[Python-Dev] Please reconsider PEP 479.

[Petr Viktorin]

On Wed, Nov 26, 2014 at 12:24 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
> On 26 November 2014 at 18:30, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:
>> Guido van Rossum wrote:
>>>
>>> Hm, that sounds like you're either being contrarian or Chris and I have
>>> explained it even worse than I thought.
>>
>> I'm not trying to be contrary, I just think the PEP could
>> explain more clearly what you're trying to achieve. The
>> rationale is too vague and waffly at the moment.
>>
>>> Currently, there are cases where list(x for x in xs if P(x)) works while
>>> [x for x in xs if P(x)] fails (when P(x) raises StopIteration). With the
>>> PEP, both cases will raise some exception
>>
>> That's a better explanation, I think.
>
> The other key aspect is that it changes the answer to the question
> "How do I gracefully terminate a generator function?". The existing
> behaviour has an "or" in the answer: "return from the generator frame,
> OR raise StopIteration from the generator frame". That then leads to
> the follow on question: "When should I use one over the other?".
>
> The "from __future__ import generator_stop" answer drops the "or", so
> it's just: "return from the generator frame".
>
> Raising *any* exception inside the generator, including StopIteration,
> then counts as non-graceful termination, bringing generators into line
> with the PEP 343 philosophy that "hiding flow control in macros makes
> your code inscrutable", where here, the hidden flow control is relying
> on the fact that a called function raising StopIteration will
> currently always gracefully terminate generator execution.
>
> The key downside is that it means relatively idiomatic code like:
>
>     def my_generator():
>         ...
>         yield next(it)
>         ...
>
> Now needs to be written out explicitly as:
>
>     def my_generator():
>         ...
>        try:
>             yield next(it)
>         except StopIteration
>             return
>         ...

It could also be written as:

     def my_generator():
        try:
            ...
            yield next(it)
            ...
        except StopIteration
             return

i.e. put the try-block around the whole body, not just the individual
yield. This emulates what's happenning in current Python, and it would
be faster than individual try blocks.