Python--28 描述符(preperty的原理)

描述符

　　描述符就是将某种特殊类型的类实的实例指派给另一个类的属性。

　　特殊类型就是实现以下三个方法的一个或多个

　　　　__get__(self, instance, owener)

　　　　　用于访问属性，它的返回属性的值

　　　　__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

property就是一个描述符类

>>> 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()
... 
>>> test = Test()
>>> test.x
getting... <__main__.MyDecriptor object at 0x7f43e3b3efd0> <__main__.Test object at 0x7f43e3b52048> <class '__main__.Test'>
>>> test
<__main__.Test object at 0x7f43e3b52048>
>>> test.x = 'X-man'
setting... <__main__.MyDecriptor object at 0x7f43e3b3efd0> <__main__.Test object at 0x7f43e3b52048> X-man
>>> del test.x
deleting... <__main__.MyDecriptor object at 0x7f43e3b3efd0> <__main__.Test object at 0x7f43e3b52048>

property的实现原理

>>> class Myproperty:
...     def __init__(self,fget = None, fset = None, fdel = None):
...         self.fget = fget
...         self.fset = fset
...         self.fdel = fdel
...     def __get__(self,instance,owener):
...         return self.fget(instance) 
...     def __set__(self,instance,value):
...         self.fset(instance,value)
...     def __del__(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
... 
>>> c =C()
>>> c.x = 'C-man'
>>> c.x
'C-man'
>>> del c.x

练习要求

　　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。

　　要求两个属性会自动转换，也就是说你可以给摄氏度这个属性赋值，然后打印的华氏度属性是自动转换后的结果

>>> class Celsius:
...     def __init__(self,value = 26.0):
...         self.value = float(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 Temperarure:
...     cel = Celsius()
...     fah = Fahrenheit()
... 
>>> temp = Temperarure()
>>> temp.cel
26.0
>>> temp.cel = 30
>>> temp.fah
86.0
>>> temp.fah =100
>>> temp.cel
37.77777777777778