test/test_contextlib_async.py
+++ b/Lib/test/test_contextlib_async.py
@@ -278,6 +278,33 @@ class AsyncContextManagerTestCase(unittest.TestCase):
         async with woohoo(self=11, func=22, args=33, kwds=44) as target:
             self.assertEqual(target, (11, 22, 33, 44))
 
+    @_async_test
+    async def test_recursive(self):
+        depth = 0
+        ncols = 0
+
+        @asynccontextmanager
+        async def woohoo():
+            nonlocal ncols
+            ncols += 1
+
+            nonlocal depth
+            before = depth
+            depth += 1
+            yield
+            depth -= 1
+            self.assertEqual(depth, before)
+
+        @woohoo()
+        async def recursive():
+            if depth < 10:
+                await recursive()
+
+        await recursive()
+
+        self.assertEqual(ncols, 10)
+        self.assertEqual(depth, 0)
+
 
 class AclosingTestCase(unittest.TestCase):