详解Python魔法方法之描述符类

•__get__(self, instance, owner)
–用于访问属性，它返回属性的值

•__set__(self, instance, value)
–将在属性分配操作中调用，不返回任何内容

•__delete__(self, instance)
–控制删除操作，不返回任何内容

>>> class MyDecriptor:
    def __get__(self,instance,owner):
        print("getting...",self,instance,owner)
    def __set__(self,instance,value):
        print("setting...",self,instance,value)
    def __delete__(self,instance):
        print("deleting...",self,instance)
>>> class Test:
    x = MyDecriptor()   #取Mydecriptor类的实例指派给Test类的属性x

>>> class MyProperty:
    def __init__(self,fget = None,fset = None,fdel = None):
        self.fget = fget
        self.fset = fset
        self.fdel = fdel
    def __get__(self,instance,owner):
        return self.fget(instance)
    def __set__(self,instance,value):
        self.fset(instance,value)
    def __delete__(self,instance):
        self.fdel(instance)

        
>>> class C:
    def __init__(self):
        self._x = None
    def getX(self):
        return self._x
    def setX(self,value):
        self._x = value
    def delX(self):
        del self._x
    x = MyProperty(getX,setX,delX)

    
>>> c = C()
>>> c.x = "HELLOW"
>>> c.x
'HELLOW'
>>> c._x
'HELLOW'
>>> del c.x
>>> c._x

ss Celsius:  #摄氏度描述符类
    def __init__(self,value = 26.0):#self为描述符类自身(此为摄氏度描述符类)的实例（此为cel)
        self.value = float(value)
    def __get__(self,instance,owner):#instance是这个描述符的拥有者所在的类的实例（此为temp)
        return self.value
    def __set__(self,instance,value):#owner是这个描述符的拥有者所在的类本身(此为温度类）
        self.value = float(value)

class Fahrenheit:   #华氏度描述符类
    def __get__(self,instance,owner):
        return instance.cel * 1.8 +32  #摄氏度转华氏度
    def __set__(self,instance,value):
        instance.cel = ((float)(value)- 32)/ 1.8 
        ## instance.cel 则会执行Temperature.cel,接着执行Celsius类中的__set__方法
        
class Temperature:   #温度类
    cel = Celsius()   #设置摄氏度属性（描述符类的实例指派给了温度类的属性）
    fah = Fahrenheit()# temp.fah ，当属性fah被赋值的时候，则会执行Fahrenheit描述符类的__set__方法
						
>>> temp = Temperature()
>>> temp.cel
26.0
>>> temp.fah
78.80000000000001
>>> temp.fah = 78.8
>>> temp.cel
25.999999999999996