Conditional Statements in Python
This implementation of absolute_value raises several important issues:
A conditional statement in Python consists of a series of headers and suites: a
required if clause, an optional sequence of elif clauses, and finally an
optional else clause:
if <expression>:
<suite>
elif <expression>:
<suite>
else:
<suite>
When executing a conditional statement, each clause is considered in order. The
computational process of executing a conditional clause follows.
1. Evaluate the header's expression.
2. If it is a true value, execute the suite. Then, skip over all subsequent clauses in
the conditional statement.
If the else clause is reached (which only happens if all if and elif expressions
evaluate to false values), its suite is executed.
Boolean contexts. Above, the execution procedures mention "a false value" and "a
true value." The expressions inside the header statements of conditional blocks are said
to be in boolean contexts: their truth values matter to control flow, but otherwise their
values are not assigned or returned. Python includes several false values, including
0, None, and the boolean value False. All other numbers are true values. In Chapter 2,
we will see that every built-in kind of data in Python has both true and false values.
Boolean values. Python has two boolean values, called True and False. Boolean
values represent truth values in logical expressions. The built-in comparison
operations, >, <, >=, <=, ==, !=, return these values.
