Chapter 16. Classes and Functions

Now that we know how to create new types, the next step is to write functions that take programmer-defined objects as parameters and return them as results. In this chapter I also present “functional programming style” and two new program development plans.

Code examples from this chapter are available from Solutions to the exercises are at


As another example of a programmer-defined type, we’ll define a class called Time that records the time of day. The class definition looks like this:

class Time:
    """Represents the time of day.
    attributes: hour, minute, second

We can create a new Time object and assign attributes for hours, minutes, and seconds:

time = Time()
time.hour = 11
time.minute = 59
time.second = 30

The state diagram for the Time object looks like Figure 16-1.

As an exercise, write a function called print_time that takes a Time object and prints it in the form hour:minute:second. Hint: the format sequence '%.2d' prints an integer using at least two digits, including a leading zero if necessary.

Write a boolean function called is_after that takes two Time objects, t1 and t2, and returns True if t1 follows t2 chronologically and False otherwise. Challenge: don’t use an if statement.

Figure 16-1. Object diagram.

Pure Functions

In the next few sections, we’ll ...

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