如何使用multiprocessing.Pool更改类属性？

Question

我想使用multiprocessing.Pool并行加载相当大的数据集。这是我用于实现这一目标的代码。

import pickle
import os
from os.path import join
from os import listdir
class db_loader:

    def __init__(self, db_path, no_of_workers):
        self.db_path = db_path
        self.no_of_workers = no_of_workers
        self.fp_dict = {}
    def one_loader(self, the_hash):
        db_path= self.db_path
        the_path = join(db_path, the_hash)
        with open(the_path, 'rb') as source:
            self.fp_dict[the_hash] = pickle.load(source)

    def pool_loader(self):
        db_path = self.db_path
        pool = mp.Pool(self.no_of_workers)
        the_files = listdir(db_path) # a list of hashes
        print(type(the_files))
        print(type(self.one_loader))
        pool.map(self.one_loader, the_files)
        print('nothing wrong')
        print(len(self.fp_dict))

t_1 = time.time()
test_loader = db_loader(db_path = db_path,
                        no_of_workers = 10)
test_loader.pool_loader()
print(len(test_loader.fp_dict))

但是，运行脚本后，属性fp_dict的长度为0。为什么multiprocessing.Pool不会更改class属性？