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General FormatIn its most complex form, the while statement consists of a header line with a testexpression, a body of one or more indented statements, and an optional else partthat is executed if control exits the loop without a break statement being encountered.Python keeps evaluating the test at the top, and executing the statementsnested in the loop body until the test returns a false value:while :else:Examples# Loop test# Loop body# Optional else# Run if didn't exit loop with breakTo illustrate, let’s look at a few simple while loops in action. The first, which consistsof a print statement nested in a while loop, just prints a message forever. Recallthat True is just a custom version of the integer 1, and always stands for a Booleantrue 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 isempty and hence false. It’s typical to test an object directly like this instead of usingthe more verbose equivalent (while x != '':). Later in this chapter, we’ll see otherways to step more directly through the items in a string with a for loop. Notice thetrailing comma in the print here—as we learned in Chapter 11, this makes all theoutputs show up on the same line:>>> x = 'spam'>>> while x: # While x is not empty... print x,... x = x[1:] # Strip first character off x...spam pam am mThe following code counts from the value of a up to, but not including, b. We’ll seean 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,... a += 1 # Or, a = a + 1...0 1 2 3 4 5 6 7 8 9while Loops | 249