Daniel Arbuckle's Mastering Python

The __prepare__method

The __prepare__ method is passed with the name of the new class and its list of parent classes as well as any keyword arguments supplied by the user. It can do whatever we want it to, but it should also return a dictionary or similar object that could be used to hold the class's attributes (refer to the previous example).

We can preassign values to the attribute dictionary from inside of __prepare__, so we can actually assign attributes to the class before it even exists. That brings us to the second thing that metaclasses can easily control-the class's namespace.

In our previous example, we returned a dict() instance from __prepare__, so instances of this metaclass use a normal dictionary to store their attributes while ...