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。
