I have a following simple code
from multiprocessing import Pool
x = []
def func(a):
print(x,a)
def main():
a = [1,2,3,4,5]
pool = Pool(1)
global x
x = [1,2,3,4]
ans = pool.map(func,a)
print(x)
It gives me the result
[] 1
[] 2
[] 3
[] 4
[] 5
[1, 2, 3, 4]
I expected the result to reflects the change in global variable x.
Which seems that the changed in global variable x is not updated before the pool call. I would like to ask what is the cause of this?
So I have done what GuangshengZuo suggested, and sadly the result was not desirable. After looking deeper into it, I realized the problem was not because of script, but rather the OS.
In windows, there is no os.fork(), hence the change in global variable is not copied. But, on Unix machine, the script works fine.
I think it is because this is multiprocess, not multithread. the main process and the new process does not share a same global variable. So the new process has the copy of the main process when x is [], and after created, main process change x's value, but it does not change to new process's x.
if change the code to this :
from multiprocessing import Pool
x = []
def func(a):
print(x,a)
def main():
a = [1,2,3,4,5]
global x
x = [1,2,3,4]
pool = Pool(1)
ans = pool.map(func,a)
print(x)
and the ouput will be what you want. Notice the pool = Pool(1) 's position
Two seperate processes will not share the same global variables. A multiprocessing pool abstracts away the fact that you are using two seperate processes which makes this tough to recognise.
xwith pool in any way here.Which seems that the global variable x is not used in the pool at all: This is because x is not used in pool at all. This is what I am getting at. Maybe your wording is misleading to your problem.