'type' 是 python built-in metaclass
其他继承自 ‘type’的class都可以是 Metaclass
子类可以继承父类的metaclass
然而 __metaclass__属性不能继承
__metaclass__可以定义成任何返回instance of 'type' callable
__new__(cls, ...) : create instance of cls
__init__(self, ...) ：instance already created and set properties to this instance
