execute parallel commands in python

parallel_exec

def parallel_exec(commands_arr):
    processes = set()
    max_processes = 8
    for cmd in commands_arr:
        print(cmd)
        processes.add(subprocess.Popen( cmd ) )
        while len(processes) >= max_processes:
            time.sleep(1)
            diffP = [p for p in processes if p.poll() is not None]
            processes.difference_update(diffP)

    # wait for unfinished processes
    while len(processes):
        time.sleep(.5)
        diffP = [p for p in processes if p.poll() is not None]
        processes.difference_update(diffP)

    print('DONE')
    sys.exit(0)