[Python-Dev] PEP 340 -- concept clarification

[Guido van Rossum]

On 5/3/05, Phillip J. Eby <pje at telecommunity.com> wrote:
> At 04:41 PM 5/3/05 -0700, Guido van Rossum wrote:
> >Given all that, it's not infeasible to add a close() method to
> >generators as a shortcut for this:
> >
> >     def close(self):
> >         try:
> >             self.__exit__(StopIteration)
> >         except StopIteration:
> >             break
> >         else:
> >             # __exit__() didn't
> >             raise RuntimeError("or some other exception")
> >
> >I'd like the block statement to be defined exclusively in terms of
> >__exit__() though.

(So do you want this feature now or not? Earlier you said it was no big deal.)

> Sure.  PEP 325 proposes a "CloseGenerator" exception in place of
> "StopIteration", however, because:
> 
>      """
>      Issues: should StopIteration be reused for this purpose?  Probably
>      not.  We would like close to be a harmless operation for legacy
>      generators, which could contain code catching StopIteration to
>      deal with other generators/iterators.
>      """
> 
> I don't know enough about the issue to offer either support or opposition
> for this idea, though.

That would be an issue for the generator finalization proposed by the
PEP as well.

But I kind of doubt that it's an issue; you'd have to have a
try/except catching StopIteration around a yield statement that
resumes the generator before this becomes an issue, and that sounds
extremely improbable. If at all possible I'd rather not have to define
a new exception for this purpose.

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