Passing The Buck - Python

If one exception handler is nested within another, Python typically usesthe one closest to where the exception occurs. To illustrate this, considerthe following code snippet:

#!/usr/bin/python



# nested_handlers.py



def popeye():
try:
olive()
except NameError:
print "Error in popeye"



def olive():
try:
print someUnnamedVar
except NameError:
print "Error in olive"



try:
popeye()
except NameError:
print "Error in main"

In this case, there are three exception handlers defined for the same typeof exception. The outermost one is in the main program body, the next iswithin the popeye() function called from the main script, and the third iswithin the olive() function called by popeye().

When olive() runs, it generates a "NameError" exception, which isimmediately handled by its own "try" block.

$ nested_handlers.py
Error in olive

However, if olive() didn't have a "try" block, or its "try" block didn'taccount for "NameError" exceptions,

#!/usr/bin/python



def popeye():
try:
olive()
except NameError:
print "Error in popeye"



def olive():
print someUnnamedVar



try:
popeye()
except NameError:
print "Error in main"

the exception would be passed up to the previous level, the callingpopeye() function, which would generate

$ nested_handlers.py
Error in popeye

If popeye()'s exception handler didn't have the ability to handle theexception, the exception would move up even further, to the main body ofthe script,

#!/usr/bin/python



def popeye():
try:
olive()
# this wouldn't handle NameError
except IndexError:
print "Error in popeye"



def olive():
print someUnnamedVar



try:
popeye()
except NameError:
print "Error in main"

which would result in

$ nested_handlers.py
Error in main

And if there weren't any handlers capable of resolving the error,

#!/usr/bin/python



def popeye():
olive()



def olive():
print someUnnamedVar



popeye()

the exception would be processed by the default handlers, which would killthe script and print a stack trace.

Traceback (innermost last):
File "./test.py", line 9, in ?
popeye()
File "./test.py", line 4, in popeye
olive()
File "./test.py", line 7, in olive
print someUnnamedVar
NameError: someUnnamedVar

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

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