Learning Python, 2nd Edition

Chapter 9. if Tests

This chapter presents the Python if statement—the main statement used for selecting from alternative actions based on test results. Because this is our first exposure to compound statements—statements which embed other statements—we will also explore the general concepts behind the Python statement syntax model here. And because the if statement introduces the notion of tests, we’ll also use this chapter to study the concepts of truth tests and Boolean expressions in general.

if Statements

In simple terms, the Python if statement selects actions to perform. It’s the primary selection tool in Python and represents much of the logic a Python program possesses. It’s also our first compound statement; like all compound Python statements, the if may contain other statements, including other ifs. In fact, Python lets you combine statements in a program both sequentially (so that they execute one after another), and arbitrarily nested (so that they execute only under certain conditions).

General Format

The Python if statement is typical of most procedural languages. It takes the form of an if test, followed by one or more optional elif tests (meaning “else if”), and ends with an optional else block. Each test and the else have an associated block of nested statements indented under a header line. When the statement runs, Python executes the block of code associated with the first test that evaluates to true, or the else block if all tests prove false. The general form ...