Learning Python

if Tests

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 of an if looks like this:

if <test1>:               # if test 
    <statements1>         # associated block
elif <test2>:             # optional elif's 
    <statements2>
else:                     # optional else
    <statements3>

Examples

Here are two simple examples of the if statement. All parts are optional except the initial if test and its associated statements. Here’s the first:

>>> if 1:
...     print 'true'
...
true
>>> if not 1:
...     print 'true'
... else:
...     print 'false'
...
false

Now, here’s an example of the most complex kind of if statement—with all its optional parts present. ...