Multiprocessing shared variables with a loop

Question

function1 generates generates two variables called daydate and numbers. What is to be achieved is that function2 receives this variables, prints and store them in a dataframe. It is very important that the process stays intact.

import random
from  multiprocessing import Process
import time
from datetime import datetime

def function1():
    while True:
        daydate = datetime.now()
        numbers = random.randrange(1,215)
        print(daydate, numbers)
        time.sleep(10)

def function2():
    while True:
        print("Recevied values from function1: (daydate, numbers)")
        time.sleep(10)

if __name__ == "__main__":
    a =Process(target=function1, args=())
    a.start() 
    b =Process(target=function2, args=())
    b.start()
    a.join()
    b.join()

Roland Smith · Accepted Answer · 2017-12-31 10:15:38Z

Probably the most versatile way is to use a multiprocessing.Manager because it can transfer list and dict types. And unlike e.g. Value or Array those are not bound to one type. I have re-worked your code into the example below.

import random
from multiprocessing import Process, Manager
import time
from datetime import datetime


def function1(d):
    while True:
        daydate = datetime.now()
        number = random.randrange(1, 215)
        print('Sent do function2: ({}, {})'.format(daydate, number))
        d['date'] = daydate
        d['number'] = number
        time.sleep(2)


def function2(d):
    while True:
        print("Recevied values from function1: ({}, {})".format(d['date'], d['number']))
        time.sleep(2)


if __name__ == "__main__":
    with Manager() as manager:
        d = manager.dict()
        a = Process(target=function1, args=(d,))
        a.start()
        b = Process(target=function2, args=(d,))
        b.start()
        a.join()
        b.join()

This produces the following output:

> python3 manager.py
Sent do function2: (2017-12-31 10:52:33.405475, 80)
Recevied values from function1: (2017-12-31 10:52:33.405475, 80)
Recevied values from function1: (2017-12-31 10:52:33.405475, 80)
Sent do function2: (2017-12-31 10:52:35.466549, 71)
Sent do function2: (2017-12-31 10:52:37.566320, 138)
Recevied values from function1: (2017-12-31 10:52:35.466549, 71)
Sent do function2: (2017-12-31 10:52:39.601367, 124)
Recevied values from function1: (2017-12-31 10:52:37.566320, 138)
Sent do function2: (2017-12-31 10:52:41.626318, 183)
Recevied values from function1: (2017-12-31 10:52:39.601367, 124)

As you can see, this can retrieve the same value multiple times. To just receive the value once, use a multiprocessing.Queue:

import random
from multiprocessing import Process, Queue
import time
from datetime import datetime


def function1(q):
    while True:
        daydate = datetime.now()
        number = random.randrange(1, 215)
        print('Sent to function2: ({}, {})'.format(daydate, number))
        q.put((daydate, number))
        time.sleep(2)


def function2(q):
    while True:
        date, number = q.get()
        print("Recevied values from function1: ({}, {})".format(date, number))
        time.sleep(2)


if __name__ == "__main__":
    q = Queue()
    a = Process(target=function1, args=(q,))
    a.start()
    b = Process(target=function2, args=(q,))
    b.start()
    a.join()
    b.join()

This produces the following output:

> python3 qtest.py
Sent to function2: (2017-12-31 11:13:34.331509, 54)
Recevied values from function1: (2017-12-31 11:13:34.331509, 54)
Sent to function2: (2017-12-31 11:13:36.337707, 194)
Recevied values from function1: (2017-12-31 11:13:36.337707, 194)
Sent to function2: (2017-12-31 11:13:38.472709, 171)
Recevied values from function1: (2017-12-31 11:13:38.472709, 171)

Sagar T · Accepted Answer · 2017-12-31 08:31:12Z

 Your Program is Correct it shows proper output.
 I run The program and i get OUTPUT

 (datetime.datetime(2017, 12, 31, 12, 27, 19, 930988), 168)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 27, 29, 941160), 12)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 27, 39, 951035), 21)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 27, 49, 958898), 191)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 27, 59, 963084), 118)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 28, 9, 971400), 194)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 28, 19, 978968), 170)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 28, 29, 986960), 40)
   Recevied values from function1: (daydate, numbers)
 (datetime.datetime(2017, 12, 31, 12, 28, 39, 995139), 94)

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