Python in a Nutshell, 2nd Edition

Control Flow Statements

A program’s control flow is the order in which the program’s code executes. The control flow of a Python program is regulated by conditional statements, loops, and function calls. (This section covers the if statement and for and while loops; functions are covered in Functions.) Raising and handling exceptions also affects control flow; exceptions are covered in Chapter 6.

The if Statement

Often, you need to execute some statements only if some condition holds, or choose statements to execute depending on several mutually exclusive conditions. The Python compound statement if, comprising if, elif, and else clauses, lets you conditionally execute blocks of statements. Here’s the syntax for the if statement:

if expression:
    statement(s)
elif expression:
    statement(s)
elif expression:
    statement(s)
...
else:
    statement(s)

The elif and else clauses are optional. Note that, unlike some languages, Python does not have a switch statement. Use if, elif, and else for all conditional processing.

Here’s a typical if statement with all three kinds of clauses:

if x < 0: print "x is negative"
elif x % 2: print "x is positive and odd"
else: print "x is even and non-negative"

When there are multiple statements in a clause (i.e., the clause controls a block of statements), the statements are placed on separate logical lines after the line containing the clause’s keyword (known as the header line of the clause), indented rightward from the header line. The block terminates when the indentation ...