Python如何在子类里扩展父类的property？

《python cookbook》8.8节讨论子类扩展property时，一开始都晕了，思考了半天才勉强弄懂一点，赶快记下来。废话不多说，先上代码：

class Person:
	def __init__(self, name):
		self.name = name
	@property
	def name(self):
		print("I am in the Person's name getter")
		return self._name
	@name.setter
	def name(self, value):
		print("I am in the Person's name setter")
		if not isinstance(value, str):
			raise TypeError('Expected a string')
		self._name = value

class SubPerson(Person):
	@property
	def name(self):
		print("I am in the SubPerson's name getter")
		super().name
	@name.setter
	def name(self, value):
		print("I am in the SubPerson's name setter")
		super(SubPerson, SubPerson).name.__set__(self, value)

我知道property其实就是特殊的描述符，但是为啥在setter里面必须显式调用父类name的__set__函数呢？直接super().name = value难道不能触发__set__函数吗？试试看：

class SubPerson(Person):
	@property
	def name(self):
		print("I am in the SubPerson's name getter")
		super().name
	@name.setter
	def name(self, value):
		print("I am in the SubPerson's name setter")
		super().name = value

>>> sp = SubPerson('shy')
I am in the SubPerson's name setter
Traceback (most recent call last):
  File "<pyshell#25>", line 1, in <module>
    sp = SubPerson('shy')
  File "<pyshell#11>", line 3, in __init__
    self.name = name
  File "<pyshell#24>", line 9, in name
    super().name = value
AttributeError: 'super' object has no attribute 'name'

果然报错，提示super对象没有name属性，WTF！为什么可以get但是不能set？一直没有查到答案，最后help(super)，才发现蛛丝马迹：

>>> help(super)

Help on class super in module builtins:

class super(object)
 |  super() -> same as super(__class__, <first argument>)
 |  super(type) -> unbound super object
 |  super(type, obj) -> bound super object; requires isinstance(obj, type)
 |  super(type, type2) -> bound super object; requires issubclass(type2, type)
 |  Typical use to call a cooperative superclass method:
 |  class C(B):
 |      def meth(self, arg):
 |          super().meth(arg)
 |  This works for class methods too:
 |  class C(B):
 |      @classmethod
 |      def cmeth(cls, arg):
 |          super().cmeth(arg)
 |
 |  Methods defined here:
 |
 |  __get__(self, instance, owner, /)
 |      Return an attribute of instance, which is of type owner.
 |
 |  __getattribute__(self, name, /)
 |      Return getattr(self, name).
 |
 |  __init__(self, /, *args, **kwargs)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |
 |  __repr__(self, /)
 |      Return repr(self).
 |
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |
 |  __self__
 |      the instance invoking super(); may be None
 |
 |  __self_class__
 |      the type of the instance invoking super(); may be None
 |
 |  __thisclass__
 |      the class invoking super()

super本身只有__getattribute__，没有__setattr__，只对获取属性做了代理。因此设置的时候，会直接设置super()对象本身的属性，所以出现如上的错误提示，因此只能够显式调用name的__set__方法。。。。

另外一个坑就是如果子类全面扩展父类的property，可以用上面的方法，但是如果只是扩展get或者set方法，就不行了，如下：

>>> class SubPerson(Person):
	@property
	def name(self):
		print("I am in SubPerson's getter")
		super().name

>>> sp = SubPerson('shy')

Traceback (most recent call last):
  File "<pyshell#48>", line 1, in <module>
    sp = SubPerson('shy')
  File "<pyshell#11>", line 3, in __init__
    self.name = name
AttributeError: can't set attribute

父类的setter方法消失了，这里比较好理解，property是描述符，是get,set,delete的集合，子类仅仅只设置了get，set和delete相当于根本没有设置。如果想要继承父类的property，只能显式的用父类的property来装饰，如下：

>>> class SubPerson(Person):
	@Person.name.getter
	def name(self):
		print("I am in SubPerson's getter")
		return super().name

>>> sp = SubPerson('shy')

I am in the Person's name setter
>>> sp.name

I am in SubPerson's getter
I am in the Person's name getter
'shy'

此时返回的name特性，其实是复制了Person.name描述符所有方法的一个新的描述符。。

扩展子类的property，需要对描述符和super的机制有比较深入的了解，现在只是模模糊糊弄了个半懂，mark在此，随时修改。