Concurrency async issue with python

Question

I am new to python, and my trying to resolve a simple issue: I have two blocks of operations (Block A and Block B), every block execute different shell commands that should be executed async inside the block (in the same block I can execute a new operation, without having to wait the result of the first one).

I can start running the operations of 'Block B' only when all the operations of 'Block A' are finished.

import asyncio
import subprocess
import sys
import threading


async def run_command(number, timeSleep):
    cmd = "(echo '"+ number +" Start -->' $(date) ;sleep "+timeSleep +" ;echo '"+ number +" End -->' $(date) ) >> /tmp/log.txt"

    p = subprocess.Popen(cmd, stderr=subprocess.PIPE, shell=True)

async def block_a():
    await asyncio.gather(
        run_command("Block A: Operation 1","8"),
        run_command("Block A: Operation 2","4"),
        run_command("Block A: Operation 3","2"))

async def block_b():
    await asyncio.gather(
        run_command("Block B Operation 1","5"),
        run_command("Block B Operation 2","4"),
        run_command("Block B Operation 3","1"))


async def main():
    await asyncio.gather(
        block_a(),
        block_b())

if __name__ == '__main__':
    # Create the asyncio event loop
    loop = asyncio.get_event_loop()
    try:
        loop.run_until_complete(main())
    finally:
        # Shutdown the loop even if there is an exception
        loop.close()

I am getting the following output:

Block A: Operation 2 Start --> Thu Apr 9 21:21:27 CEST 2020
Block A: Operation 3 Start --> Thu Apr 9 21:21:27 CEST 2020
Block A: Operation 1 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 1 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 2 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 3 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 3 End --> Thu Apr 9 21:21:28 CEST 2020
Block A: Operation 3 End --> Thu Apr 9 21:21:29 CEST 2020
Block A: Operation 2 End --> Thu Apr 9 21:21:31 CEST 2020
Block B Operation 2 End --> Thu Apr 9 21:21:31 CEST 2020
Block B Operation 1 End --> Thu Apr 9 21:21:32 CEST 2020
Block A: Operation 1 End --> Thu Apr 9 21:21:35 CEST 2020

I am expecting to start operations of Block B after that operations of Block A are completed.
The output that I was expecting is something like:

Block A: Operation 2 Start --> Thu Apr 9 21:21:27 CEST 2020
Block A: Operation 3 Start --> Thu Apr 9 21:21:27 CEST 2020
Block A: Operation 1 Start --> Thu Apr 9 21:21:27 CEST 2020
Block A: Operation 3 End --> Thu Apr 9 21:21:29 CEST 2020
Block A: Operation 2 End --> Thu Apr 9 21:21:31 CEST 2020
Block A: Operation 1 End --> Thu Apr 9 21:21:35 CEST 2020
Block B Operation 1 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 2 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 3 Start --> Thu Apr 9 21:21:27 CEST 2020
Block B Operation 3 End --> Thu Apr 9 21:21:28 CEST 2020
Block B Operation 2 End --> Thu Apr 9 21:21:31 CEST 2020
Block B Operation 1 End --> Thu Apr 9 21:21:32 CEST 2020

edd · Accepted Answer · 2020-04-09 19:37:23Z

2

You have to use the asyncio module to create subprocesses, rather than the subprocess module directly.
From the docs:

import asyncio

async def run(cmd):
    proc = await asyncio.create_subprocess_shell(
        cmd,
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.PIPE)

    stdout, stderr = await proc.communicate()

    print(f'[{cmd!r} exited with {proc.returncode}]')
    if stdout:
        print(f'[stdout]\n{stdout.decode()}')
    if stderr:
        print(f'[stderr]\n{stderr.decode()}')

asyncio.run(run('ls /zzz'))

answered Apr 9, 2020 at 19:37

edd

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