Sharing State Between Processes

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Code running in different processes do not, by default, share the same data. However, the multiprocessing module contains primitives to help share values across multiple processes.

import multiprocessing

plain_num = 0
shared_num = multiprocessing.Value('d', 0)
lock = multiprocessing.Lock()

def increment():

global plain_num with lock: # ordinary variable modifications are not visible across processes plain_num += 1 # multiprocessing.Value modifications are shared_num.value += 1

ps = [multiprocessing.Process(target=increment) for n in range(4)]
for p in ps:
    p.start()
for p in ps:
    p.join()

print("plain_num is %d, shared_num is %d" % (plain_num, shared_num.value))

# Out: plain_num is 0, shared_num is 4

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