104 Chapter 8. Classes and Object-Oriented Programming

Figure 8.5 contains a class that can be used to keep track of the grades of a collection of students. Instances of class Grades are implemented using a list and a dictionary. The list keeps track of the students in the class. The dictionary maps a students identification number to a list of grades.

Notice that getGrades returns a copy of the list of grades associated with a student, and getStudents returns a copy of the list of students. The computational cost of copying the lists could have been avoided by simply returning the instance variables themselves. Doing so, however, is likely to lead to problems. Consider the code

allStudents = course1.getStudents() allStudents.extend(course2.getStudents())

If getStudents returned self.students, the second line of code would have the (probably unexpected) side effect of changing the set of students in course1.

The instance variable isSorted is used to keep track of whether or not the list of students has been sorted since the last time a student was added to it. This allows the implementation of getStudents to avoid sorting an already sorted list.

Figure 8.6 contains a function that uses class Grades to produce a grade report for some students taking 6.00, the MIT course for which this book was developed.