Bad Boys - Python

Most of what you've just learned also applies to Python's otherexception-handlng construct, the "try-finally" statement. The "try-finally"statement block differs from "try-except-else" in that it merely detectserrors; it does not provide for a mechanism to resolve them. It istypically used to ensure that certain statements are always executed whenan error (regardless of type) is encountered.

The "try-finally" statement block looks like this:

try:
execute this block
finally:
if exceptions generated, execute this block

If an exception is encountered when running the code within the "try"block, Python will stop execution at that point; jump to the "finally"block; execute the statements within it; and then pass the exceptionupwards, to the parent "try" block, if one exists, or to the defaulthandler, which terminates the program and displays a stack trace.

Here's an example:

#!/usr/bin/python



dessert = ('apple pie', 'chocolate fudge cake', 'icecream')



try:
# generate error by accessing index out of range
print dessert[10]
finally:
print "Something bad happened"

When this program runs, an IndexError exception will be generated and the"finally" block will execute, printing an error message. Control will thenflow to the parent exception handler, which is the Python interpreter inthis case; the interpreter will terminate the program and print a stacktrace.

$ dessert.py
Something bad happened
Traceback (innermost last):
File "dessert.py", line 7, in ?
print dessert[10]
IndexError: tuple index out of range

Since "try-finally" blocks simply detect errors, passing the resolutionbuck upwards to the parent "try" block, it's possible to nest them within"try-except-else" blocks. Take a look:

#!/usr/bin/python



try:
dessert = ('apple pie', 'chocolate fudge cake', 'icecream')



try:
# generate error by accessing index out of range
print dessert[10]
finally:
print "Something bad happened"



except IndexError:
print "You attempted to access a non-existent element. Bad boy!"
except NameError:
print "You attempted to access a non-existent object. What are you
thinking?"

Here's what'll happen when you run it:

Something bad happened
You attempted to access a non-existent element. Bad boy!

>>> More Python Articles          >>> More By Vikram Vaswani, (c) Melonfire

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