gh-155433: Fix TaskGroup losing outside cancellation after cancel()

Author: deadlovelll

Lib/asyncio/taskgroups.py

@@ -151,6 +151,9 @@ async def _aexit(self, et, exc):
                 # If there are no pending cancellations left,
                 # don't propagate CancelledError.
                 propagate_cancellation_error = None
+            else:
+                # gh-155433: the remaining cancellation is not ours, don't drop it
+                propagate_cancellation_error = exceptions.CancelledError()
 
         # Propagate CancelledError if there is one, except if there
         # are other errors -- those have priority.

Lib/test/test_asyncio/test_taskgroups.py

@@ -1154,6 +1154,26 @@ async def test_taskgroup_cancel_before_create_task(self):
             with self.assertRaises(RuntimeError):
                 tg.create_task(asyncio.sleep(1))
 
+    async def test_taskgroup_cancel_keeps_outer_cancellation(self):
+        # gh-155433: any cancellation from outside the group must propagate.
+        async def child():
+            try:
+                await asyncio.sleep(10)
+            finally:
+                await asyncio.sleep(0.1)
+
+        async def body():
+            async with asyncio.TaskGroup() as tg:
+                tg.create_task(child())
+                await asyncio.sleep(0)
+                tg.cancel()
+
+        task = asyncio.create_task(body())
+        await asyncio.sleep(0.01)
+        task.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await task
+
     async def test_taskgroup_cancel_before_exception(self):
         async def raise_exc(parent_tg: asyncio.TaskGroup):
             parent_tg.cancel()

Misc/NEWS.d/next/Library/2026-08-09-16-14-38.gh-issue-155433.KL7tHV.rst

@@ -0,0 +1,2 @@
+Fix :class:`asyncio.TaskGroup` losing outside cancellation after
+``cancel()``.