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

[M.-A. Lemburg]

Antoine Pitrou wrote:
> On Mon, 28 Jun 2010 13:14:21 +0200
> "M.-A. Lemburg" <mal at egenix.com> wrote:
>>
>> 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.
> 
> Probably because the proxy would then have an additional attribute
> which isn't on the proxied object. Or, worse, it could also shadow
> one of the proxied object's existing attributes.

That's a very weak argument, IMHO. It all depends on the
naming of the attribute. Also note that the proxied object
won't know anything about that attribute, so it doesn't have
any side-effects.

We've used such an approach on our mxProxy object for years without
any problems or naming conflicts so far:

http://www.egenix.com/products/python/mxBase/mxProxy/
http://www.egenix.com/products/python/mxBase/mxProxy/doc/#_Toc162774452

>> Perhaps someone could highlight a use case where the traceback
>> is needed outside the except clause ?!
> 
> Well, it's needed if you want delayed error reporting and still display
> a comprehensive stack trace (rather than just the exception message).
> Frameworks often need this kind of behaviour; Twisted was already
> mentioned in this thread. But, even outside of frameworks, there are
> situations where you want to process a bunch of data and present all
> processing errors at the end.

I had already given that example myself, but in those cases I had
in mind the stack trace is not really needed: instead, you add the
relevant information to the list of errors directly from the
except clause, since the error information needed to report
the issues is not related to programming errors, but instead to
data errors.

> However, as the OP argued, most often you need the traceback in order
> to display file names and line numbers, but you don't need the attached
> variables (locals and globals).

I guess all this just needs to be highlighted in the documentation
to make programmers aware of the fact that they cannot just store
exception objects away without considering the consequences of this
first.

-- 
Marc-Andre Lemburg
eGenix.com

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