dict attribute for built-in types

Thu Oct 27 06:08:45 EDT 2011

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 ?

__dict__ attribute for built-in types

dict attribute for built-in types