Metaclasses Are Subclasses of Type

So why do we care that classes are instances of a type class in 3.0? It turns out that this is the hook that allows us to code metaclasses. Because the notion of type is the same as class today, we can subclass type with normal object-oriented techniques and class syntax to customize it. And because classes are really instances of the type class, creating classes from customized subclasses of type allows us to implement custom kinds of classes. In full detail, this all works out quite naturally—in 3.0, and in 2.6 new-style classes:

 

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  • type is a class that generates user-defined classes.
  • Metaclasses are subclasses of the type class.
  • Class objects are instances of the type class, or a subclass thereof.
  • Instance objects are generated from a class.

In other words, to control the way classes are created and augment their behavior, all we need to do is specify that a user-defined class be created from a user-defined metaclass instead of the normal type class.

Notice that this type instance relationship is not quite the same as inheritance: user-defined classes may also have superclasses from which they and their instances inherit attributes (inheritance superclasses are listed in parentheses in the class statement and show up in a class’s __bases__ tuple). The type from which a class is created, and of which it is an instance, is a different relationship. The next section describes the procedure Python follows to implement this instance-of type relationship.