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

[Guido van Rossum]

On Fri, Mar 27, 2009 at 8:45 PM, P.J. Eby <pje at telecommunity.com> wrote:
> At 12:53 PM 3/28/2009 +1200, Greg Ewing wrote:
>> Guido van Rossum wrote:
>>> Perhaps the crux is that *if* you accidentally use "return <value>" in
>>> a vanilla generator expecting the value to show up somewhere, you are
>>> probably enough of a newbie that debugging this will be quite hard.
>>> I'd like not to have such a newbie trap lying around.
>>
>> Okay, so would you be happy if the for-loop were to
>> raise an exception if it gets a StopIteration whose
>> value is not None?
>
> Wouldn't it have to be more than just for-loops?  What about list(), map(),
> ...?
>
> It seems a lot simpler to just make it use a different exception, as nothing
> else has to change for that to work correctly; the new construct can just
> catch it, and everywhere else it's an error.

Exactly. Making StopIteration() with a non-None value invalid is not
backwards compatible.

The new exception could either be a designated (built-in) subclass of
StopIteration, or not; I haven't thought enough about which choice
would be more useful or easier. I think in either case a check in
PyIter_Next() would cover most cases -- though there are probable a
few other places in the interpreter where tp_next is called that
should also have this check added.

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