描述符
•描述符就是将某种特殊类型的类的实例指派给另一个类的属性。
•__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
实例：
1 >>> class MyDecriptor:
2 def __get__(self,instance,owner):
3 print("getting...",self,instance,owner)
4 def __set__(self,instance,value):
5 print("setting...",self,instance,value)
6 def __delete__(self,instance):
7 print("deleting...",self,instance)
8
9
10
11 >>> class Test:
12 x = MyDecriptor() #取Mydecriptor类的实例指派给Test类的属性x
13
14 >>> test = Test()
15 >>> test.x
16 getting... <__main__.MyDecriptor object at 0x00000000033467F0> <__main__.Test object at 0x000000000335EF98> <class '__main__.Test'>
17 >>> test
18 <__main__.Test object at 0x000000000335EF98>
19 >>> test.x = "X-man"
20 setting... <__main__.MyDecriptor object at 0x00000000033467F0> <__main__.Test object at 0x000000000335EF98> X-man
21 >>> del test.x
22 deleting... <__main__.MyDecriptor object at 0x00000000033467F0> <__main__.Test object at 0x000000000335EF98>
实例2：
1 >>> class MyProperty:
2 def __init__(self,fget = None,fset = None,fdel = None):
3 self.fget = fget
4 self.fset = fset
5 self.fdel = fdel
6 def __get__(self,instance,owner):
7 return self.fget(instance)
8 def __set__(self,instance,value):
9 self.fset(instance,value)
10 def __delete__(self,instance):
11 self.fdel(instance)
12
13
14 >>> class C:
15 def __init__(self):
16 self._x = None
17 def getX(self):
18 return self._x
19 def setX(self,value):
20 self._x = value
21 def delX(self):
22 del self._x
23 x = MyProperty(getX,setX,delX)
24
25
26 >>> c = C()
27 >>> c.x = "HELLOW"
28 >>> c.x
29 'HELLOW'
30 >>> c._x
31 'HELLOW'
32 >>> del c.x
33 >>> c._x
34 Traceback (most recent call last):
35 File "<pyshell#70>", line 1, in <module>
36 c._x
37 AttributeError: 'C' object has no attribute '_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进行转换，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。
实例3：
1 ss Celsius: #摄氏度描述符类
2 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)
3 self.value = float(value)
4 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)
5 return self.value
6 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）
7 self.value = float(value)
8
9 class Fahrenheit: #华氏度描述符类
10 def __get__(self,instance,owner):
11 return instance.cel * 1.8 +32 #摄氏度转华氏度
12 def __set__(self,instance,value):
13 instance.cel = ((float)(value)- 32)/ 1.8 #华氏度转摄氏度
14
15 class Temperature: #温度类
16 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）
17 fah = Fahrenheit()#设置华氏度属性
18
19 >>> temp = Temperature()
20 >>> temp.cel
21 26.0
22 >>> temp.fah
23 78.80000000000001
24 >>> temp.fah = 78.8
25 >>> temp.cel
26 25.999999999999996
