Understanding New-Style and Old-Style Classes

Earlier in this chapter, in "Defining a Class," you got a hint that something was afoot in the evolution of classes and objects in the Python language. Beginning in Python 2.2, a new type of class was introduced, called new-style classes. A new-style class is a class that is directly or indirectly based on the new, built-in object introduced in Python 2.2. All of the classes you've seen in this chapter are based on object and are therefore new-style classes. An old-style class is a class that is not based on object, directly or indirectly. If you removed the (object) from any of the Critter class headers in this chapter, you'd have an old-style class. To drive the point home, here's an example of a new-style class header:

 class Critter(object): 

This is the header of an old-style class:

 class Critter: 

Every program you've seen so far in this chapter will work equally well with either new-style or old-style classes. However, new-style classes offer significant improvements over old-style classes. In fact, you'll see one of those improvements at work in the next chapter program, the Property Critter.

HINT

Create new-style classes instead of old-style classes whenever possible. New-style classes can do everything old-style classes can, plus more. Besides, old style classes will cease to exist beginning in Python 3.0.