Why is "type" not present in mro of new style class?

Question

Why is "type" not present in mro of new style class?

1. In below code "type" is the meta class of class "A", which means "type" "type" should be part of method resolution order of "A". Why?

2. Also for new style class why are we inheriting "object", why not "type"?

3. A.__class__ is "type", but it's not part of mro.

4. Please let me know the difference between "object" and "type", in fact "type" also inherits "object" by definition. I mean difference between "object" and metaclass.

5. Also in old style class is "classobj" a metaclass?

Example:

>>> class A(object):pass
...
>>>
>>> A.__mro__
(<class '__main__.A'>, <type 'object'>)
>>>
>>> A.__class__
<type 'type'>

Answer 1

type is the “type” that “types” inherit from. Any defined type is a type, so isinstance(x, type) is valid for any x that is a type. – Confused yet? ^^

When you instantiate a type, you get an object of that type. That object is not necessarily a type; unless the type was a metaclass because instances of metaclasses are types (which I’m going to ignore here).

1. A is a type. Types inherit from type (by definition), and in the common case the metaclass for types is also type. Instances of type are types, so type will be included in the method resolution order for the type instance A. That is why e.g. calling A.mro() works. mro() is defined on type and that method is inherited when the type instance A gets created.

   However note that A.mro() gives you the method resolution order for instances of A. Not for A itself (the type instance).

2. You inherit object because when instantiating an object of A, that object should be instanceof(x, A) and instanceof(x, object). But it is not a type because that would mean that you could instantiate it again. It is an object, so you should inherit from object (directly or indirectly).

3. A.__class__ gives you the type that was used to create the instance A. If you do A().__class__, you will get A which is the type that was used to create the instance A().

4. object is the base type for any object in Python. Everything is an object, so any object is a direct or indirect instance of object. type is the type of types that can be instantiated to create objects.

   For information on why object is a type, and why type is in turn an object, see this answer.

5. For old-style classes, classobj is the kind-of metaclass. Note that old-style classes have a very different object model, so you can only apply parts of the new-style mentality there. You should try to use only new-style classes now (or possibly even switch to Python 3) since the object model is a lot more consistent and in my opinion easier to reason about.