元类

"""
元类 - 描述类的类 - metaclass
"""
import sys


class SingletonMeta(type):
    
    def __init__(cls, *args, **kwargs):
        cls.__instance = None
        super().__init__(*args, **kwargs)

    def __call__(cls, *args, **kwargs):
        if cls.__instance is None:
            cls.__instance = super().__call__(*args, **kwargs)
        return cls.__instance


class President(object, metaclass=SingletonMeta):
    __slots__ = ('name', 'year')

    def __init__(self, name, year):
        self.name = name
        self.year = year


def main():
    p1 = President('奥巴马', 4)
    p2 = President('特朗普', 4)
    # p2.wife = '刘美丽'
    print(p1 == p2)
    print(id(p1), id(p2))
    print(p1.name, p2.name)
    print(sys.getsizeof(p1))


if __name__ == '__main__':
    main()