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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"refcycles in exceptions raised from asyncio.TaskGroup","articleBody":"# Bug report\n\n### Bug description:\n\nasyncio.TaskGroup attempts to avoid refcycles in raised exceptions by deleting `self._errors` but when I reviewed the code it doesn't actually achieve this:\n\nsee https://github.com/python/cpython/blob/5e9e50612eb27aef8f74a0ccc234e5cfae50c4d7/Lib/asyncio/taskgroups.py#L152-L156\n\nThere's a refcycle in `me is me.__traceback__.tb_next.tb_frame.f_locals[\"me\"]`\n\nI wrote a few tests to route out all the refcycles in tracebacks\n\n```python\nimport asyncio\nimport gc\nimport unittest\n\nclass TestTaskGroup(unittest.IsolatedAsyncioTestCase):\n\n    async def test_exception_refcycles_direct(self):\n        \"\"\"Test that TaskGroup doesn't keep a reference to the raised ExceptionGroup\"\"\"\n        tg = asyncio.TaskGroup()\n        exc = None\n\n        class _Done(Exception):\n            pass\n\n        try:\n            async with tg:\n                raise _Done\n        except ExceptionGroup as e:\n            exc = e\n\n        self.assertIsNotNone(exc)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n\n    async def test_exception_refcycles_errors(self):\n        \"\"\"Test that TaskGroup deletes self._errors, and __aexit__ args\"\"\"\n        tg = asyncio.TaskGroup()\n        exc = None\n\n        class _Done(Exception):\n            pass\n\n        try:\n            async with tg:\n                raise _Done\n        except* _Done as excs:\n            exc = excs.exceptions[0]\n\n        self.assertIsInstance(exc, _Done)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n\n    async def test_exception_refcycles_parent_task(self):\n        \"\"\"Test that TaskGroup deletes self._parent_task\"\"\"\n        tg = asyncio.TaskGroup()\n        exc = None\n\n        class _Done(Exception):\n            pass\n\n        async def coro_fn():\n            async with tg:\n                raise _Done\n\n        try:\n            async with asyncio.TaskGroup() as tg2:\n                tg2.create_task(coro_fn())\n        except* _Done as excs:\n            exc = excs.exceptions[0].exceptions[0]\n\n        self.assertIsInstance(exc, _Done)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n    async def test_exception_refcycles_propagate_cancellation_error(self):\n        \"\"\"Test that TaskGroup deletes propagate_cancellation_error\"\"\"\n        tg = asyncio.TaskGroup()\n        exc = None\n\n        try:\n            async with asyncio.timeout(-1):\n                async with tg:\n                    await asyncio.sleep(0)\n        except TimeoutError as e:\n            exc = e.__cause__\n\n        self.assertIsInstance(exc, asyncio.CancelledError)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n    async def test_exception_refcycles_base_error(self):\n        \"\"\"Test that TaskGroup deletes self._base_error\"\"\"\n        class MyKeyboardInterrupt(KeyboardInterrupt):\n            pass\n\n        tg = asyncio.TaskGroup()\n        exc = None\n\n        try:\n            async with tg:\n                raise MyKeyboardInterrupt\n        except MyKeyboardInterrupt as e:\n            exc = e\n\n        self.assertIsNotNone(exc)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n```\n\n\nin writing all these tests I noticed refcycles in PyFuture:\n\nhttps://github.com/python/cpython/blob/58f7763b88d4e81662b9d212f6beddcb42437fe3/Lib/asyncio/futures.py#L197-L198\nhttps://github.com/python/cpython/blob/58f7763b88d4e81662b9d212f6beddcb42437fe3/Lib/asyncio/futures.py#L215-L216\n\n```python\nclass BaseFutureTests:\n    def test_future_cancelled_result_refcycles(self):\n        f = self._new_future(loop=self.loop)\n        f.cancel()\n        exc = None\n        try:\n            f.result()\n        except asyncio.CancelledError as e:\n            exc = e\n        self.assertIsNotNone(exc)\n        self.assertListEqual(gc.get_referrers(exc), [])\n\n    def test_future_cancelled_exception_refcycles(self):\n        f = self._new_future(loop=self.loop)\n        f.cancel()\n        exc = None\n        try:\n            f.exception()\n        except asyncio.CancelledError as e:\n            exc = e\n        self.assertIsNotNone(exc)\n        self.assertListEqual(gc.get_referrers(exc), [])\n```\n\n\n### CPython versions tested on:\n\n3.12, 3.13\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-124959\n* gh-125463\n* gh-125466\n* gh-125486\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/graingert","@type":"Person","name":"graingert"},"datePublished":"2024-10-04T08:40:39.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/124958/cpython/issues/124958"}