How do I access the return value from an async function in Python?

Question

If I try running the following code in Python, I get the following error:

async def foo():
    yield 1

async def bar():
    x = await foo()
b=bar()
b.send(None)

TypeError: object async_generator can't be used in 'await' expression

On the other hand, the following code works (and throws a StopIteration, but this is expected):

async def foo():
    pass

async def bar():
    await foo()
b=bar()
b.send(None)

Why doesn't this work?

I can make it work if I replace foo with:

@coroutine
def foo():
    yield 1

The problem here is that this seems weird enough that I'm pretty sure this isn't the recommended way of getting this behaviour. Then in most languages you just need async and await, not @coroutine too!

Daniele Bacarella · Accepted Answer · 2017-09-29 08:38:56Z

1

I'm assuming here that you are using asyncio library.
I think that the use of the decorator @coroutine is suggested for compatibility reasons with async def functions,

although this is not strictly enforced.

asyncio.coroutine documentation

Also

@asyncio.coroutine Decorator to mark generator-based coroutines. This enables the generator use yield from to call async def coroutines, and also enables the generator to be called by async def coroutines, for instance using an await expression.

There is no need to decorate async def coroutines themselves.

answered Sep 29, 2017 at 8:38

Daniele Bacarella

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