We've looked at using decorators to modify function and class definitions. We've also looked at mixins that allow us to decompose a larger class into components that are knitted together.

The idea of both of these techniques is to separate application-specific features from generic features such as security, audit, or logging. We're going to distinguish between the inherent features of a class and aspects that aren't inherent but are additional concerns. The inherent features are part of the explicit design. They're part of the inheritance hierarchy; they define what an object is. The other aspects can be mixins or decorations; they define how an object might also act.