How to append values to the same array from two different functions?

Question

Current Code

import multiprocessing as mu
import time

global_array=[]

def add_array1(array):
    while True:
        time.sleep(2.5)
        global_array.append(1)

        print(global_array)

def add_array2(array):
    while True:
        time.sleep(3)
        global_array.append(2)

        print(global_array)

def runInParallel(*fns):
      if __name__=='__main__':
        proc = []
      for fn in fns:
        p = mu.Process(target=fn)
        p.start()
        proc.append(p)
      for p in proc:
        p.join()

runInParallel(
    add_array1(global_array),
    add_array2(global_array)
    )

When running my code above only the first function add_array1() is appending the value to the array and printing instead of both functions providing the wrong output:

[1]
[1,1]
[1,1,1]

When the actual desired output for the following code is:

[1]
[1,2]
[1,2,1]
[1,2,1,2]

you are calling the functions, you probably need to provide them only: — Patrick Artner
– Patrick Artner, Commented Oct 25, 2020 at 23:17
what do you mean? I need them to run simultaneously as well. — Philip Julian
– Philip Julian, Commented Oct 25, 2020 at 23:19
runInParallel(add_array1(global_array),add_array2(global_array)) EXEcutes the functions not in parallel — Patrick Artner
– Patrick Artner, Commented Oct 25, 2020 at 23:21

Patrick Artner · Accepted Answer · 2020-10-25 23:35:39Z

3

Your problem is that the function call

runInParallel( add_array1(global_array), add_array2(global_array))

executes the functions and provides the return value of the function calls as parameters to runInParallel. As add_array1 is an endless loop, it never returns from the execution. You need to provide your functions as functions - not the returnvalue of the functions as parameters to runInParallel(...)

Start with

runInParallel( add_array1, add_array2) # name of the functions, dont execute em

and change

def runInParallel(*fns): 
    proc = []
    for fn in fns:
        p = mu.Process(target=fn, args=(global_array,)) # provide param here
        p.start()
        proc.append(p)
    for p in proc:
        p.join()

and then fix the "not joining" problem due to your threaded functions never returning.

Example from the official documentation of multiprocessing.Process:

from multiprocessing import Process 
import os

def info(title):
    print(title)
    print('module name:', __name__)
    print('parent process:', os.getppid())
    print('process id:', os.getpid())

# Function name is f
def f(name):
    info('function f')
    print('hello', name)

if __name__ == '__main__':
    info('main line')
    # f is provided, and args is provided - not f("bob")
    p = Process(target=f, args=('bob',))
    p.start()
    p.join()

edited Oct 25, 2020 at 23:35

answered Oct 25, 2020 at 23:23

Patrick Artner

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3 Comments

Philip Julian Over a year ago

so are you suggesting that I don't give it parameters just call the function and let it append to the global_array?

Patrick Artner Over a year ago

@Philip See the quoted example from the documentation for how to call/provide a function that takes parameters

Patrick Artner Over a year ago

@philip Your function have endless loops, they will never return - so they will also never join. You need to fix that as well

wind · Accepted Answer · 2020-10-25 23:36:05Z

1

You can use the the code below to get your desired output:

import threading
import time

global_array = []


def add_array1():
    while True:
        time.sleep(2.5)
        global_array.append(1)

        print(global_array)


def add_array2():
    while True:
        time.sleep(3)
        global_array.append(2)

        print(global_array)


def runInParallel(*fns):
    if __name__ == '__main__':
        proc = []
    for fn in fns:
        p = threading.Thread(target=fn)
        p.start()
        proc.append(p)
    for p in proc:
        p.join()


if __name__ == '__main__':
    runInParallel(
        add_array1,
        add_array2
    )

edited Oct 25, 2020 at 23:36

answered Oct 25, 2020 at 23:14

wind

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1 Comment

Philip Julian Over a year ago

I already have the if __name__=='__main__': in my runInParallell() function.

erfan_msv_0081 · Accepted Answer · 2020-10-26 01:23:34Z

0

you can use this simple code to get your desired output:

'''

import time


global_array = []


def add_array1(array):


    while True:
        time.sleep(2.5)
        if len(global_array) % 2 == 0:

            global_array.append(1)
            print(global_array)
    
        else:
            global_array.append(2)
            print(global_array)


add_array1(global_array)

answered Oct 26, 2020 at 1:23

erfan_msv_0081

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