Nesting Code Three Levels Deep

Let’s look at one last mutation of our script. Nesting can take us even further if we need it to—we could, for example, branch to one of a set of alternatives based on the relative magnitude of a valid input:

while True:
    reply = input('Enter text:')
    if reply == 'stop':
        break
    elif not reply.isdigit():
        print('Bad!' * 8)
    else:
        num = int(reply)
        if num < 20:
            print('low')
        else:
            print(num ** 2)
print('Bye')

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This version includes an if statement nested in the else clause of another if statement, which is in turn nested in the while loop. When code is conditional, or repeated like this, we simply indent it further to the right. The net effect is like that of the prior versions, but we’ll now print “low” for numbers less than 20:

Enter text:19
low
Enter text:20
400
Enter text:spam
Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad!
Enter text:stop
Bye