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[Python-Dev] What is the precise problem? [was: Reference cycles in Exception.__traceback__]

Maciej Fijalkowski fijall at gmail.com
Sat Mar 8 16:30:12 CET 2014 
On Sat, Mar 8, 2014 at 5:14 PM, 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.
>
> It looks like a "exception summary" containing only data to format the
> traceback would fit asyncio needs. If you don't want it in the
> traceback module, I will try to implement it in asyncio.
>
> It would be nice to provide an "exception summary" in the traceback
> module, because it looks like reference cycles related to exception
> and/or traceback is a common issue (see the list of links I gave in a
> previous email).
>
> Victor

How about fixing cyclic gc to deal with __del__ instead? That sounds
like an awful change to the semantics.
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