Python: why is __dict__ attribute not in built-in class instances -

if understanding of python data model correct, both classes , class instances have associated __dict__ objects contain attributes. however, i'm little confused why class instances, such instances of str example, don't have __dict__ attribute.

if create custom class:

class foo:      def __init__(self):              self.firstname = "john"              self.lastname = "smith" 

then can instance variables saying:

>>> f = foo() >>> print(f.__dict__) {'lastname': 'smith', 'firstname': 'john'} 

but if try same instance of built-in str, get:

>>> s = "abc" >>> print(s.__dict__) traceback (most recent call last):   file "<stdin>", line 1, in <module> attributeerror: 'str' object has no attribute '__dict__' 

so, why don't instances of str have __dict__ attribute?

instances of types defined in c don't have __dict__ attribute default.