Python基本特殊方法之__new__

__new__()和不可变对象

　　__new__方法的一个用途是初始化不可变对象，__new()__方法中允许创建未初始化的对象，这允许我们在__init__()方法被调用之前先设置对象的属性

　　例：为float对象定义一个包含单位信息的属性

　　（1）重载__init__()方法

#coding = utf-8
 
class myfloat(float):
    def __init__(self, value, unit):
        self.value = value
        self.unit = unit
 
if __name__ == "__main__":
    f = myfloat(3, "hello")

说明对于内置的float类不能简单的重载__init__()方法，对于其他的内置不可变类型也是同样的问题，我们不能在不可变对象self上设置新的属性值

　　（2）重载__new__()方法

#coding = utf-8
 
class myfloat(float):
    def __new__(cls, value, unit):
        obj = super().__new__(cls, value)
        #obj.unit = unit
        return obj
 
    def __init__(self, value, unit):
        self.value = value
        self.unit = unit 
 
if __name__ == "__main__":
    f = myfloat(3, "kg")
    print (f.value, f.unit)
    print (f, f.unit)

__new__()和元类型

　　例：实现有序属性

#coding=utf-8
 
class myclass(type):
    @classmethod
    def __prepare__(mcs, name, bases, **kwargs):
        return super().__prepare__(name,bases,**kwargs)
 
    def __new__(cls, name, bases, namespace, **kwargs):
        print ("name:",name)
        print ("bases:",bases)
        print ("namespace:",namespace)
        a = super().__new__(cls, name, bases, namespace)
        a._order = tuple(c for c in namespace if not c.startswith("__"))
        print (a)
        print (type(a))
        return a
    def __init__(self, name, bases,namespace, **kwargs):
        super().__init__(name, bases, namespace)
 
class people(metaclass=myclass):
    name = "zhanglin"
    age = ""
 
    def sayhello(self):
        self.color="red"
        print ("say hello")
 
if __name__ == "__main__":
    p = people()
    print (p._order)