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1 February 2015

Handling Python Exceptions

Exception handling is a core part of the Python language.

There are a number of reasons why using exceptions is a good idea:

(a) exceptions offer protection against unexpected conditions
(b) exceptions can deliver on-screen messages for warning and error reporting
(c) exceptions are an alternative to the print statement for debugging

Exceptions are especially useful in code associated with error-prone processes, for example data input, file operations and wireless communications.

So, what does a Python exception handler look like?

Here's a simple example:

try:
    x = int(raw_input("Please enter a number: ")
    break
except ValueError:
    print "Not a valid number. Please try again..."

The try part of the exception asks the user for a number. While the exception part tests for non-numeric values and reports problems back to the user. So this is a combination of the '(a)' an '(b)' scenarios above.

Here's a more elaborate example:

try:
    f = open(fname, 'r')
except IOError:
    print 'Can't open file: ', fname
else:
    print fname, ' contains ', len(f.readlines()), ' lines '
    f.close()

This time we only try to do something with the file fname if the open() operation is successful. Otherwise we notify the user of the problem.

Take a look at some non-trivial Python source code to see if you spot the exception handlers and work out why they exist.

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