In this chapter, we meet Python's two main
looping constructs—statements that repeat
an action over and over. The first of these, the
while loop, provides a way to code general loops;
the second, the
for statement, is designed for
stepping through the items in a sequence object and running a block
of code for each item.
There are other kinds of looping operations in Python, but the two
statements covered here are the primary syntax provided for coding
repeated actions. We'll also study a few unusual
statements such as
continue here, because they are used within loops.
while statement is its most general iteration
construct. In simple terms, it repeatedly executes a block of
indented statements, as long as a test at the top keeps evaluating to
a true value. When the test becomes false, control continues after
all the statements in the
while block; the body
never runs if the test is false to begin with.
while statement is one of two looping
statements (along with the
for). It is called a
loop because control keeps looping back to the start of the
statement, until the test becomes false. The net effect is that the
loop's body is executed repeatedly while the test at
the top is true. Besides statements, Python also provides a handful
of tools that implicitly loop (iterate): the
in membership test; list comprehensions; and
more. We'll explore most of these in Chapter 14.