Lesson 19 Handling Exceptions

1. Lesson 19Handling Exceptions Python Mini-Course University of Oklahoma Department of Psychology Python Mini-Course: Lesson 19

2. Lesson objectives • Understand how exceptions are generated and handled in Python • Use the raise statement to generate exceptions • Use the try…except statement to intercept and handle exceptions • List the common built-in exceptions Python Mini-Course: Lesson 19

3. What is an exception? • Exceptions are run-time errors • Whenever the interpreter has a problem it notifies the user/programmer by raising an exception Python Mini-Course: Lesson 19

4. Handling exceptions • By default, the interpreter handles exceptions by stopping the program and printing an error message • However, we can override this behavior by catching the exception Python Mini-Course: Lesson 19

5. Example: nocatch.py fin = open('bad_file') for line in fin: print line fin.close() Python Mini-Course: Lesson 19

6. Example: catch.py try: fin = open('bad_file') for line in fin: print line fin.close() except: print 'Something went wrong.' Python Mini-Course: Lesson 19

7. Catching specific problems • There are a number of kinds of exceptions, including: • IndexError • EOFError • KeyError • SyntaxError • http://docs.python.org/library/exceptions.html#bltin-exceptions Python Mini-Course: Lesson 19

8. Example: catch2.py try: fin = open('bad_file') for line in fin: print line fin.close() except IOError: print 'Something went wrong.' Python Mini-Course: Lesson 19

9. Error handling • Once you catch the error, you need to handle it • Perform an alternate action • Generate a customized error message and gracefully end the program Python Mini-Course: Lesson 19

10. Raising exceptions • You can also create your own error generating and handling routines by raising an exception • Example: • study147.py line 146 Python Mini-Course: Lesson 19

11. Exercise • Write a short program to divide two numbers • Include a routine to handle division by zero Python Mini-Course: Lesson 19