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The “New-Style” Class Model

In Release 2.2, Python introduced a new flavor of classes, known as “new-style” classes; classes following the original model became known as “classic classes” when compared to the new kind. In 3.0 the class story has merged, but it remains split for Python 2.X users:

 

As of Python 3.0, all classes are automatically what we used to call “new-style,” whether they explicitly inherit from object or not. All classes inherit from object, whether implicitly or explicitly, and all objects are instances of object.In Python 2.6 and earlier, classes must explicitly inherit from object (or another built-in type) to be considered “new-style” and obtain all new-style features.

Because all classes are automatically new-style in 3.0, the features of new-style classes are simply normal class features. I’ve opted to keep their descriptions in this section separate, however, in deference to users of Python 2.X code—classes in such code acquire new-style features only when they are derived from object.

In other words, when Python 3.0 users see descriptions of “new-style” features in this section, they should take them to be descriptions of existing features of their classes. For 2.6 readers, these are a set of optional extensions.

In Python 2.6 and earlier, the only syntactic difference for new-style classes is that they are derived from either a built-in type, such as list, or a special built-in class known as object. The built-in name object is provided to serve as a superclass for new-style classes if no other built-in type is appropriate to use:

class newstyle(object):
    ...normal code...

Any class derived from object, or any other built-in type, is automatically treated as a new-style class. As long as a built-in type is somewhere in the superclass tree, the new class is treated as a new-style class. Classes not derived from built-ins such as object are considered classic.

New-style classes are only slightly different from classic classes, and the ways in which they differ are irrelevant to the vast majority of Python users. Moreover, the classic class model still available in 2.6 works exactly as it has for almost two decades.

In fact, new-style classes are almost completely backward compatible with classic classes in syntax and behavior; they mostly just add a few advanced new features. However, because they modify a handful of class behaviors, they had to be introduced as a distinct tool so as to avoid impacting any existing code that depends on the prior behaviors. For example, some subtle differences, such as diamond pattern inheritance search and the behavior of built-in operations with managed attribute methods such as __getattr__, can cause some legacy code to fail if left unchanged.

The next two sections provide overviews of the ways the new-style classes differ and the new tools they provide. Again, because all classes are new-style today, these topics represent changes to Python 2.X readers but simply additional advanced class topics to Python 3.0 readers.