1.10 Exercises

This is a practical book. As such, we’re not assigning a bunch of fake object-oriented analysis problems to create designs for you to analyze and design. Instead, we want to give you some ideas that you can apply to your own projects. If you have previous object-oriented experience, you won’t need to put much effort into this chapter. However, they are useful mental exercises if you’ve been using Python for a while, but have never really cared about all that class stuff.

First, think about a recent programming project that you’ve completed. Identify the most prominent object in the design. Try to think of as many attributes for this object as possible. Did it have the following: Color? Weight? Size? Profit? Cost? Name? ID number? Price? Style?

Think about the attribute types. Were they primitives or classes? Were some of those attributes actually behaviors in disguise? Sometimes, what looks like data is actually calculated from other data on the object, and you can use a method to do those calculations. What other methods or behaviors did the object have? Which objects called those methods? What kinds of relationships did they have with this object?

Now, think about an upcoming project. It doesn’t matter what the project is; it might be a fun free-time project or a multi-million-dollar contract. It doesn’t have to be a complete application; it could just be one subsystem. Perform a basic object-oriented analysis. Identify the requirements and the interacting objects. Sketch out a class diagram featuring the highest level of abstraction on that system. Identify the major interacting objects. Identify minor supporting objects. Go into detail about the attributes and methods of some of the most interesting ones.

The goal is not to design a system right now (although you’re certainly welcome to do so if inclination meets both ambition and available time). The goal is to think about object-oriented design from the perspective of class, responsibility, and collaboration among objects. It can help to focus on projects that you have worked on, or are expecting to work on in the future; this makes it less of a hypothetical exercise and more practical.

Lastly, visit your favorite search engine and look up some tutorials on UML. There are dozens, so find one that suits your preferred method of study. Sketch some class diagrams or a sequence diagram for the objects you identified earlier. Don’t get too hung up on memorizing the syntax (after all, if it is important, you can always look it up again); just get a feel for the language. Something will stay lodged in your brain, and it can make communicating a bit easier if you can quickly sketch a diagram for your next OOP discussion.

The UML diagrams in this book were prepared with the PlantUML tool. The documentation for this tool includes numerous example diagrams that can help show how to describe objects and their relationships. Some people like to use Mermaid ( https://mermaid.live/) to create UML diagrams.