user2133814 user2133814 - 7 months ago 22
Python Question

python multiprocessing pool with expensive initialization

Here is a complete simple working example

import multiprocessing as mp
import time
import random

class Foo:
def __init__(self):
# some expensive set up function in the real code
self.x = 2

def run(self, y):
time.sleep(random.random() / 10.)
return self.x + y

def f(y):
foo = Foo()

def main():
pool = mp.Pool(4)
for result in, range(10)):

if __name__ == '__main__':

How can I modify it so Foo is only initialized once by each worker, not every task? Basically I want the init called 4 times, not 10. I am using python 3.5


The intended way to deal with things like this is via the optional initializer and initargs arguments to the Pool() constructor. They exist precisely to give you a way to do stuff exactly once when a worker process is created. So, e.g., add:

def init():
    global foo
    foo = Foo()

and change the Pool creation to:

pool = mp.Pool(4, initializer=init)

If you needed to pass arguments to your per-process initialization function, then you'd also add an appropriate initargs=... argument.

Note: of course you should also remove the

foo = Foo()

line from f(), so that your function uses the global foo created by init().