[Python-Dev] Tricky way of of creating a generator via a comprehension expression (original) (raw)

Steve Holden steve at holdenweb.com
Thu Nov 23 04:14:56 EST 2017

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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