[Python-Dev] What is the precise problem? [was: Reference cycles in Exception.traceback] (original) (raw)
Antoine Pitrou solipsis at pitrou.net
Sat Mar 8 18:24:21 CET 2014
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On Sat, 8 Mar 2014 16:14:23 +0100 Victor Stinner <victor.stinner at gmail.com> wrote:
2014-03-08 14:33 GMT+01:00 Antoine Pitrou <solipsis at pitrou.net>: > 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.
Well, the problem is more general than this specific example. I would like to implement a general solution which would not hold references to local variables, to destroy objects when Python exits the except block.
How about the following patch:
diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py --- a/Lib/asyncio/futures.py +++ b/Lib/asyncio/futures.py @@ -315,6 +315,7 @@ class Future: self._schedule_callbacks() if _PY34: self._log_traceback = True
self._loop.call_soon(traceback.clear_frames, exception.__traceback__) else: self._tb_logger = _TracebackLogger(exception, self._loop) # Arrange for the logger to be activated after all callbacks
That said, I agree an automated mechanism would be useful. It is overwhelmingly rare to want to analyze local variables in a traceback, yet Python always keeps a reference to those. Perhaps tracebacks could have a debug attribute which, when sent to False, would prevent the locals from being kept alive (but how?).
Regards
Antoine.
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