The process of attribute lookup

1) If attrname is a special (i.e. Python-provided) attribute for objectname, return it.
(The attribute of Python-provided are something like 'this': 
__lolcat__, __doc__, __weakref__ and so on)

2)Check objectname.__class__.__dict__ for attrname. 
If it exists and is a data-descriptor, return the descriptor result. Search all bases of objectname.__class__ for the same case.

3)Check objectname.__dict__ for attrname, and return if found.
If objectname is a class, search its bases too. 
If it is a class and a descriptor exists in it or its bases, 
return the descriptor result.

4)Check objectname.__class__.__dict__ for attrname. 
If it exists and is a non-data descriptor, return the descriptor result. 
If it exists, and is not a descriptor, just return it. 
(If it exists and is a data descriptor, we shouldn't be here because we would have returned at point 2.) 
Search all bases of objectname.__class__ for same case.

5)Raise AttributeError.

如果objectname是实例：
1)Check objectname.__class__.__dict__ for attrname.
If it exists and is a data-descriptor, return the descriptor result. Search all bases of objectname.__class__ for the same case.
(在父类，以及父类的基类中查找data-descriptor属性，找到data-descriptor才返回)
2)Check objectname.__dict__ for attrname, and return if found.
(在实例自身中 查找，只要找到就返回，不管是不是descriptor属性)
3)Check objectname.__class__.__dict__ for attrname.
似乎还要加上objectname.__class__.__basses__.__dict__
If it exists and is a non-data descriptor, return the descriptor result.
If it exists, and is not a descriptor, just return it.
(If it exists and is a data descriptor, we shouldn't be here because we would have returned at point 2.)
(在父类中查找non-data descriptor和非descriptor属性，找到就返回)

如果objectname是类对象：
1)Check objectname.__class__.__dict__ for attrname.
If it exists and is a data-descriptor, return the descriptor result. Search all bases of objectname.__class__ for the same case.
(在父类，以及父类的基类中查找data-descriptor属性，找到data-descriptor才返回)
2)Search objectname.__dict__ and objectname.__bases__ for attrname. If a descriptor exists in objectname or objectname.__bases__, return the descriptor result.
(在自身，以及自身的基类中查找descriptor属性，只有找到descriptor属性，才返回)
3)Check objectname.__class__.__dict__ for attrname.
似乎还要加上objectname.__class__.__basses__.__dict__
If it exists and is a non-data descriptor, return the descriptor result.
If it exists, and is not a descriptor, just return it.
(If it exists and is a data descriptor, we shouldn't be here because we would have returned at point 2.)
(在父类中查找non-data descriptor和非descriptor属性，找到就返回)