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

[Ivan Levkivskyi]

On 22 November 2017 at 21:07, Yury Selivanov <yselivanov.ml at gmail.com>
wrote:

> On Wed, Nov 22, 2017 at 2:37 PM, Ivan Levkivskyi <levkivskyi at gmail.com>
> wrote:
> > On 22 November 2017 at 20:33, Guido van Rossum <guido at python.org> wrote:
> >>
> >> On Wed, Nov 22, 2017 at 11:12 AM, Ivan Levkivskyi <levkivskyi at gmail.com
> >
> >> wrote:
> >>>
> >>> On 22 November 2017 at 20:05, Guido van Rossum <guido at python.org>
> wrote:
> >>>>
> >>>> On Wed, Nov 22, 2017 at 10:54 AM, Jelle Zijlstra
> >>>> <jelle.zijlstra at gmail.com> wrote
> >>>>>
> >>>>> 2017-11-22 9:58 GMT-08:00 Guido van Rossum <guido at python.org>:
> >>>>>
> >>>>> (OTOH, await in the same position must keep working since it's not
> >>>>> broken and not unintuitive either.)
> >>>>
> >>>>
> >>>
> >>>
> >>> This is very questionable IMO.
> >>> So do you think that [await x for y in z] and list(await x for y in z)
>
> Comprehensions are declarative, and that's why [], and {} work with
> async/await.  When you're using parens () you *explicitly* tell Python
> compiler that you want a generator expression.
>
> And the distinction between comprehensions and generator expressions
> also exists for synchronous code:
>
>    x = [a for a in range(10)]
>    x[0]
>
> and
>
>    x = (a for a in range(10))
>    x[0]  # TypeError
>
> Is the above "intuitive" for all Python users?  Probably not.  Write
> it once, get your TypeError, read the error message and you understand
> what's going on here.
>
> Is the difference between "[await x for y in z ]" and "list(await x
> for y in z)" intuitive for all Python users?  Again, probably not.
> But for those who write async code it is.
>

Just found another example of intuitive behaviour:

>>> async def f():
...     for i in range(3):
...         yield i
...
>>> async def g():
...     return [(yield i) async for i in f()]
...
>>> g().send(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in g
TypeError: object async_generator can't be used in 'await' expression

of course it is obvious for anyone who writes async code, but anyway an
interesting example.

--
Ivan
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