__getattr__ and __getattribute__

[Gabriel Genellina]

En Tue, 08 May 2007 08:22:03 -0300, km <srikrishnamohan at gmail.com>  
escribió:

> i find it difficult to understand the difference between the magic  
> methods
> __getattr__ and __getattribute__
> and so donot know when to use former or later.
> can someone  brief me on it ?

This is better understood with a bit of history.
On earlier Python versions (before 2.2) the only object model available  
was what we now call "classic classes".
Classic instances hold their attributes in a dictionary, called __dict__.  
Attribute lookup starts at this instance dictionary; if not found,  
continues in the class, and its parent class, all along the inheritance  
tree. If still not found, __getattr__ (if it exists) is called, and should  
return the attribute value or raise AttributeError. That is, __getattr__  
is called *last*, and *only* when the attribute was not previously found  
in the usual places.

Since Python 2.2, there are "new style classes" available; they inherit  
directly or indirectly from object. A new style instance may not even have  
a __dict__. An existing __getattribute__ method is tried *first*; it  
should return the attribute value or raise AttributeError. If no custom  
__getattribute__ exists, the default object.__getattribute__ is used. As a  
last resort, if __getattr__ is defined, it is called.

OTOH, there is a single version of __setattr__, which is always invoked  
when setting an attribute.

-- 
Gabriel Genellina