python Descriptor (描述符)

简介:

python 描述符是新式类(继承自object)中的语言协议，基于描述符可以提供更佳优雅的解决方案。

python的classmethod, staticmethod, property都是基于描述符建立的。

描述符的协议:

定义了__set__, __get__, __delete__3个方法中任何一个方法的object可以作为描述符.

描述符分类:

同时定义了__set__,__get__被叫做data descriptor.

只定义了__get__被叫做no-data descriptor.

2种描述符的区别:

 Data and non-data descriptors differ in how overrides are calculated with respect to entries in an instance’s dictionary. If an instance’s dictionary has an entry with the same name as a data descriptor, the data descriptor takes precedence. If an instance’s dictionary has an entry with the same name as a non-data descriptor, the dictionary entry takes precedence.

在 attrubuite lookup过程中 ： 

如果对象属性有与data-descriptor同名的属性，data-descriptor优先于对象属性.

 如果对象属性有与no-data descriptor同名的属性，对象属性优先。

触发描述符的调用:

A descriptor can be called directly by its method name. For example, d.__get__(obj).

Alternatively, it is more common for a descriptor to be invoked automatically upon attribute access. For example, obj.d looks up d in the dictionary of obj. If d defines the method __get__(), thend.__get__(obj) is invoked according to the precedence rules listed below.

The details of invocation depend on whether obj is an object or a class. Either way, descriptors only work for new style objects and classes. A class is new style if it is a subclass of object.

For objects, the machinery is in object.__getattribute__() which transforms b.x into type(b).__dict__['x'].__get__(b, type(b)). The implementation works through a precedence chain that gives data descriptors priority over instance variables, instance variables priority over non-data descriptors, and assigns lowest priority to __getattr__() if provided

The important points to remember are:

• descriptors are invoked by the __getattribute__() method
• overriding __getattribute__() prevents automatic descriptor calls
• __getattribute__() is only available with new style classes and objects
• object.__getattribute__() and type.__getattribute__() make different calls to __get__().
• data descriptors always override instance dictionaries.
• non-data descriptors may be overridden by instance dictionaries.