[Python-Dev] Using async/await in place of yield expression

[David Mertz]

Changing subject line because this is way off to the side.  Guido and
Nathaniel point out that you can do everything yield expressions do with
async/await *without* an explicit event loop.  While I know that is true,
it feels like the best case is adding fairly considerable ugliness to the
code in the process.


> On Sat, Nov 25, 2017 at 3:37 PM, Guido van Rossum <guido at python.org>
> wrote:
> > Maybe you didn't realize async/await don't need an event loop? Driving an
> > async/await-based coroutine is just as simple as driving a
> yield-from-based
> > one (`await` does exactly the same thing as `yield from`).
>


> On Sun, Nov 26, 2017 at 12:29 PM, Nathaniel Smith <njs at pobox.com> wrote:
> Technically anything you can write with yield/yield from could also be
> written using async/await and vice-versa, but I think it's actually
> nice to have both in the language.
>

Here is some code which is definitely "toy", but follows a pattern pretty
similar to things I really code using yield expressions:

In [1]: from itertools import takewhile
In [2]: def injectable_fib(a=1, b=2):
   ...:     while True:
   ...:         new = yield a
   ...:         if new is not None:
   ...:             a, b = new
   ...:         a, b = b, a+b
   ...:
In [3]: f = injectable_fib()
In [4]: list(takewhile(lambda x: x<200, f))
Out[4]: [1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144]
In [5]: f.send((100,200))
Out[5]: 200
In [6]: list(takewhile(lambda x: x<1000, f))
Out[6]: [300, 500, 800]


Imagining that 'yield' vanished from the language tomorrow, and I wanted to
write the same thing with async/await, I think the best I can come up with
is... actually, I just don't know who to do it without any `yield`.

I can get as far as a slightly flawed:

In [9]: async def atakewhile(pred, coro):
   ...:     l = []
   ...:     async for x in coro:
   ...:         if pred(x):
   ...:             return l
   ...:         l.append(x)


But I just have no idea what would go in the body of

async def afib_injectable():


(that is, if I'm prohibited a `yield` in there)

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