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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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8.6 Defining Clean-up Actions

The try statement has another optional clause which is intended to define clean-up actions that must be executed under all circumstances. For example:

    >>> try:
    ...     raise KeyboardInterrupt
    ... finally:
    ...     print 'Goodbye, world!'
    Goodbye, world!
    Traceback (most recent call last):
      File "<stdin>", line 2, in ?

A finally clause is always executed before leaving the try statement, whether an exception has occurred or not. When an exception has occurred in the try clause and has not been handled by an except clause (or it has occurred in a except or else clause), it is re-raised after the finally clause has been executed. The finally clause is also executed "on the way out" when any other clause of the try statement is left via a break, continue or return statement. A more complicated example:

    >>> def divide(x, y):
    ...     try:
    ...         result = x / y
    ...     except ZeroDivisionError:
    ...         print "division by zero!"
    ...     else:
    ...         print "result is", result
    ...     finally:
    ...         print "executing finally clause"
    >>> divide(2, 1)
    result is 2
    executing finally clause
    >>> divide(2, 0)
    division by zero!
    executing finally clause
    >>> divide("2", "1")
    executing finally clause
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 3, in divide
    TypeError: unsupported operand type(s) for /:
     'str' and 'str'

As you can see, the finally clause is executed in any event. The TypeError raised by dividing two strings is not handled by the except clause and therefore re-raised after the finally clauses has been executed.

In real world applications, the finally clause is useful for releasing external resources (such as files or network connections), regardless of whether the use of the resource was successful.

ISBN 0954161769An Introduction to PythonSee the print edition