How to increment counter in different process python?

Question

I'm going to increase start global variable in the multi-process python,

Source :

from multiprocessing import Process, Lock

start = 0

def printer(item, lock):
    """
    Prints out the item that was passed in
    """
    global start
    lock.acquire()
    try:
        start = start + 1
        print(start)
    finally:
        lock.release()

if __name__ == '__main__':
    lock = Lock()
    items = ['tango', 'foxtrot', 10]
    for item in items:
        p = Process(target=printer, args=(item, lock))
        p.start()

Output:

1
1
1

I use lock for start counter but it doesn't work, I was expecting to see this output:

1 # ==> start = 1
2 # ==> start = start + 1 = 2
3 # ==> start = start + 1 = 3

Possible duplicate of Python multiprocessing global variable updates not returned to parent — a_guest
– a_guest, Commented Sep 26, 2018 at 12:20

kabanus · Accepted Answer · 2018-09-26 12:12:56Z

3

You need to explicitly share the memory to make it work properly:

from multiprocessing import Process, Lock, Value

start = Value('I',0)

def printer(item):
    """
     Prints out the item that was passed in
    """
    with start.get_lock():
        start.value+=1
        print(start.value)

Note the multiprocessing Value wrapper comes with its own lock.

answered Sep 26, 2018 at 12:12

kabanus

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