This course introduces classes, instances, and inheritance. You will learn how to use classes to represent data in concise and natural ways. You'll also learn how to override built-in methods and how to create "inherited" classes that reuse functionality. You'll also learn about how to design classes. Finally, you will be introduced to the good programming habit of writing automated tests for their own code. The course is best-suited for you if you are already familiar with Python fundamentals, which are covered in the "Python Basics" and "Python Functions, Files, and Dictionaries" courses (courses 1 and 2 of the Python 3 Programming Specialization). It is optional to have taken the "Data Collection and Processing with Python" course (course 3 of the specialization), but knowledge of retrieving and processing complex nested data is helpful. This is the fourth of five courses in the Python 3 Programming Specialization.