Passing variable while using thread(multiprocessing) in python

Question

I'm trying to change 'double for loop' to 'for + multiprocessing' in python.

(I cannot come up with an idea for change double for loop to one multiprocessing code, not for + multiprocessing)

functiona gives different results for different i and j (first and second for loop)

I changed second loop(j) to multiprocessing.Pool() and it works.

However if I use first for loop with second loop(multiprocessing), in functiona the index of first loop does not change.

If I run code below, multiprocessing works, however the y in functiona does not change.

I tried to use global, it does not work. What should I do?

from multiprocessing import Pool
import multiprocessing

y = 0

def functiona(i):
    # global y
    print('y', y, i)

if __name__=='__main__':
    start_time = datetime.now()
    pool = multiprocessing.Pool()    # pool = Pool(processes=16)

    for t in range(0, 10):
        print('t : ', t)
        y = t
        result = pool.map(functiona, range(0, 10))
    print("- %s seconds -" % (datetime.now() - start_time))

You aren't using a thread. Multiple processes don't share state, unlike multiple threads — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 2, 2020 at 20:00

curious · Accepted Answer · 2020-12-02 20:16:01Z

1

Is this what you trying to achieve?:

import multiprocessing
from datetime import datetime

y = 0

def functiona(i,y):
    # global y
    print('y', y, i)

if __name__=='__main__':
    start_time = datetime.now()
    pool = multiprocessing.Pool()    # pool = Pool(processes=16)

    functiona_args = []
    for t in range(0, 10):
        print('t : ', t)
        y = t
        for i in range(0,10):
            functiona_args.append((i,y,))
    result = pool.starmap(functiona, functiona_args)

answered Dec 2, 2020 at 20:16

curious

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1 Comment

Wooni Over a year ago

This is what I wanted! Thanks :)

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