[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Steve Holden]

On Wed, Nov 22, 2017 at 8:48 PM, Sven R. Kunze <srkunze at mail.de> wrote:

> Isn't yield like a return?
>
​Enough like it to make a good case, I'd say.​


> A return in a list/dict/set comprehension makes no sense to me.
>
​Nor me, nor the vast majority of instance. But nowadays yield is more of a
synchronisation point. If something is valid syntax we should presumably
like to have defined semantics.

> So, +1 on SyntaxError from me too.
>
​I'd tend to agree. This would give more time to discuss the intended
semantics: giving it meaning later might be a more cautious approach that
would allow decisions to be made in the light of further experience.

I would urge developers, in their ​improvements to the language to support
asynchronous programming, to bear in mind that this is (currently) a
minority use case. Why the rush to set complex semantics in stone?

>
regards
 Steve​


> Cheers.
>
> On 22.11.2017 21:29, David Mertz wrote:
>
> Inasmuch as I get to opine, I'm +1 on SyntaxError. There is no behavior
> for that spelling that I would find intuitive or easy to explain to
> students. And as far as I can tell, the ONLY time anything has ever been
> spelled that way is in comments saying "look at this weird edge case
> behavior in Python."
>
> On Nov 22, 2017 10:57 AM, "Jelle Zijlstra" <jelle.zijlstra at gmail.com>
> wrote:
>
>
>
> 2017-11-22 9:58 GMT-08:00 Guido van Rossum <guido at python.org>:
>
>> Wow, 44 messages in 4 hours. That must be some kind of record.
>>
>> If/when there's an action item, can someone summarize for me?
>>
>> The main disagreement seems to be about what this code should do:
>
>     g = [(yield i) for i in range(3)]
>
> Currently, this makes `g` into a generator, not a list. Everybody seems to
> agree this is nonintuitive and should be changed.
>
> One proposal is to make it so `g` gets assigned a list, and the `yield`
> happens in the enclosing scope (so the enclosing function would have to be
> a generator). This was the way things worked in Python 2, I believe.
>
> Another proposal is to make this code a syntax error, because it's
> confusing either way. (For what it's worth, that would be my preference.)
>
> There is related discussion about the semantics of list comprehensions
> versus calling list() on a generator expression, and of async semantics,
> but I don't think there's any clear point of action there.
>
>
>> --
>> --Guido van Rossum (python.org/~guido <http://python.org/%7Eguido>)
>>
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