[Python-Dev] PEP 380 (yield from a subgenerator) comments

[Guido van Rossum]

On Sat, Mar 28, 2009 at 4:34 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> I still think raise is out due to the fact that it would trigger
> subsequent except clauses. Guido has (sensibly) ruled out raising
> StopIteration and complaining if it has value in old code, since there
> is too much code which cases StopIteration *without* performing such a
> check.
>
> If those two points are accepted as valid, then that leaves the two
> options as being:
>
> 1. Add a new GeneratorReturn exception that will escape from existing
> code that only traps StopIteration. The only real downside of this is
> that either "return" and "return None" will mean different things in
> generators (unlike functions) or else "return None" will need to be
> special cased to raise StopIteration in the calling code rather than
> raising GeneratorReturn(None). The latter approach is probably
> preferable if this option is chosen - any code for dealing with
> "generators as coroutines" is going to have to deal with the possibility
> of bare returns and falling off the end of the function anyway, so the
> special case really wouldn't be that special.

It seems so indeed.

> 2. In addition to the "yield from" syntax for delegating to a
> subgenerator, also add new syntax for returning values from
> subgenerators so that the basic "return X" can continue to trigger
> SyntaxError.
>
> Since option 2 would most likely lead to a bikeshed discussion of epic
> proportions, I'm currently a fan of option 1 ;)

Me too. It also seems option 2 doesn't help us decide what it should
do: I still think that raising StopIteration(value) would be
misleading to vanilla users of the generators.

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