Dealing with the imperfect (or how to handle errors)

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Subsections

Exercises

So you now have the perfect program, it runs flawlessly, except for one detail, it will crash on invalid user input. Have no fear, for Python has a special control structure for you. It's called try and it tries to do something. Here is an example of a program with a problem:

print "Type Control C or -1 to exit"
number = 1
while number != -1:
    number = int(raw_input("Enter a number: "))
    print "You entered: ",number

Notice how when you enter @#& it outputs something like:

Traceback (innermost last):
  File "try_less.py", line 4, in ?
    number = int(raw_input("Enter a number: "))
ValueError: invalid literal for int(): @#&

As you can see the int function is unhappy with the number @#& (as well it should be). The last line shows what the problem is; Python found a ValueError. How can our program deal with this? What we do is first: put the place where the errors occurs in a try block, and second: tell Python how we want ValueErrors handled. The following program does this:

print "Type Control C or -1 to exit"
number = 1
while number != -1:
    try:
        number = int(raw_input("Enter a number: "))
        print "You entered: ",number
    except ValueError:
        print "That was not a number."

Now when we run the new program and give it @#& it tells us ``That was not a number.'' and continues with what it was doing before.

When your program keeps having some error that you know how to handle, put code in a try block, and put the way to handle the error in the except block.

Exercises

Update at least the phone numbers program so it doesn't crash if a user doesn't enter any data at the menu.

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Josh Cogliati jjc@honors.montana.edu Wikibooks Version: Wikibooks Non-programmers Python Tutorial Contents