为什么基类singleton仍然多次调用构造函数?

我将定义一个python singleton,如下所示:

class Database:
    initialized = False

    def __init__(self):
        self.id = random.randint(1,101)
        print("Generated an id of ", self.id)
        print("Loading database from file")

    _instance = None

    def __new__(cls, *args, **kwargs):
        if not cls._instance:
            cls._instance = super(Database, cls)\
                .__new__(cls, *args, **kwargs)
        return cls._instance

这就意味着每一个电话 Database() 实际上只返回一个实例。然而, __init__() 方法是 仍然 每次调用时调用。例如,

database = Database()

if __name__ == '__main__':
    d1 = Database()
    d2 = Database()

    print(d1.id, d2.id)
    print(d1 == d2)
    print(database == d1)

产生输出

Generated an id of  8
Loading database from file
Generated an id of  89
Loading database from file
Generated an id of  81
Loading database from file
81 81
True
True

为什么会这样?我该怎么做才能避免初始化器被多次调用?