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By Root 1405 0

a few simple while loops in action. The first, which consists of a print statement nested in a while loop, just prints a message forever. Recall that True is just a custom version of the integer 1 and always stands for a Boolean true value; because the test is always true, Python keeps executing the body forever, or until you stop its execution. This sort of behavior is usually called an infinite loop:

>>> while True:

... print('Type Ctrl-C to stop me!')

The next example keeps slicing off the first character of a string until the string is empty and hence false. It’s typical to test an object directly like this instead of using the more verbose equivalent (while x != '':). Later in this chapter, we’ll see other ways to step more directly through the items in a string with a for loop.

>>> x = 'spam'

>>> while x: # While x is not empty

... print(x, end=' ')

... x = x[1:] # Strip first character off x

...

spam pam am m

Note the end=' ' keyword argument used here to place all outputs on the same line separated by a space; see Chapter 11 if you’ve forgotten why this works as it does. The following code counts from the value of a up to, but not including, b. We’ll see an easier way to do this with a Python for loop and the built-in range function later:

>>> a=0; b=10

>>> while a < b: # One way to code counter loops

... print(a, end=' ')

... a += 1 # Or, a = a + 1

...

0 1 2 3 4 5 6 7 8 9

Finally, notice that Python doesn’t have what some languages call a “do until” loop statement. However, we can simulate one with a test and break at the bottom of the loop body:

while True:

...loop body...

if exitTest(): break

To fully understand how this structure works, we need to move on to the next section and learn more about the break statement.

break, continue, pass, and the Loop else

Now that we’ve seen a few Python loops in action, it’s time to take a look at two simple statements that have a purpose only when nested inside loops—the break and continue statements. While we’re looking at oddballs, we will also study the loop else clause here, because it is intertwined with break, and Python’s empty placeholder statement, the pass (which is not tied to loops per se, but falls into the general category of simple one-word statements). In Python:

break

Jumps out of the closest enclosing loop (past the entire loop statement)

continue

Jumps to the top of the closest enclosing loop (to the loop’s header line)

pass

Does nothing at all: it’s an empty statement placeholder

Loop else block

Runs if and only if the loop is exited normally (i.e., without hitting a break)

General Loop Format

Factoring in break and continue statements, the general format of the while loop looks like this:

while :

if : break # Exit loop now, skip else

if : continue # Go to top of loop now, to test1

else:

# Run if we didn't hit a 'break'

break and continue statements can appear anywhere inside the while (or for) loop’s body, but they are usually coded further nested in an if test to take action in response to some condition.

Let’s turn to a few simple examples to see how these statements come together in practice.

pass

Simple things first: the pass statement is a no-operation placeholder that is used when the syntax requires a statement, but you have nothing useful to say. It is often used to code an empty body for a compound statement. For instance, if you want to code an infinite loop that does nothing each time through, do it with a pass:

while True: pass # Type Ctrl-C to stop me!

Because the body is just an empty statement, Python gets stuck in this loop. pass is roughly to statements as None is to objects—an explicit nothing. Notice that here the while loop’s body is on the same line as the header, after the colon; as with if statements, this only works if the body isn’t a compound statement.

This example does nothing forever. It probably isn’t the most useful Python program ever written (unless you want to warm up your laptop computer