This chapter presents the Python
if statement, which is the main statement used for selecting from alternative actions based on test results. Because this is our first in-depth look at compound statements—statements that embed other statements—we will also explore the general concepts behind the Python statement syntax model here in more detail than we did in the introduction in Chapter 10. And, because the
if statement introduces the notion of tests, this chapter will also deal with Boolean expressions, and fill in some details on truth tests in general.
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 statement may contain other statements, including other
ifs. In fact, Python lets you combine statements in a program sequentially (so that they execute one after another), and in an arbitrarily nested fashion (so that they execute only under certain conditions).
if statement is typical of
if statements in most procedural languages. It takes the form of an
if test, followed by one or more optional
elif ("else if") tests, and a final optional
else block. The tests and the
else part each have an associated block of nested statements, indented under a header line. When the
if statement runs, Python executes the block of code ...