A class is also an object, both in Python's object system and the system we are implementing here. For simplicity, we say that classes do not themselves have a class. (In Python, classes do have classes; almost all classes share the same class, called type.) A class can respond to get and set messages, as well as the new message:

>>> def make_class(attributes, base_class=None):
        """Return a new class, which is a dispatch dictionary."""
        def get_value(name):
            if name in attributes:
                return attributes[name]
            elif base_class is not None:
                return base_class['get'](name)
        def set_value(name, value):
            attributes[name] = value
        def new(*args):
            return init_instance(cls, *args)
        cls = {'get': get_value, 'set': set_value, 'new': new}
        return cls

Unlike an instance, the get function for classes does not query its class when an attribute is not found, but instead queries its base_class. No method binding is required for classes.

Initialization. The new function in make_class calls init_instance, which first makes a new instance, then invokes a method called __init__.

>>> def init_instance(cls, *args):
        """Return a new object with type cls, initialized with args."""
        instance = make_instance(cls)
        init = cls['get']('__init__')
        if init:
            init(instance, *args)
        return instance

This final function completes our object system. We now have instances, which set locally but fall back to their classes on get. After an instance looks up a name in its class, it binds itself to function values to create methods. Finally, classes can create new instances, and they apply their __init__ constructor function immediately after instance creation.

In this object system, the only function that should be called by the user is make_class. All other functionality is enabled through message passing. Similarly, Python's object system is invoked via the class statement, and all of its other functionality is enabled through dot expressions and calls to classes.