How to not await in a loop with asyncio?

Question

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 ?

Answer 1

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.