Python asyncio function call inside loop

Question

I have a server and client where multiple clients will be connecting to multiple server and does something. I want to make client connection to server async.

clients = [1,2,3]
for c in clients:
  connect_to_server()

right now the above code connects to server but waits for first iteration to be executed and then second. how to make it async function call to connect_to_server() method so that second iteration will not wait for first to be executed ? And which function has to be async either connect_to_server or for loop function and which has to be awaiting ?


def connect_to_server(client_id):
    print(client_id)
    time.sleep(3)


async def main():
    clients = [1, 2, 3, 4]
    for client in clients:
       await connect_to_server(client)

asyncio.run(main())

Khushal · Accepted Answer · 2021-08-20 17:40:11Z

1

Here wrap the code you want to be async in an async function,

import asyncio

async def main():

    clients = [1,2,3]

    for c in clients:
        await connect_to_server()

asyncio.run(main())

Explaination : here asyncio calls your async "MAIN" function and "AWAIT" in the function makes the process proceed to the next iteration without waiting for connect_to_server() function to finish !

answered Aug 20, 2021 at 17:40

Khushal

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4 Comments

user12797847 Over a year ago

I tried the above and I get this error TypeError: object NoneType can't be used in 'await' expression

Khushal Over a year ago

@VimalanE what does the connect_to_server function does ? Is it even defined or are you running this snippet as it is ?

user12797847 Over a year ago

I have updated the code for you in my question have a look

user12797847 Over a year ago

just for learning am printing the argument in connect_to_server which is not async function

Tarik · Accepted Answer · 2024-05-22 11:47:15Z

Your code will run at least for as much time is necessary to run the syncronous code. The following code illustrates this:

import asyncio
import time

async def connect_to_server(client_id):
    print(client_id)
    print(f'sleep for client_id {client_id}')
    time.sleep(1)
    print(f'async sleep for client_id {client_id}')
    await asyncio.sleep(1)
    print(f'done for client_id {client_id}')

async def main():
    clients = range(10)
    
    # Launch all of the coroutines in parallel
    coroutines = [connect_to_server(client) for client in clients]
    
    # Wait for all corountines to complete
    await asyncio.gather(*coroutines)

start = time.time()
asyncio.run(main())
print(f'Time elapsed: {time.time() - start} seconds')

The code above takes no less than 11 seconds to complete.

Beware! The following code calls await connect_to_server in a loop:

import asyncio
import time

async def connect_to_server(client_id):
    print(client_id)
    print(f'sleep for client_id {client_id}')
    time.sleep(1)
    print(f'async sleep for client_id {client_id}')
    await asyncio.sleep(1)
    print(f'done for client_id {client_id}')

async def main():
    clients = range(10)
    
    # Launch all of the coroutines in parallel
    for client in clients:
        await connect_to_server(client)
    
start = time.time()
asyncio.run(main())
print(f'Time elapsed: {time.time() - start} seconds')

The above code takes no less than 20 seconds to complete.

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