7.8 Exercises

Since we're at the end of Part I, we'll just work on a few short exception exercises to give you a chance to play with the basics. Exceptions really are a simple tool, so if you get these, you've got exceptions mastered.

1. try/except. Write a function called oops that explicitly raises a IndexError exception when called. Then write another function that calls oops inside a try/except statement to catch the error. What happens if you change oops to raise KeyError instead of IndexError? Where do the names KeyError and IndexError come from? (Hint: recall that all unqualified names come from one of three scopes, by the LGB rule.)

2. Exception lists. Change the oops function you just wrote to raise an exception you define yourself, called MyError, and pass an extra data item along with the exception. Then, extend the try statement in the catcher function to catch this exception and its data in addition to IndexError, and print the extra data item.

3. Error handling. Write a function called safe(func, *args) that runs any function using apply, catches any exception raised while the function runs, and prints the exception using the exc_type and exc_value attributes in the sys module. Then, use your safe function to run the oops function you wrote in Exercises 1 and/or 2. Put safe in a module file called tools.py, and pass it the oops function interactively. What sort of error messages do you get? Finally, expand safe to also print a Python stack trace when an error occurs by calling the built-in print_exc() function in the standard traceback module (see the Python library reference manual or other Python books for details).