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.

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.)
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.
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).