Python 类属性为dict,第二次修改时，为什么出现两个不同的值

Python 类属性为dict,第二次修改时，为什么出现两个不同的值？？
1、当这个类属性修改几次后，每次查看该类属性时，可能得到的结果不一样，请问是为什么？

2、使用add_user_data_access_rule这个方法修改后，每次获取data_access_rule_dict的结果不一样

3、调用clear_data_access_rule_dict方法后，有时候会得到之前的结果，有时候会得到空字典

import logging
from threading import Lock
import gc
logger = logging.getLogger(_name)

class DataAccessRuleCache(object):
data_access_rule_dict = {}
lock = Lock()

def __new__(cls, *args, **kw):
    if not hasattr(cls, '_instance'):
        ori = super(DataAccessRuleCache, cls)

    cls._instance = ori.__new__(cls, *args, **kw)
    cls._instance.__dict__ = cls.data_access_rule_dict

    return cls._instance

@classmethod
def clear_data_access_rule_dict(cls):
    cls.data_access_rule_dict = {}

@classmethod
def add_user_data_access_rule(cls, which_database, user_id, data_access_rule):
    if not cls.__check_which_database_user_data_access_rule_exists(which_database):
        cls.data_access_rule_dict.update({which_database: {}})

    if str(user_id) not in cls.data_access_rule_dict[which_database]:
        cls.data_access_rule_dict[which_database].update({str(user_id): data_access_rule})
    else:
        cls.data_access_rule_dict[which_database][str(user_id)] = data_access_rule

@classmethod
def __check_which_database_user_data_access_rule_exists(cls, which_database):
    if which_database not in cls.data_access_rule_dict.keys():
        return False

    return True

@classmethod
def check_user_data_access_rule_exists(cls, which_database, user_id):
    if which_database not in cls.data_access_rule_dict.keys():
        return False

    user_data_access_rule_dict = cls.data_access_rule_dict[which_database]
    if str(user_id) not in user_data_access_rule_dict:
        return False

    return True

@classmethod
def get_user_data_access_rule(cls, which_database, user_id):
    if not cls.check_user_data_access_rule_exists(which_database, user_id):
        return []

    user_data_access_rule_list = cls.data_access_rule_dict[which_database][str(user_id)]

    return user_data_access_rule_list