Python Mini-Course

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Lesson 19Handling Exceptions

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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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 tryexcept 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 errors
• Whenever 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 line
• fin.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
• IndexError
• EOFError
• KeyError
• SyntaxError
• http//docs.python.org/library/exceptions.htmlblt
  in-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 it
• Perform an alternate action
• 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 numbers
• Include a routine to handle division by zero