[Python-Dev] Using async/await in place of yield expression (original) (raw)

Chris Angelico rosuav at gmail.com
Sun Nov 26 22:43:55 EST 2017

On Mon, Nov 27, 2017 at 2:20 PM, David Mertz <mertz at gnosis.cx> wrote:

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 injectablefib(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 = injectablefib() 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 afibinjectable(): (that is, if I'm prohibited a yield in there)

Honestly, this is one of Python's biggest problems when it comes to async functions. I don't know the answer to that question, and I don't know where in the docs I'd go looking for it. In JavaScript, async functions are built on top of promises, so you can just say "well, you return a promise, tada". But in Python, this isn't well documented. Snooping the source code for asyncio.sleep() shows that it uses @coroutine and yield, and I have no idea what magic @coroutine does, nor how you'd use it without yield.

ChrisA

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