Handling Errors by Testing Inputs

So far so good, but notice what happens when the input is invalid:

Enter text:xxx
...error text omitted...
ValueError: invalid literal for int() with base 10: 'xxx'

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The built-in int function raises an exception here in the face of a mistake. If we want our script to be robust, we can check the string’s content ahead of time with the string object’s isdigit method:

>>> S = '123'
>>> T = 'xxx'
>>> S.isdigit(), T.isdigit()
(True, False)

This also gives us an excuse to further nest the statements in our example. The following new version of our interactive script uses a full-blown if statement to work around the exception on errors:

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

We’ll study the if statement in more detail in Chapter 12, but it’s a fairly lightweight tool for coding logic in scripts. In its full form, it consists of the word if followed by a test and an associated block of code, one or more optional elif (“else if”) tests and code blocks, and an optional else part, with an associated block of code at the bottom to serve as a default. Python runs the block of code associated with the first test that is true, working from top to bottom, or the else part if all tests are false.

The if, elif, and else parts in the preceding example are associated as part of the same statement because they all line up vertically (i.e., share the same level of indentation). The if statement spans from the word if to the start of the print statement on the last line of the script. In turn, the entire if block is part of the while loop because all of it is indented under the loop’s header line. Statement nesting is natural once you get the hang of it.

When we run our new script, its code catches errors before they occur and prints an (arguably silly) error message to demonstrate:

Enter text:5
25
Enter text:xyz
Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad!
Enter text:10
100
Enter text:stop