Adding method functions to a class

A class decorator creates new method functions using a two-step process: by creating the method function and then inserting it into the class definition. This is often better done via a mixin class than a decorator. The obvious and expected use of a mixin is to insert methods. Inserting methods another way is less obvious and can be astonishing.

In the example of the Total_Ordering decorator, the exact method functions inserted were flexible and depended on what was already provided. This was a kind of special case that was typical but also very clever.

We might want to define a standardized memento() method. We'd like to include this standard method function in a variety of classes. We'll look at the decorator and mixin versions of this design. The following is the decorator version of adding a standard method:

def memento( class_ ):
    def memento( self ):
        return "{0.__class__.__qualname__}({0!r})".format(self)
    class_.memento= memento
    return...