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

[Yury Selivanov]

On Sat, Nov 25, 2017 at 3:27 PM, Guido van Rossum <guido at python.org> wrote:
> 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.)

Great!

As for PEP 530, after reading this discussion I realized how many
things in it are underspecified.  I'll be working on PEP 550 successor
next week and will also try to update PEP 530 to make it clearer.

> 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.

+1 from me.

Yury