[Python-Dev] What is the precise problem? [was: Reference cycles in Exception.traceback] (original) (raw)

Antoine Pitrou solipsis at pitrou.net
Sat Mar 8 14:33:02 CET 2014

On Sat, 8 Mar 2014 23:16:07 +1000 Nick Coghlan <ncoghlan at gmail.com> wrote:

On 8 March 2014 23:01, Victor Stinner <victor.stinner at gmail.com> wrote: > 2014-03-08 12:45 GMT+01:00 Antoine Pitrou <solipsis at pitrou.net>: >>> Attached script: neverdeleted2.py, it's almost the same but it >>> explains better the problem. The script creates MyObject and Future >>> objects which are never deleted. Calling gc.collect() does not break >>> the reference cycle (between the future, the exception, traceback and >>> frames). Stopping the event loop does not remove Future nor MyObject >>> objects. Only exiting Python does remove the Future object. >> >> So clearly the coroutine must be kept alive by something. > > It's a reference cycle. Something like that: > > Future -> Exception -> Traceback -> Frames -> Local variables -> > {Future, MyObject}

It seems unlikely we could have an uncollectable reference cycle that doesn't end up in gc.garbage. Are you sure there are no external references to one of the objects in the cycle, thus keeping the whole thing alive?

Ok, it's actually quite trivial. The whole chain is kept alive by the "fut" global variable. If you arrange for it to be disposed of:

fut = asyncio.Future() asyncio.Task(func(fut)) del fut [etc.]

then the problem disappears: as soon as gc.collect() happens, the MyObject instance is destroyed, the future is collected, and the future's traceback is printed out.

Regards

Antoine.

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