Exception Handling in Python

by Phillip Watts

July 23, 2009

Introduction

All modern high level computer languages, and most ancient ones, have built-in constructs and methods for the handling of errors or exceptions. I will loosely define an error as anything which could happen, though you hope that it does not, which will cause the program to fail. "Fail" could mean the program crashes, or it could mean that the program is going to produce meaningless results.

I must say this to the beginning programmer: once you begin using networks, servers, databases, or even text files you have built yourself, you are wide open to errors. And the possibilities become endless. If you like, you can wait until errors occur then patch in the solutions. It is much wiser to anticipate the conditions which bad data could cause, and make this a part of your program plan.

Python has robust and elegant error handling methods, and this article will provide an introduction to those methods. The reader of this article should have at least a minimum familiarity with Python, or something comparable such as Java or Ruby.

Let us start with a simple example, a try/except/else:

First a simple prgoram:

#!/usr/bin/env python
a = 2
b = 0
c = a / b

The division operation, in this example, is doomed to failure. Python "crashes" on divide by zero.

Traceback (most recent call last):
  File "./sample.py", line 4, in <module>
    c = a / b
ZeroDivisionError: integer division or modulo by zero

Now let's try that with exception handling:

#!/usr/bin/env python
import sys

a = 2
b = 0
try:
    print 'a=',a
    print 'b=',b
    c = a / b
    print 'c=',c
except:
    print 'division failed miserably'
    c = 0
    print 'c defaults to',c
else  :
    print 'division went very well'

This time the program will not crash, we will get an error message printed instead. Our program will continue to execute.

The statements in the try: block are attempted.
The first failure jumps to the except: block
If nothing in the try block fails, the program jumps to the else: block.
The 1st two print statements execute, but then the try block fails, so - print 'c=',c - does not.
The except block executes.
The else block does not execute.
And our output looks like this:

a= 2
b= 0
division failed miserably
c defaults to 0

In this simple example, I set a default value for c in the except block. I am presuming that an undefined value for c will cause problems later. I could have set a default value for c before the stament which failed. This would have worked just as well. The statement which failed would not have altered the value of c.

Exception Handling in Python
Exception Handling in Python - Cont.