call multiprocessing in class method Python

Initially, I have a class to store some processed values and re-use those with its other methods.

The problem is when i tried to divide the class method into multiple process to speed up, python spawned processes but it seems didn't work (as I saw in Task Manager that only 1 process was running) and result is never delivered.

I did couple of search and found that pathos.multiprocessing can do this instead but I wonder if standard library can solve this problems?

from multiprocessing import Pool

class A():
    def __init__(self, vl):
        self.vl = vl
    def cal(self, nb):
        return nb * self.vl
    def run(self, dt):
        t = Pool(processes=4)
        rs = t.map(self.cal, dt)
        t.close()
        return t

a = A(2)

a.run(list(range(10)))