What is the difference between old style and new style classes in Python?
What is the difference between old style and new style classes in Python? When should I use one or the other?
Answer
From New-style and classic classes:
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
xis an instance of an old-style class, thenx.__class__designates the class ofx, buttype(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, called instance.
New-style classes were introduced in Python 2.2 to unify the concepts of class and type. A new-style class is simply a user-defined type, no more, no less.
If x is an instance of a new-style class, then
type(x)is typically the same asx.__class__(although this is not guaranteed – a new-style class instance is permitted to override the value returned forx.__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 behaviour 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.
Python 3 only has new-style classes.
No matter if you subclass from
objector not, classes are new-style in Python 3.