|An Introduction to Python|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
RRP £12.95 ($19.95)
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8.6 Defining Clean-up Actions
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 ? KeyboardInterrupt
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
else clause), it is re-raised after the
finally clause has been executed. The
is also executed "on the way out" when any other clause of the
try statement is left via a
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
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 0954161769||An Introduction to Python||See the print edition|