Question

If my understanding of the Python data model is correct, both classes and class instances have associated __dict__ objects which contain all the attributes. However, I’m a little confused as to why certain class instances, such as instances of str for example, don’t have a __dict__ attribute.

If I create a custom class:

class Foo:
     def __init__(self):
             self.firstname = "John"
             self.lastname = "Smith"

Then I can get the instance variables by saying:

>>> f = Foo()
>>> print(f.__dict__)
{'lastname': 'Smith', 'firstname': 'John'}

But if I try to do the same with an instance of the built-in str, I 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 a __dict__ attribute?

Answer 1

Instances of types defined in C don’t have a __dict__ attribute by default.