Simple example of stream with Asyncio operations

Start by reproducing the Python example from the issue with its aiohttp, asyncio, and Stream(asynchronous=True) pipeline. Read the streamz asyncio-related implementation and existing tests, if present, to determine whether async map operations are supported. Done means the reported pipeline runs correctly or the supported limitation is documented.

Hi,

I've been following the docs and reading through the tests, and I cannot get streamz working with Asyncio :/

Here's a very minimal example of a stream comprising of two async operation and one sync :

1. we retrieve content via an aiohttp call
2. return content length
3. simulate DB write with an Asyncio sleep
4. sink to stdout

import asyncio

import aiohttp
from streamz import Stream


async def fetch(url):
    print("fetching url {}", url)
    async with aiohttp.ClientSession() as session:
        async with session.get(url) as resp:
            print(resp.status)
            body = await resp.text()
    print("Finished w/ url {}", url)
    return body


def count(x):
    print("I", x)
    return len(x)


async def write(x):
    await asyncio.sleep(0.2)
    print("O", x)
    return x

async def f():
    print("Starting stream")
    source = Stream(asynchronous=True)
    source.map(fetch).map(count).map(write).sink(print)
    urls = [
        'https://httpstatus.io/?i=1',
        'https://httpstatus.io/?i=2',
        'https://httpstatus.io/?i=3',
        'https://httpstatus.io/?i=4',
        'https://httpstatus.io/?i=5',
        'https://httpstatus.io/?i=6',
    ]
    for u in urls:
        await source.emit(u)


if __name__ == '__main__':
    asyncio.run(f())

I've tried a lot of combinations using tornado event loop etc. but didn't manage to get anything working.
Is this supposed to be possible or is the Asyncio support still behind?
Am I missing something obvious?

Thanks for the help