[Python-3000] Pre-peps on raise and except changes

[Collin Winter]

On 2/9/07, Guido van Rossum <guido at python.org> wrote:
> On 2/9/07, Phillip J. Eby <pje at telecommunity.com> wrote:
> > At 01:33 PM 2/10/2007 +1300, Greg Ewing wrote:
> > >Collin Winter wrote:
> > >
> > > > I believe the original proposal was something like
> > > >
> > > > raise E(V).with_traceback(T)
> > >
> > >Does this mean you're not intending to have any syntactic
> > >variant of the raise statement that includes a traceback
> > >in 3.0?
> >
> > That *is* the variant.  ;)
> >
> >
> > >Or is this just so that forward-compatible code
> > >can be written in 2.6?
> >
> > Actually, forward compatible code would be easier with something syntactic,
> > like your 'raise e with t' idea.  It would allow the implementation to be
> > different in 2.6 and 3.0, while using the same syntax.  (In 2.6 it could
> > use the existing machinery, while in 3.0 it could call the
> > .with_traceback() method.
> >
> > Hm.  Actually, that's not necessary.  We could include .with_traceback(T)
> > in 2.6, and just have old-style except: clauses delete the traceback from
> > the returned objects.  New-style except: clauses would work just as they
> > would in 3.0.
> >
> > To summarize, in 2.6 we could support .with_traceback() and create
> > exception instances with traceback attributes, but the old-style except:
> > clauses could discard them to prevent cycles.  Raising an exception
> > instance with a __traceback__ attribute would get some special handling so
> > that it's equivalent to 3-argument raise in today's Python.  Likewise,
> > generator.throw() would need the same special handling in 2.6.  Meanwhile,
> > sys.exc_info() still lives in both versions.
> >
> > To write 3.0-compatible code, you just use the 3.0 spellings of raise,
> > throw(), and except.  Sounds like a plan!
>
> Can't see anything wrong with this either. Collin, do you have enough
> to update your PEPs?

I think so. I've already got language ready for the section on using
BaseException.with_traceback() in the 2->3 raise translations, and
I'll work up additional language for the transition plan sometime this
weekend.

> I wonder if we should try to keep PEP 344 up to date, or if we should
> just do this in the Py3k PEPs; I'm okay with adding some notes about
> 2.6 to Py3k PEPs so I guess the latter would work.

If with_traceback() is going to be added in 2.6, I think at least that
much should go in PEP 344. The rest falls under "transitioning to
3.0", so it should probably go in PEP 3109.

Collin Winter