On Mon, Nov 27, 2017 at 2:20 PM, 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
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
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)
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