CODEWITHSUNDEEP
python-3.xpython-asyncio
rumdrums
rumdrums

Reputation: 1382

Understanding asyncio in python

I've been having a hard time understanding Python's asyncio module and how to not block on asynchronous calls. For instance, given this code snippet:

import aiohttp
import asyncio
import async_timeout

async def fetch(session, url):
    with async_timeout.timeout(10):
        async with session.get(url) as response:
            return await response.text()

async def main(loop):
    print(1)
    async with aiohttp.ClientSession(loop=loop) as session:
        html = await fetch(session, 'http://python.org')
        print(html)
    print(2)

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

I would expect, similar to how this would work in Javascript, the output to be 

1
2
<!doctype html>
<...>

Instead, the function prints 1, blocks until it gets back the html, then prints 2. Why does it block, and how / can I avoid the blocking? Thanks for your help.

Upvotes: 2

Views: 1036

Answers (2)

dekassert
dekassert

Reputation: 31

To understanding asyncio in python 3.6 async def style I suggest you to run this simple example:

import asyncio
import time

def write(msg):
    print(msg, flush=True)

async def say1():
    await asyncio.sleep(1)
    write("Hello 1!")

async def say2():
    await asyncio.sleep(1)
    write("Hello 2!")

write("start")
loop = asyncio.get_event_loop()
loop.run_until_complete(asyncio.gather(
    say1(),
    say2()
))
write("exit")

loop.close()

Source: https://maketips.net/tip/146/parallel-execution-of-asyncio-functions See link for detailed explanations of this code with call diagram .

Upvotes: 3

etlsh
etlsh

Reputation: 701

Your problem is this line: 

html = await fetch(session, 'http://python.org')

this means - wait until getting a response from fetch, Or in other word's it's blocking the current coroutine until the fetch coroutine return's the response text.

To make things simpler:

  • The fetch operation by itself isn't blocking, only the await.
  • The await is blocking only for the current coroutine, not the thread.
  • Like vincent said, calling the operation by itself doesn't do anything (we don't start the fetching process)

from asyncio documentation

Calling a coroutine does not start its code running – the coroutine object returned by the call doesn’t do anything until you schedule its execution. There are two basic ways to start it running: call await coroutine or yield from coroutine from another coroutine (assuming the other coroutine is already running!), or schedule its execution using the ensure_future() function or the AbstractEventLoop.create_task() method.

And a fix (for getting the desired output):

import asyncio
import aiohttp
import async_timeout


async def fetch(session, url):
    with async_timeout.timeout(10):
        async with session.get(url) as response:
            return await response.text()


async def main(loop):
    print(1)
    async with aiohttp.ClientSession(loop=loop) as session:
        future = asyncio.ensure_future(fetch(session, 'http://python.org')) # we start the fetch process
        print(2)
        html = await future # we wait for getting the fetch response
        print(html)


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

Upvotes: 3

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