Executing multiple functions simultaneously

I’m trying to run two functions simultaneously in Python. I have tried the below code which uses multiprocessing but when I execute the code, the second function starts only after the first is done.

from multiprocessing import Process
def func1:
     #does something

def func2:
     #does something

if __name__=='__main__':
     p1 = Process(target = func1)
     p1.start()
     p2 = Process(target = func2)
     p2.start()

Answers:

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Method 1

You are doing it correctly. 🙂

Try running this silly piece of code:

from multiprocessing import Process
import sys

rocket = 0

def func1():
    global rocket
    print 'start func1'
    while rocket < sys.maxint:
        rocket += 1
    print 'end func1'

def func2():
    global rocket
    print 'start func2'
    while rocket < sys.maxint:
        rocket += 1
    print 'end func2'

if __name__=='__main__':
    p1 = Process(target = func1)
    p1.start()
    p2 = Process(target = func2)
    p2.start()

You will see it print ‘start func1’ and then ‘start func2’ and then after a (very) long time you will finally see the functions end. But they will indeed execute simultaneously.

Because processes take a while to start up, you may even see ‘start func2’ before ‘start func1’.

Method 2

This is just what i needed. I know it wasn’t asked but i modified shashank’s code to suit Python 3 for anyone else looking 🙂

from multiprocessing import Process
import sys

rocket = 0

def func1():
    global rocket
    print ('start func1')
    while rocket < sys.maxsize:
        rocket += 1
    print ('end func1')

def func2():
    global rocket
    print ('start func2')
    while rocket < sys.maxsize:
        rocket += 1
    print ('end func2')

if __name__=='__main__':
    p1 = Process(target=func1)
    p1.start()
    p2 = Process(target=func2)
    p2.start()

Substitute sys.maxsize for an number then print(rocket)and you can see it count up one at a time. Get to a number and stop

Method 3

This can be done elegantly with Ray, a system that allows you to easily parallelize and distribute your Python code.

To parallelize your example, you’d need to define your functions with the @ray.remote decorator, and then invoke them with .remote.

import ray

ray.init()

# Define functions you want to execute in parallel using 
# the ray.remote decorator.
@ray.remote
def func1():
    #does something

@ray.remote
def func2():
    #does something

# Execute func1 and func2 in parallel.
ray.get([func1.remote(), func2.remote()])

If func1() and func2() return results, you need to rewrite the code as follows:

ret_id1 = func1.remote()
ret_id2 = func1.remote()
ret1, ret2 = ray.get([ret_id1, ret_id2])

There are a number of advantages of using Ray over the multiprocessing module. In particular, the same code will run on a single machine as well as on a cluster of machines. For more advantages of Ray see this related post.

Method 4

This is a very good example by @Shashank. I just want to say that I had to add join at the end, or else the two processes were not running simultaneously:

from multiprocessing import Process
import sys

rocket = 0

def func1():
    global rocket
    print 'start func1'
    while rocket < sys.maxint:
        rocket += 1
    print 'end func1'

def func2():
    global rocket
    print 'start func2'
    while rocket < sys.maxint:
        rocket += 1
    print 'end func2'

if __name__=='__main__':
    p1 = Process(target = func1)
    p1.start()
    p2 = Process(target = func2)
    p2.start()
    # This is where I had to add the join() function.
    p1.join()
    p2.join()

Furthermore, Check this thread out:
When to call .join() on a process?

Method 5

Here is another version, if a dynamic list of processes need to be run.
I am including the two shell scripts, if you want to try it:

t1.sh

for i in {1..10}
  do 
     echo "1... t.sh i:"$i
     sleep 1
  done

t2.sh

   for i in {1..3}
   do
       echo "2.. t2.sh i:"$i
       sleep 1
   done

np.py

import os
from multiprocessing import Process, Lock

def f(l, cmd):
    os.system(cmd)

if __name__ == '__main__':
    lock = Lock()

    for cmd in ['sh t1.sh', 'sh t2.sh']:
        Process(target=f, args=(lock, cmd)).start()

output

1... t.sh i:1
2.. t2.sh i:1
1... t.sh i:2
2.. t2.sh i:2
1... t.sh i:3
2.. t2.sh i:3
1... t.sh i:4
1... t.sh i:5
1... t.sh i:6
1... t.sh i:7
1... t.sh i:8
1... t.sh i:9
1... t.sh i:10

“lock” left there can be acquired before task “l.acquire()” and released after “l.release()“

Method 6

#Try by using threads instead of multiprocessing

#from multiprocessing import Process
#import sys

import time
import threading
import random

rocket = 0

def func1():
    global rocket
    print('start func1')
    while rocket < 100:
        print("Im in func1")
        rocket += 1
        value = "Im global var "+str(rocket)+" from fun1"
        print(value)

    print ('end func1')

def func2():
    global rocket
    print ('start func2')
    while rocket < 100:
        print("Im in func2")
        rocket += 1
        value = "Im global var " + str(rocket) + " from fun2"
        print(value)
    print ('end func2')

if __name__=='__main__':
    p1 = threading.Thread(target=func1)
    p2 = threading.Thread(target=func2)
    p1.start();p2.start()

#Hope it works

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0