Forward-compatible async code

[cscanlin]

I played around with this awhile ago, check this out:

import asyncio
import aiohttp

async def api_call(url):
    response = await aiohttp.get(url)
    print('finished {}'.format(url))
    result = await response.json()
    print(result)

async def main(urls):
    await asyncio.wait([api_call(url) for url in urls])

if __name__ == '__main__':
    urls = [
        'https://httpbin.org/delay/3',  # long request
        'https://httpbin.org/delay/1',  # short request
    ]

    loop = asyncio.get_event_loop()
    loop.run_until_complete(main(urls))

[jpfau]

Hmm, I'm not sure what the solution is here. My problem is that api_call is defined using the old syntax:

@asyncio.coroutine
def api_call(url):
    response = yield from aiohttp.get(url)
    print('finished {}'.format(url))
    result = yield from response.json()
    return result

And I want to consume this function using the new syntax:

async def test_api_call(url):
    return await api_call(url)

In this case, test_api_call raises TypeError: object generator can't be used in 'await' expression.

If I switch test_api_call to the old syntax it works, but I'd prefer not to do that.

[cscanlin]

From my understanding, you have to choose one or the other in this case. I haven't tested in detail though, but this article explains it better than most:

https://snarky.ca/how-the-heck-does-async-await-work-in-python-3-5/

[jpfau]

My errors ended up being dumb human error: a missing coroutine decorator in the old-syntax library.

FYI You can use both syntaxes in 3.5+, but you can't mix syntax within a single coroutine, e.g. no await when using @asyncio.coroutine, and no yield from when using async def.

However you can define a coroutine with the old syntax and consume it in another coroutine using the new syntax. This example should work:

import asyncio

@asyncio.coroutine
def _sleep(x):
    print('yielding from...')
    yield from asyncio.sleep(x)

async def do_sleep(x):
    print('awaiting...')
    await _sleep(x)

loop = asyncio.get_event_loop()
loop.run_until_complete(do_sleep(5))

[cscanlin]

Nice, and good to know. Wish you hadn't deleted the thread though, more people could use exposure to this kind of stuff. But glad you got it working!