[Python-Dev] PEP 344 (was: with_traceback)

[Guido van Rossum]

On 3/2/07, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
> Guido van Rossum wrote:
>
> > I'm afraid we're back at square zero; perhaps we should keep the
> > existing (type, value, traceback) API
>
> Whatever happens, we should be able to get that down
> to at most two things: (exception, context) where
> exception is either a class or an instance, and
> context includes the traceback and anything else
> that varies from one raising to another.
>
> > rather than going for radical
> > (and unproven) innovation like "except E as v with tb: <block>".
>
> The "with something" clause isn't mutually exclusive
> with that -- rather it goes hand-in-hand as a nicer
> way of getting hold of the context than calling
> sys.exc_info(). I'd consider it an optional extra.
>
> > "except E as v with tb in context for cause" anyone? :-)
>
> I was thinking more of
>
>    except E as v with context:
>
> and context would have attributes such as traceback,
> cause, etc.
>
> (Although calling it a "context" could be seriously
> confusing, considering we also have a "with" statement
> having something called a context that's completely
> different.)
>
> > The one thing that makes me not want to give up yet is that having the
> > traceback, context, cause etc. as attributes of the exception object
> > would seem to make the API for handling exceptions that much cleaner,
>
> If we can get it down to a 2-tuple, with no fudging
> around to allow for sometimes having the args separate
> from the type but not always, that should simplify it
> considerably.
>
> One thing I would like to get out of it if possible
> is the chance to raise and catch an exception class
> without instantiating it at all under favourable
> circumstances. That should be possible if we allow
> the first item of the tuple to be either a class or
> instance, and not bother instantiating if it's a
> class and gets caught with
>
>    except E:
>
> or
>
>    except E with context:

But what's the advantage of not instantiating the exception if we
instantiate the context instead? I really think that it would be nicer
if the exception object *was* the context. We'd still not have to
instantiate if it was just "except E:" but that can be handled behind
the scenes.

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)