[Python-Dev] Reference cycles in Exception.__traceback__

[Guido van Rossum]

But inspect is in the stdlib. Surely changing inspect.py is less
controversial than amending the semantics of frame objects.


On Thu, Mar 6, 2014 at 10:10 AM, Xavier Morel <python-dev at masklinn.net>wrote:

> On 2014-03-06, at 16:52 , Antoine Pitrou <solipsis at pitrou.net> wrote:
> > Le 06/03/2014 16:03, Yury Selivanov a écrit :
> >>
> >> On 2014-03-06, 8:42 AM, Antoine Pitrou wrote:
> >>> Le 05/03/2014 23:53, Nick Coghlan a écrit :
> >>>>
> >>>> __traceback__ wouldn't change [...]
> >>>
> >>> Uh, really? If you want to suppress all reference cycles, you *have*
> >>> to remove __traceback__.
> >>>
> >>> The problem is to make computation of the traceback summary
> >>> lightweight enough that it doesn't degrade performance in the common
> >>> case where you don't have to print the traceback later.
> >>
> >> So why can't we allow instantiation of types.TracebackType &
> >> types.FrameType?
> >
> > IMO it is absolutely out of question to allow creation of arbitrary
> frames from Python code, because the structure and initialization of frames
> embody too many low-level implementation details.
> >
> > We might allow the creation of traceback objects, but without any custom
> frame objects it is unclear how useful that would be.
>
> Some bits of the standard library (and probably third-party libraries
> transitively relying on getinnerframes) really, really want traceback
> objects and can't be used with a stack or frames extracted via
> inspect.currentframe() or whatever. For instance cgitb.text calls
> inspect.getinnerframes which accesses param.tb_frame then calls
> getframeinfo(param). getframeinfo blows up if it does not get either an
> actual traceback object or an actual frame object.
>
> A frame object does not have a tb_frame attribute, and will thus fail
> in getinnerframes, a fake traceback object will fail in getframeinfo.
>
> Therefore it's not possible to call cgitb.text outside of an exception
> handling context (that is, not possible to use it as a
> traceback.print_stack providing extra information). If it were possible
> to create traceback objects, there would be no issue there at least.
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-- 
--Guido van Rossum (python.org/~guido)
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