Python descriptor and Attribute lookup order

1.1(ref:http://hbprotoss.github.io/posts/python-descriptor.html) first introduced the next __dict__

Example:

class A (object):     = 1    def__init__(self, a_l):        = a_l    def  foo (self):         Pass

Print a.__dict__

A = a (5)
Print a.__dict__

The output is as follows:

{'__module__':'__main__',   'A_g': 1,    '__dict__': <attribute'__dict__'Of'A'Objects>,    'Foo': <function foo at 0x10bdfb668>,   '__weakref__': <attribute'__weakref__'Of'A'Objects>,   '__doc__': None,'__init__': <function__init__At 0x10bdfb050>}{'_a_l': 5}

As you can see, a contains all the methods and the global variables of the class (these methods can also be considered global),

A (5) contains only the variables passed in at the time of initialization (__init__ self.xx that type of attribute)

The 1.2 instance does not contain a method of the class, but when we call A.foo (), we can find the correct method, which is why?

>> A.foo

<bound method A.foo of <__main__. A Object at 0x1094504d0>>

>> a.__dict__[' foo ']

<function Foo at 0x10944d668>

>>> a.__dict__[' foo '].__get__ (A, a)
<bound method A.foo of <__main__. A Object at 0x1094504d0>>

When the Foo attribute is obtained from a, it is followed in a certain order. This depends on which class of Foo is the object.

The concept of descriptor is introduced here:

A descriptor is actually an object of a particular class.

If this class defines both the __get__ and __set__ methods, then the object of this class is called data descriptor.

If this class only defines the __get__ method, then the object of this class is called Non-data descriptor

If a descriptor a_d as a property of Class A, then when object A of a accesses A_d, A_d's __get__ is called, __set__ method

In fact, a.__dict__[' foo ' is actually a descriptor, and when we a.foo, we actually call the __get__ method of this descriptor.

1.3 (ref:http://www.cnblogs.com/xybaby/p/6270551.html)

The order of instance property lookups is:

obj = Clz (), then the obj.attr (equivalent to obj.attr) order is as follows:

(1) If "attr" appears in the __dict__ of CLZ or its base class, and attr is data descriptor, call its __get__ method, otherwise

(2) If "attr" appears in the __dict__ of obj, then return directly to obj.__dict__[' attr '), otherwise

(3) If "attr" appears in the __dict__ of CLZ or its base class

(3.1) If attr is Non-data descriptor, then call its __get__ method, otherwise

(3.2) return __dict__[' attr ']

(4) If CLZ has a __getattribute__ method, call the __getattribute__ method, and if the attributeerror is thrown, continue calling

__getattr__. Otherwise throws Attributeerror end.

Python descriptor and Attribute lookup order