Using asyncio in Python

Question

I have just started learning asyncio in Python, and wrote the following code:

import asyncio
loop = asyncio.get_event_loop()


async def hello():
    print("Hello")
    await asyncio.sleep(3)
    print("World")

if __name__ == '__main__':
    for i in range(3):
        loop.run_until_complete(hello())
    loop.close()

This gives me the following output:

Hello
<waits for 3 seconds>
World
Hello
<waits for 3 seconds>
World
Hello
<waits for 3 seconds>
World

How can I modify the program so that the same is handled asynchronously (i.e. once the first task sleeps, the second one begins executing), essentially giving output similar to this:

Hello
Hello
Hello
World
World
World

Yuval Pruss · Accepted Answer · 2019-01-22 16:33:22Z

5

You can asyncio.gather them.

import asyncio


async def hello():
    print("Hello")
    await asyncio.sleep(3)
    print("World")

async def lots_of_hello():
    cur_list = []
    for _ in range(3):
        cur_list.append(hello())
    await asyncio.gather(*cur_list)

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    loop.run_until_complete(lots_of_hello())
    loop.close()

edited Jan 22, 2019 at 16:33

answered Jan 22, 2019 at 12:23

Yuval Pruss

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2 Comments

Mihika Over a year ago

Don't we need to perform loop.close()?

Yuval Pruss Over a year ago

We do, added :) @Mihika

Stanowczo · Accepted Answer · 2019-01-22 12:26:56Z

3

I'm also new to asyncio, but I got curious reading your question and tried to explore it a little bit. This is what I got, something different than the previous answer, with an extra candy of variable sleep time ;) :

import asyncio


async def hello(timeToSleep):
    print("Hello")
    await asyncio.sleep(timeToSleep)
    print("World")

async def main():
    tasks = []
    for i in range(3):
        tasks.append(asyncio.create_task(hello(i)))
    for task in tasks:
        await task

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

answered Jan 22, 2019 at 12:26

Stanowczo

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