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New-style and classic classes

Classes and instances come in two flavors: old-style or classic, and new-style.

Up to Python 2.1, old-style classes were the only flavour available to the user. The concept of (old-style) class is unrelated to the concept of type: if x is an instance of an old-style class, then x.__class__ designates the class of x, but type(x) is always <type 'instance'>. This reflects the fact that all old-style instances, independently of their class, are implemented with a single built-in type (see type-instance).

New-style classes were introduced in Python 2.2 to unify classes and types. A new-style class neither more nor less than a user-defined type. If x is an instance of a new-style class, then type(x) is the same as x.__class__.

The major motivation for introducing new-style classes is to provide a unified object model with a full meta-model. It also has a number of immediate benefits, like the ability to subclass most built-in types, or the introduction of descriptors, which enable computed properties.

For compatibility reasons, classes are still old-style by default. New-style classes are created by specifying another new-style class (i.e. a type) as a parent class, or the top-level type object if no other parent is needed. The behavior of new-style classes differs from that of old-style classes in a number of important details in addition to what type returns. Some of these changes are fundamental to the new object model, like the way special methods are invoked. Others are fixes that could not be implemented before for compatibility concerns, like the method resolution order in case of multiple inheritance.

This manual is not up-to-date with respect to new-style classes. For now, please see http://www.python.org/doc/newstyle.html for more information.

The plan is to eventually drop old-style classes, leaving only the semantics of new-style classes. This change will probably only be feasible in Python 3.0.