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

[Maciej Fijalkowski]

On Mon, Mar 10, 2014 at 7:35 PM, Guido van Rossum <guido at python.org> wrote:
> On Mon, Mar 10, 2014 at 10:30 AM, Maciej Fijalkowski <fijall at gmail.com>
> wrote:
>>
>> On Mon, Mar 10, 2014 at 3:23 PM, Victor Stinner
>> <victor.stinner at gmail.com> wrote:
>> > 2014-03-10 13:11 GMT+01:00 Maciej Fijalkowski <fijall at gmail.com>:
>> >> It was agreed long time ago that the immediate finalization is an
>> >> implementation specific detail and it's not guaranteed. You should not
>> >> rely on __del__s being called timely one way or another. Why would you
>> >> require this for the program to work correctly in the particular
>> >> example of __traceback__?
>> >
>> > For asyncio, it's very useful to see unhandled exceptions as early as
>> > possible. Otherwise, your program is blocked and you don't know why.
>> >
>> > Guido van Rossum suggests to run gc.collect() regulary:
>> > http://code.google.com/p/tulip/issues/detail?id=42
>> >
>> > Victor
>>
>> twisted goes around it by attaching errback by hand. Would that work for
>> tulip?
>
>
> Can you describe that idea in more detail?

Essentially, instead of relying on deferred to be garbage collected,
you attach an errback like this:

deferred.addErrback(callback_that_writes_to_log)

so in case of a failure, you get a traceback directly in the callback
immediately, without relying on garbage collection.

I'm sorry if I'm using twisted nomenclature here (it's also awfully
off-topic for python-dev), but making programs rely on refcounting
sounds like a bad idea for us (PyPy).

Cheers,
fijal