课时46：魔法方法：描述符（property的原理）

__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 MyDescriptor:
    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 = MyDescriptor()

>>> test = Test()
>>> test.x
getting... <__main__.MyDescriptor object at 0x000001559C681CF8> <__main__.Test object at 0x000001559C67A588> <class '__main__.Test'>

>>> test.x = 'X-man'
setting... <__main__.MyDescriptor object at 0x000001559C681CF8> <__main__.Test object at 0x000001559C67A588> X-man

>>> del test.x
deleting... <__main__.MyDescriptor object at 0x000001559C681CF8> <__main__.Test object at 0x000001559C67A588>

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
>>> c.x
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    c.x
  File "C:\Users\14158\Desktop\MyProperty.py", line 8, in __get__
    return self.fget(instance)
  File "C:\Users\14158\Desktop\MyProperty.py", line 21, in getX
    return self._x
AttributeError: 'C' object has no attribute '_x'

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。

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 = 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

class Temperature:
    cel = Celsius()
    fah = Fahrenheit()

>>> temp = Temperature()
>>> temp.cel
26.0
>>> temp.fah
78.80000000000001