__dict__:实例化类时将实例属性分配到__dict__
__slots__:阻止实例分配属性到__dict__
class Base(object): __slots__ = ['y', 'x'] def __init__(self): self.y = 'aa' self.x = 'xx' b = Base() print b.__dict__ #抛错 print b.y b.x = 30 print b.x