描述符的运用

描述符

描述符就是一个类，一个至少拥有get(self,instance,owner),set(self,instance,value),delete(self,instance) 三个方法的任意一个或几个

• get(self,instance,owner)
  用于访问属性，它返回属性的值
• set(self,instance,value)
  属性被赋值中调用，不返回任何值
• 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()

    
>>> text = Test()
>>> text.x
getting <__main__.MyDecriptor object at 0x000001B84F911E80> <__main__.Test object at 0x000001B84F8989E8> <class '__main__.Test'>
>>> text.x = 'love'
setting <__main__.MyDecriptor object at 0x000001B84F911E80> <__main__.Test object at 0x000001B84F8989E8> love
>>> del text.x
deleting <__main__.MyDecriptor object at 0x000001B84F911E80> <__main__.Test object at 0x000001B84F8989E8>
>>> text
<__main__.Test object at 0x000001B84F8989E8>
>>> Test
<class '__main__.Test'>

注意instance(拥有者的实例对象） 是Test()的实例化对象test, owner 是Test

自己定义一个 Property

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=1
print(c._x)

1

先定义一个温度类，然后定义两个描述符类用于描述摄氏度和华氏度两个属性

class Celsius:
    def __init__(self,value = 26.0):
        self.value = value
    def __get__(self, instance, owner):
        return self.value
    def __set__(self, instance, value):
        self.value = value

class Fahrenheit:
    def __get__(self, instance, owner):
        return instance.cel*1.8+32
    def __set__(self, instance, value):
        instance.cel = (float(value)-32)/1.8
class Temperature:
    cel = Celsius()
    fah = Fahrenheit()
temp = Temperature()
print(temp.cel)

temp.cel = 100
print(temp.cel)

temp.fah = 70
print(temp.cel)
print(temp.fah)

26.0
100
21.11111111111111
70.0