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

Answer 1

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()

• Mihika: Don't we need to perform loop.close()?
• Yuval Pruss: We do, added :) @Mihika

Answer 2

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())