How to not await in a loop with asyncio?

 

Here is a toy example that downloads the home page from several websites using asyncio and aiohttp: 

import asyncio
import aiohttp

sites = [
    "http://google.com",
    "http://reddit.com",
    "http://wikipedia.com",
    "http://afpy.org",
    "http://httpbin.org",
    "http://stackoverflow.com",
    "http://reddit.com"
]


async def main(sites):
    for site in sites:
        download(site)


async def download(site):
    response = await client.get(site)
    content = await response.read()
    print(site, len(content))


loop = asyncio.get_event_loop()
client = aiohttp.ClientSession(loop=loop)
content = loop.run_until_complete(main(sites))
client.close()

If I run it, I get: 

RuntimeWarning: coroutine 'download' was never awaited

But I don't want to await it.

In twisted I can do: 

for site in sites:
    download(site)

And If I don't explicitly "yield" or add a callback to the returned Deferred, it just runs without blocking nor complaining. I can't access the result, but in this case I don't need it.

In JS I can do: 

site.forEarch(site){
    donwload(site)
}

And again, it doesn't block nor does it requires anything from my part.

I found a way to do: 

async def main(sites):
    await asyncio.wait([download(site) for site in sites])

But:

  • this is really not obvious to find it out. I it's hard to remember.
  • it's hard to understand what it does. "waits" seems to say "i block", but does not convey clearly it block for the entire list of coroutine to finish.
  • you can't pass in a generator, it needs to be a real list, which i feels really unatural in Python.
  • what if I have only ONE awaitable ?
  • what if I don't want to wait at all on my tasks, and just schedule them for execution then carry on with the rest of my code ?
  • it's way more verbose thant twisted and JS solution.

    • It there a better way ?

  • this is really not obvious to find it out. I it's hard to remember.

    • The documentation on coroutines does make it pretty clear what asyncio.wait 's purpose is.

  • it's hard to understand what it does. "waits" seems to say "i block", but does not convey clearly it block for the entire list of coroutine to finish.

    • Again, see the documentation.

  • you can't pass in a generator, it needs to be a real list, which i feels really unatural in Python.

    • Again, see the documentation, specifically asyncio.as_completed

  • what if I have only ONE awaitable ?

    • It should still work.

  • what if I don't want to wait at all on my tasks, and just schedule them for execution then carry on with the rest of my code ?

    • Then you can use asyncio.ensure_furture . In fact, asyncio.wait is a convenience function around asyncio.ensure_future (and some other logic).

  • it's way more verbose thant twisted and JS solution.

    • Maybe, but that's not a bad thing (from my perspective).

    In order to schedule a coroutine as a task, use asyncio.ensure_future: 

    for site in sites:
    coro = download(site)
    future = asyncio.ensure_future(coro)

    It replaces the deprecated function asyncio.async in version 3.4.4.

    Then you can manage those futures using await , asyncio.wait or asyncio.gather.

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