How can a global variable be accessed within a multiprocessing pool in Python?

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There are a few ways to achieve this:

1. Using the multiprocessing.Manager() class:

from multiprocessing import Pool, Manager

global_var = None

def func(x):
    global global_var
    global_var += x

if __name__ == '__main__':
    global_var = 0
    with Manager() as manager:
        global_var = manager.Value('i', global_var)
        p = Pool()
        p.map(func, [1, 2, 3])
        p.close()
        p.join()
        print(global_var.value)

In this example, the Manager() class is used to create a shared variable global_var. The variable is passed to the function as an argument and can be accessed like any other shared variable.

1. Using the multiprocessing.sharedctypes module:

from multiprocessing import Pool, sharedctypes, RawArray

global_var = None

def func(x):
    global global_var
    global_var[0] += x

if __name__ == '__main__':
    global_var = RawArray('i', [0])
    global_var = sharedctypes.RawArray('i', global_var)
    p = Pool()
    p.map(func, [1, 2, 3])
    p.close()
    p.join()
    print(global_var[0])

In this example, a RawArray object is created and then converted into a shared ctypes object. The shared variable is accessed by indexing the shared array.

Note: It is important to note that changes made to global variables in one process will not affect the values of the same variable in other processes. Therefore, the global variable should be shared among all the processes using one of the methods described above.