Chapter 12. if Tests
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.
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
statement may contain other statements, including other if
s. 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).
General Format
The Python 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 ...
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