I have a fairly simple program that I am writing in python that does not work for some reason. It will output the print statement above the 'join' line but not anything after it, including in change. Why?
from multiprocessing import Process
import time
x = 0
def main():
global x
p = Process(target=change)
p.start()
print(x)
p.join()
# time.sleep(5)
def change():
global x
x = x +1
print(x)
if __name__ == '__main__':
main()
Im guessing this is what you want:
from threading import Lock, Thread
x = 0
lock = Lock()
def main():
global x
print(x)
p = Thread(target=change, args=(1,))
p2 = Thread(target=change, args=(2,))
p.start()
p2.start()
p.join()
p2.join()
def change(number):
global x
with lock:
x = x + 1
print('{} - coming from process #{}'.format(x, number))
if __name__ == '__main__':
main()
output:
0
1 - coming from process #1
2 - coming from process #2
this you will notice will not work:
from multiprocessing import Process
from threading import Lock
x = 0
lock = Lock()
def main():
global x
print(x)
p = Process(target=change, args=(1,))
p2 = Process(target=change, args=(2,))
p.start()
p2.start()
p.join()
p2.join()
def change(number):
global x
with lock:
x = x + 1
print('{} - coming from process #{}'.format(x, number))
if __name__ == '__main__':
main()
output:
0
1 - coming from process #1
1 - coming from process #2
thats because Process is like your exwife - she doesnt care about you, and will keep 100% of the stuff when she last saw it. So its 0 first, and it will copy 0 and will never look at x again. Unlike Thread she likes to share.
