__dict__ attribute for built-in types

[candide]

I realize that built-in types objects don't provide a __dict__ attribute 
and thereby i can't set an attribute to a such object, for instance


 >>> a=[42,421]
 >>> a.foo="bar"
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute 'foo'
 >>> a.__dict__
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
AttributeError: 'list' object has no attribute '__dict__'
 >>>


So, i was wondering :

-- why this behaviour ?
-- where the official documentation refers to this point ?