Python multiprocessing: processes do not run in parallel

Question

Im new at learning a multiprocessing module in python. I tried to run a following simple code:

import multiprocessing, time

def print_squares(number):
    for i in range(number):
        print("square of {0} is {1}".format(i , i*i))
        time.sleep(0.1)

def print_cubes(number):
    for j in range(number):
        print("cube of {0} is {1}".format(j, j*j*j))
        time.sleep(0.1)

if __name__ == "__main__":
    process_1 = multiprocessing.Process(target = print_squares, args = (10,))
    process_2 = multiprocessing.Process(target = print_cubes, args = (10,))

    process_1.start()
    process_2.start()

    process_1.join()
    process_2.join()

So, I encountered a following trouble: I expected that two processes will print cubes and squares mixed, by working in parallel, like

square of 0 is 0

cube of 0 is 0

square of 1 is 1

cube of 1 is 1

and so on. Instead of behaving like described, my script prints:

cube of 0 is 0
cube of 1 is 1
cube of 2 is 8
cube of 3 is 27
cube of 4 is 64
cube of 5 is 125
cube of 6 is 216
cube of 7 is 343
cube of 8 is 512
cube of 9 is 729
square of 0 is 0
square of 1 is 1
square of 2 is 4
square of 3 is 9
square of 4 is 16
square of 5 is 25
square of 6 is 36
square of 7 is 49
square of 8 is 64
square of 9 is 81

It is obvious that processes do not run in parallel, and second process starts only after first one has finished.

Also, when i run a similar code but using Threads instead of Processes, it works as I want.

Im using windows 10 and python 3.8. I will be grateful for any information how to solve this problem and make two processes work at the same time, thanks in advance!

Answer 1

The code is correct, it should run in parallel. Here's the output on my machine (linux mint + python3.8):

square of 0 is 0
cube of 0 is 0
square of 1 is 1

Maybe there's some stdout buffering, and print calls happen at the right time, but the streams are flushed at the end, try:

print("cube of {0} is {1}".format(j, j * j * j), flush=True)

or

sys.stdout.flush()