When working in the object-oriented programming paradigm, we use the object metaphor to guide the organization of our programs. Most logic about how to represent and manipulate data is expressed within class declarations. In this section, we see that classes and objects can themselves be represented using just functions and dictionaries. The purpose of implementing an object system in this way is to illustrate that using the object metaphor does not require a special programming language. Programs can be object-oriented, even in programming languages that do not have a built-in object system.

In order to implement objects, we will abandon dot notation (which does require built-in language support), but create dispatch dictionaries that behave in much the same way as the elements of the built-in object system. We have already seen how to implement message-passing behavior through dispatch dictionaries. To implement an object system in full, we send messages between instances, classes, and base classes, all of which are dictionaries that contain attributes.

We will not implement the entire Python object system, which includes features that we have not covered in this text (e.g., meta-classes and static methods). We will focus instead on user-defined classes without multiple inheritance and without introspective behavior (such as returning the class of an instance). Our implementation is not meant to follow the precise specification of the Python type system. Instead, it is designed to implement the core functionality that enables the object metaphor.