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On 27 November 2017 at 14:53, Yury Selivanov <yselivanov.ml@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.get\_event\_loop()
>>> loop.run\_until\_complete(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.

rgds

Caleb