Python multiprocessing's Pool constants

Question

I'm beginner with the multiprocessing module in python and I want to use concurrent execution ONLY for my def func. Moreover I'm using some constants in my code and I have problem with them. The code is (python 3.6.8):

from multiprocessing import Pool

FIRST_COUNT=10
print("Enter your path")
PATH=input()

some_list=[]
for i in range(10000):
    some_list.append(i)  


def func(some_list):
    .....


if __name__ == "__main__":
chunks = [some_list[i::4] for i in range(4)]
pool = Pool(processes=4)

pool.map(func,chunks)

When I try to start this programm, I see the message Enter your path 5 times and 5 times I need to input my path. i.e this code execute 1 + 4(for each processes) times.

I want to use FIRST_COUNT, PATH and some_list like a constants, and use multiprocesseing only for func. How can i do this. Please, help me.

Answer 1

You should put code inside if __name__ == "__main__": to execute it only once

if __name__ == "__main__":
    FIRST_COUNT = 10
    PATH = input("Enter your path: ")

    some_list = list(range(10000))
    #some_list = []
    #for i in range(10000):
    #    some_list.append(i)  

    chunks = [some_list[i::4] for i in range(4)]
    pool = Pool(processes=4)

    results = pool.map(func, chunks)

    print(results)

If you want to use FIRST_COUNT, PATH then better send it to func as arguments.

You will have to create tuples with FIRST_COUNT, PATH in chunks

chunks = [(FIRST_COUNT, PATH, some_list[i::4]) for i in range(4)]

and function will have to get it as tuple and unpack it

def func(args):
    first_count, paht, some_list = args

---

Working example

from multiprocessing import Pool

def func(args):
    first_count, path, some_list = args

    result = sum(some_list)

    print(first_count, path, result)

    return result 


if __name__ == "__main__":
    FIRST_COUNT = 10
    PATH = input("Enter your path: ")

    some_list = list(range(10000))
    #some_list = []
    #for i in range(10000):
    #    some_list.append(i)  

    chunks = [(FIRST_COUNT, PATH, some_list[i::4]) for i in range(4)]
    pool = Pool(processes=4)

    all_results = pool.map(func, chunks)

    print('all results:', all_results)

---

EDIT: You can also use starmap() instead of map()

    all_results = pool.starmap(func, chunks)

and then you can use (without unpacking arguments)

def func(first_count, path, some_list):