[Python-ideas] feature to make traceback objects usable without references to frame locals and globals

[M.-A. Lemburg]

Antoine Pitrou wrote:
> On Sun, 27 Jun 2010 19:20:07 +0200
> "M.-A. Lemburg" <mal at egenix.com> wrote:
>>
>> Not necessarily roll back the feature, but an implementation
>> that deliberately introduces circular references is not really
>> ideal.
>>
>> Since tracebacks on exceptions are rarely used by applications,
>> I think it would be better to turn them into weak references.
> 
> How do you manage to get a strong reference before the traceback object
> gets deleted?

IIUC, the traceback object will still be alive during processing
of the except clause, so all you'd have to do is turn the weak reference
into a real one. Let's assume that the weakref object is called
.__traceback_weakref__ and the proxy called .__traceback__ (to assure
compatibility).

...
except TypeError as exc:

    # Replace the weakref object with the referenced object
    exc.__traceback__ = exc.__traceback_weakref__()

    # Set the weakref object to None to have it collected and to
    # signal this operation to other code knowing about this
    # strategy.
    exc.__traceback_weakref__ = None

BTW: I wonder why proxy objects don't provide a direct access to
the weakref object they are using. That would make keeping that
extra variable around unnecessary.

> Besides, an API which gives some information in an unreliable manner
> does not seem very user-friendly to me.

The argument so far has been that most error processing happens
in the except clause itself, making it unnecessary to deal with
possible circular references. That is certainly true in many cases.

Now under that argument, using the traceback stored on an
exception outside the except clause is even less likely to be
needed, so I don't follow your concern that using a weak
reference is less user-friendly.

Perhaps someone could highlight a use case where the traceback
is needed outside the except clause ?!

> I think I like the OP's idea better: allow to release the references to
> local and global variables from the frames in the traceback. This is
> keeps a lot of potentially large objects alive - some of which may also
> keep some OS resources busy.

It's certainly a good idea to pay extra attention to this in
Python3.

-- 
Marc-Andre Lemburg
eGenix.com

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