Chapter 16. Classes and Functions

Code examples from this chapter are available from


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

class Time(object):
    """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.

Exercise 16-1.

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.

Exercise 16-2.

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.

Object diagram.

Figure 16-1. Object diagram.

Pure Functions

In the next few sections, we’ll write two functions that add time values. They demonstrate two kinds of functions: pure functions and modifiers. They also demonstrate a development plan I’ll call prototype and patch, which is a way of tackling a complex problem by starting with a simple prototype and incrementally dealing with the complications.

Here is ...

Get Think Python now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.