Python Mini-CourseUniversity of Oklahoma

Department of Psychology

Lesson 19Handling Exceptions

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Lesson objectives

1. Understand how exceptions are generated and handled in Python

2. Use the raise statement to generate exceptions

3. Use the try…except statement to intercept and handle exceptions

4. List the common built-in exceptions

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What is an exception?

Exceptions are run-time errorsWhenever the interpreter has a problem it notifies the user/programmer by raising an exception

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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

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Example: nocatch.py

fin = open('bad_file')for line in fin: print linefin.close()

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Example: catch.py

try: fin = open('bad_file') for line in fin: print line fin.close()except: print 'Something went wrong.'

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Catching specific problems

There are a number of kinds of exceptions, including:IndexErrorEOFErrorKeyErrorSyntaxError

http://docs.python.org/library/exceptions.html#bltin-exceptions

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Example: catch2.py

try: fin = open('bad_file') for line in fin: print line fin.close()except IOError: print 'Something went wrong.'

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Error handling

Once you catch the error, you need to handle it1. Perform an alternate action2. Generate a customized error

message and gracefully end the program

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Raising exceptions

You can also create your own error generating and handling routines by raising an exception

Example:study147.py line 146

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Exercise

Write a short program to divide two numbersInclude a routine to handle division by zero

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