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

Caleb Hattingh caleb.hattingh at gmail.com
Mon Nov 27 00:33:51 EST 2017

On 27 November 2017 at 14:53, Yury Selivanov <yselivanov.ml at gmail.com> wrote:

It is correct. While 'yield from coro()', where 'coro()' is an 'async def' coroutine would make sense in some contexts, it would require coroutines to implement the iteration protocol. That would mean that you could write 'for x in coro()', which is meaningless for coroutines in all contexts. Therefore, coroutines do not implement the iterator protocol.

The two worlds (iterating vs awaiting) collide in an interesting way when one plays with custom Awaitables.

From your PEP, an awaitable is either a coroutine, or an object implementing await, and that await returns an iterator.

The PEP only says that await must return an iterator, but it turns out that it's also required that that iterator should not return any intermediate values. This requirement is only enforced in the event loop, not in the await call itself. I was surprised by that:

class A: ... def await(self): ... for i in range(3): ... yield i # <--- breaking the rules, returning a value ... return 123

async def cf(): ... x = await A() ... return x

c = cf() c.send(None) 0 c.send(None) 1 c.send(None) 2 c.send(None) Traceback (most recent call last): File "", line 1, in StopIteration: 123 123

So we drive the coroutine manually using send(), and we see that intermediate calls return the illegally-yielded values. I broke the rules because my await iterator is returning values (via yield i) on each iteration, and that isn't allowed because the event loop wouldn't know what to do with these intermediate values; it only knows that "awaiting" is finished when a value is returned via StopIteration. However, you only find out that it isn't allowed if you use the loop to run the coroutine function:

import asyncio loop = asyncio.geteventloop() loop.rununtilcomplete(f()) Traceback (most recent call last): File "", line 1, in File "/usr/lib64/python3.6/asyncio/base_events.py", line 467, in run_until_complete return future.result() File "", line 2, in f File "", line 4, in await RuntimeError: Task got bad yield: 0 Task got bad yield: 0

I found this quite confusing when I first came across it, before I understood how asyncio/async/await was put together. The await method implementation must return an iterator that specifically doesn't return any intermediate values. This should probably be explained in the docs. I'm happy to help with any documentation improvements if help is desired.

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