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

[Caleb Hattingh]

On 27 November 2017 at 13:20, David Mertz <mertz at gnosis.cx> wrote:
>
> 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 recently had to look into these things with quite some detail. When using
`yield` for *iteration* specifically, you cannot use async/await to replace
it. I find it easiest to think about all this in the context of the ABC
table:

https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes

The `Generator` ABC also implement the `Iterator` protocol so this allows
"normal" iteration to work, i.e. for-loop, while-loop, and comprehensions
and so on. In contrast, the `Coroutine` ABC implements only send(), throw()
and close().  This means that if you want to iterate a coroutine,
*something* must drive send(), and Python's iteration syntax features don't
do that.  async/await is only useful when an event loop drives coroutines
using the Coroutine ABC protocol methods. Note that AsyncIterable and
AsyncIterator doesn't help because objects implementing these protocols may
only legally appear inside a coroutine, i.e. an `async def` coroutine
function, which you still cannot drive via the Iterator protocol (e.g.,
from a for-loop).

The Coroutine ABC simply doesn't implement the Iterator protocol, so it
seems it cannot be a replacement for generators. It is however true that
`async/await` completely replaces `yield from` for *coroutines*, but both
of those required a loop of some kind.

I'd be very grateful if anyone can point out if my understanding of the
above is incorrect.  Private email is fine if you prefer not to post to the
list.

rgds
Caleb
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