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

[Tim Delaney]

On 28 June 2010 04:00, Antoine Pitrou <solipsis at pitrou.net> 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?
>

At the beginning of the 'except' block, a strong local (but hidden)
reference is obtained to the traceback (if it exists). This is deleted at
the end of the 'except' block.

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


I agree, with a variation - keep a weak reference to the frame in the
traceback, and have a way for the application to specify that it wants to
retain strong references to frames (so unittest for example can guarantee
access to locals and globals). Possibly a context manager could be used for
this, and decorators could be used to wrap an entire method in the context
manager.

A dummy frame would also be stored that contained enough info to replicate
the existing stack trace (file, line number, etc). A strong reference could
be obtained via the existing attribute, converted to a property, which does:

a. return the internal reference if it is not a dummy frame;
b. return the result of the weak reference if it still exists;
c. return the dummy frame reference.

I think this gives us the best of all worlds:

1. No strong reference to locals/globals in tracebacks by default;

2. Able to force strong references to frames;

3. We don't lose the ability to compose a full and complete stack trace.

Tim Delaney
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