sedawkgrep sedawkgrep - 25 days ago 9
Python Question

Asyncio doesn't execute the tasks asynchronously

I'm playing around with

asyncio
module of Python and I don't know what's the problem with my simple code. It doesn't execute the tasks asynchronously.

#!/usr/bin/env python3

import asyncio
import string


async def print_num():
for x in range(0, 10):
print('Number: {}'.format(x))
await asyncio.sleep(1)

print('print_num is finished!')

async def print_alp():
my_list = string.ascii_uppercase

for x in my_list:
print('Letter: {}'.format(x))
await asyncio.sleep(1)

print('print_alp is finished!')


async def msg(my_msg):
print(my_msg)
await asyncio.sleep(1)


async def main():
await msg('Hello World!')
await print_alp()
await msg('Hello Again!')
await print_num()


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


Here's the output when the script is called:

Hello World!
Letter: A
Letter: B
Letter: C
Letter: D
Letter: E
Letter: F
Letter: G
Letter: H
Letter: I
Letter: J
Letter: K
Letter: L
Letter: M
Letter: N
Letter: O
Letter: P
Letter: Q
Letter: R
Letter: S
Letter: T
Letter: U
Letter: V
Letter: W
Letter: X
Letter: Y
Letter: Z
print_alp is finished!
Hello Again!
Number: 0
Number: 1
Number: 2
Number: 3
Number: 4
Number: 5
Number: 6
Number: 7
Number: 8
Number: 9
print_num is finished!

Answer

You are calling the functions sequentially, so the code also executes sequentially. Remember that await this means "do this and wait for it to return" (but in the meantime, if this chooses to suspend execution, other tasks which have already started elsewhere may run).

If you want to run the tasks asynchronously, you need to:

async def main():
    await msg('Hello World!')
    task1 = asyncio.ensure_future(print_alp())
    task2 = asyncio.ensure_future(print_num())
    await asyncio.gather(task1, task2)
    await msg('Hello Again!')

See also the documentation of the asyncio.gather function. Alternatively, you could also use asyncio.wait.