From 3b776cf15a585e0d2ec32e7847072ed753c964d5 Mon Sep 17 00:00:00 2001
From: Timofey Ivankov <playboicartina@gmail.com>
Date: Sun, 9 Aug 2026 16:15:52 +0300
Subject: [PATCH] gh-155433: Fix TaskGroup losing outside cancellation after
 cancel()

---
 Lib/asyncio/taskgroups.py                     |  3 +++
 Lib/test/test_asyncio/test_taskgroups.py      | 20 +++++++++++++++++++
 ...-08-09-16-14-38.gh-issue-155433.KL7tHV.rst |  2 ++
 3 files changed, 25 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-08-09-16-14-38.gh-issue-155433.KL7tHV.rst

diff --git a/Lib/asyncio/taskgroups.py b/Lib/asyncio/taskgroups.py
index a431b45a19489d..0c1d09ded2f956 100644
--- a/Lib/asyncio/taskgroups.py
+++ b/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.
diff --git a/Lib/test/test_asyncio/test_taskgroups.py b/Lib/test/test_asyncio/test_taskgroups.py
index bc246400b83e9b..88bb9510af367b 100644
--- a/Lib/test/test_asyncio/test_taskgroups.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Library/2026-08-09-16-14-38.gh-issue-155433.KL7tHV.rst b/Misc/NEWS.d/next/Library/2026-08-09-16-14-38.gh-issue-155433.KL7tHV.rst
new file mode 100644
index 00000000000000..dc8961e976abbc
--- /dev/null
+++ b/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()``.