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

[Guido van Rossum]

>
> On Sat, Nov 25, 2017 at 12:17 PM Brett Cannon <brett at python.org> wrote:
>
On Fri, Nov 24, 2017, 19:32 Guido van Rossum, <guido at python.org> wrote:
>>
>>> On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <guido at python.org>
>>> wrote:
>>>
>>>> The more I hear about this topic, the more I think that `await`,
>>>> `yield` and `yield from` should all be banned from occurring in all
>>>> comprehensions and generator expressions. That's not much different from
>>>> disallowing `return` or `break`.
>>>>
>>>
>>> From the responses it seems that I tried to simplify things too far.
>>> Let's say that `await` in comprehensions is fine, as long as that
>>> comprehension is contained in an `async def`. While we *could* save `yield
>>> [from]` in comprehensions, I still see it as mostly a source of confusion,
>>> and the fact that the presence of `yield [from]` *implicitly* makes the
>>> surrounding `def` a generator makes things worse. It just requires too many
>>> mental contortions to figure out what it does.
>>>
>>> I still propose to rule out all of the above from generator expressions,
>>> because those can escape from the surrounding scope.
>>>
>>
>> +1 from me
>>
>
On Sat, Nov 25, 2017 at 9:21 AM, Yury Selivanov <yselivanov.ml at gmail.com>
wrote:

> So we are keeping asynchronous generator expressions as long as they are
> defined in an 'async def' coroutine?
>

I would be happy to declare that `await` is out of scope for this thread.
It seems that it is always well-defined and sensible what it does in
comprehensions and in genexprs. (Although I can't help noticing that PEP
530 does not appear to propose `await` in generator expressions -- it
proposes `async for` in comprehensions and in genexprs, and `await` in
comprehensions only -- but they appear to be accepted nevertheless.)

So we're back to the original issue, which is that `yield` inside a
comprehension accidentally makes it become a generator rather than a list,
set or dict. I believe that this can be fixed. But I don't believe we
should fix it. I believe we should ban `yield` from comprehensions and from
genexprs. We don't need it, and it's confused most everyone. And the ban
should extend to `yield from` in those same contexts. I think we have a
hope for consensus on this.

(I also think that if we had invented `await` earlier we wouldn't have gone
down the path of `yield` expressions -- but historically it appears we
wouldn't have invented `await` at all if we hadn't first tried `yield` and
then `yield from` to build coroutines, so I don't think this so bad after
all. :-)

-- 
--Guido van Rossum (python.org/~guido)
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