How to do a loop with multiprocessing

Question

I want a number to go up in the background while waiting for user input but my code doesn't work anyone know why or how to fix

import multiprocessing as mp
num = 0
def add():
  global num
  while True:
    num += 1
def ask():
  global num
  x = input("Input:")
  print(x)
  print(num)
if __name__ == "__main__":
  p2 = mp.Process(target=ask())
  p1 = mp.Process(target=add())
  p1.start()
  p2.start()
  p1.join()
  p2.join()

this is what I have tried

switch to threading ... or you will need to look into multiprocessing shared memory or a different way of passing the num back to the other thing ... but ask is waiting for input and cannot print until the user presses enter ... add should probably just print in its own space ... you can use ansi codes to print at specific spots and clear lines etc `print("\e[31mRED TEXT\e[0m")` or something like that — Joran Beasley, Oct 27 '22 at 16:10
I want it so when user puts a input it prints the number I complete forgot that processes don't share memory lol — Estronoid, Oct 27 '22 at 16:14
Similar question about using `input()` with multiprocessing https://stackoverflow.com/questions/5697305/python-command-line-input-in-a-process. Also you need change `mp.Process(target=ask())` to `mp.Process(target=ask)` — u1234x1234, Oct 27 '22 at 17:32

score 1 · Answer 1 · answered Oct 28 '22 at 21:13

The target argument of the mp.Process initializer is a reference to a function that will be called within a new process. But you are calling ask and add and passing their return values, which are None, as the target arguments. This is clearly a problem.

The other two problems you have are:

You can only call the input function from the main process. Generally this also has to be done by the initial, main thread of the main process - unless you go to special trouble that should be avoided.
Variable num being referenced by the child processes you are creating are not referencing the same num instances. Each process has its own address space and either inherits a copy of num (e.g. under Linux) or creates a new num in its own address space (e.g. under Windows). For all processes to see the same instance of num it must be located in shared memory.

Note that I have modified the code so that add ultimately terminates and I have added calls to time.sleep to "slow everything down" so that the child process does not terminate before you have a chance to input a number:

import multiprocessing as mp
import time

def add(num):
    while num.value < 10:
        num.value += 1
            time.sleep(1)

if __name__ == "__main__":
    # A shared memory integer value initialized to 0:
    num = mp.Value('i', 0)
    p = mp.Process(target=add, args=(num,))
    p.start()

    x = input("Input: ")
    print(x)
    print(num.value)

    p.join()

If you were using multithreading instead, which is not appropriate if function add is mostly CPU-bound like your original version without the calls to time.sleep is, then everything would be running in the same address space of a single process and there would be no problem with using a variable num that is not in shared memory:

import threading
import time

num = 0

def add():
    global num

    while num < 10:
        num += 1
        time.sleep(1)

if __name__ == "__main__":
    t = threading.Thread(target=add)
    t.start()

    x = input("Input: ")
    print(x)
    print(num)

    t.join()

How to do a loop with multiprocessing

1 Answers1