Python入门学习笔记（魔法方法描述符 property的原理）

>>>class MyDecriptor:                        #定义一个描述符类
	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:                                #类的实例
	x = MyDecriptor()


>>> test = Test()
>>> test.x  
getting... <__main__.MyDecriptor object at 0x000000000315BC88> <__main__.Test object at 0x000000000315BBE0> <class '__main__.Test'>
>>> test
<__main__.Test object at 0x000000000315BBE0>
>>> Test
<class '__main__.Test'>
>>> test.x = "x-man"   #赋值，会调用setting
setting... <__main__.MyDecriptor object at 0x000000000315BC88> <__main__.Test object at 0x000000000315BBE0> x-man
>>> del test.x
deleting... <__main__.MyDecriptor object at 0x000000000315BC88> <__main__.Test object at 0x000000000315BBE0>

定义一个属于自己的property（）函数：

>>> 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 = 'x=man'
>>> c.x
'x=man'
>>> c._x
'x=man'
>>> del c.x

练习：

Python入门学习笔记（魔法方法描述符 property的原理）

摄氏度 * 1.8 + 32 =华氏度

class Celsius:
    def __init__(self,value = 26.0):
        self.value = float(value)   #最后设置到self.value里面去

    def __get__(self,instance,owner):
        return self.value

    def __set__(self,instance,value):
        self.value = float(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  #set方法被激发，cel对象赋值


class Temperature:
    cel = Celsius()     #即是对cel属性赋值，cel也有一个描述符类Celsius()
    fah = Fahrenheit()  #fah赋值，描述符类描述的fah属性，当属性被赋值，激发描述符类的set方法

运行测试：

>>> temp.fah
86.0
>>> temp.fah = 100
>>> temp.cel
37.77777777777778