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

Guido van Rossum guido at python.org
Sat Feb 10 02:14:47 CET 2007

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 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.

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

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