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__dict__ Attribute of Ultimate Base Class, object in Python

Following piece of code lists the attributes of a class named 'A' in a sorted order:-

>>> class A():
        def __init__(self, i):
            self.at = i

>>> sorted(vars(A))
['__dict__', '__doc__', '__init__', '__module__', '__weakref__']

Now, printing the value of key, '__dict__' results this:-

>>> vars(A)['__dict__']                 #Value of '__dict__'
<attribute '__dict__' of 'A' objects>

As per docs, vars([object])

Return the __dict__ attribute for a module, class, instance, or any other object with a __dict__ attribute.

What I am not understanding is that is the '__dict__' attribute in the list the same attribute used by vars() to return the attributes of A or is it a different attribute which has some another objective like implementing A's objects' namespace as suggested (according to me) by the value which '__dict__' holds.

Edit:-

The first part of the question is very much related to this other question (also, mentioned by @eliotness) but it's the second part (described below) for which I can't find any answers or related question and hence, changing title of the question.

Let's consider another code that produces list of attributes of ultimate base class in Python, object:-

>>> sorted(vars(object))
['__class__', '__delattr__', '__dir__', '__doc__', '__eq__', ...., '__str__', '__subclasshook__']
>>> hasattr(object, '__dict__')
True
>>> sorted(getattr(object, '__dict__')) == sorted(vars(object))
True

Another quotation from docs about object.__dict__

A dictionary or other mapping object used to store an object’s (writable) attributes.

This time, '__dict__' doesn't show up in the list of object. So, is it that the __dict__ attribute is a read-only attribute in case of object or any other reason?

Also, is it possible to get a list of read-only attributes in Python in any way?

Solution

  • The first part of your question is already answered by the linked answer: the __dict__ of instances is stored as a descriptor on the class. This is the A.__dict__['__dict__']. A.__dict__ on the other hand stores all the attributes of the A class - which itself is an instance of type. So actually it's type.__dict__['__dict__'] that provides these variables:

    >>> type.__dict__['__dict__']
<attribute '__dict__' of 'type' objects>
>>> A.__dict__ == type.__dict__['__dict__'].__get__(A)
True

    The reason why you're not seeing a __dict__ attribute on object is because it doesn't have one. This means you can't set instance variables on object instances:

    >>> o = object()
>>> o.x = 1

AttributeError: 'object' object has no attribute 'x'

    Similar behavior can be achieved for custom classes by defining __slots__:

    >>> class B:
...     __slots__ = ()
... 
>>> vars(B)
mappingproxy({'__module__': '__main__', '__slots__': (), '__doc__': None})
>>> 
>>> b = B()
>>> b.x = 1

AttributeError: 'B' object has no attribute 'x'