[Python-ideas] PEP 530: Asynchronous Comprehensions

[Andrew Svetlov]

On Wed, Sep 7, 2016 at 2:31 PM Nick Coghlan <ncoghlan at gmail.com> wrote:

> On 4 September 2016 at 09:31, Yury Selivanov <yselivanov.ml at gmail.com>
> wrote:
> > With the proposed asynchronous comprehensions syntax, the above code
> > becomes as short as::
> >
> >     result = [i async for i in aiter() if i % 2]
>
> After using it a few times in examples, while I'm prepared to accept
> the agrammatical nature of "async for" in the statement form (where
> the adjective-noun phrase can be read as a kind of compound noun
> introducing the whole statement), I think for the comprehension form,
> we should aim to put the words in the right grammatical order if we
> can:
>
>     result = [i for i in async aiter() if i % 2]
>
> Please, no.
It may be totally correct from English grammar POV but brings different
syntax from regular async for statement, e.g.
async for row in db.execute(...):
    pass
Currently regular comprehensions are pretty similar to `for` loop.

Why async comprehensions should look different from `async for` counterpart?


> I think the readability gain from that approach becomes even clearer
> with nested loops:
>
>     result = [x for aiterable in async outer() for x in async aiterable]
>
> vs the current:
>
>     result = [x async for aiterable in outer() async for x in async
> aiterable]
>
> In the first form, "async" is clearly a pre-qualifier on "outer()" and
> "aiterable", indicating they need to be asynchronous iterators rather
> than synchronous ones.
>
> By contrast, in the current form, the first "async" reads like a
> post-qualifer on "x" (it isn't, it modifies how outer() is handled in
> the outer loop), while the second looks like a post-qualifier on
> "outer()" (it isn't, it modified how aiterable is handled in the inner
> loop)
>
> If that means having to postpone full async comprehensions until
> "async" becomes a proper keyword in 3.7 and only adding "await in
> comprehensions and generator expressions" support to 3.6, that seems
> reasonable to me
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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-- 
Thanks,
Andrew Svetlov
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