Sep. 30, 2012

Try and Except in Python

Earlier I wrote about Errors and Exceptions in Python. This post will be about how to handle those. Exception handling allows us to continue our program (or terminate it) if an exception occurs.

Error Handling

Error handling in Python is done through the use of exceptions that are caught in 
try blocks and handled in except blocks. 

Try and Except

If an error is encountered, a try block code execution is stopped and transferred
down to the except block. 

In addition to using an except block after the try block, you can also use the
finally block. 

The code in the finally block will be executed regardless of whether an exception
occurs.

Raising an Exception

You can raise an exception in your own program by using the raise exception 
[, value] statement. 

Raising an exception breaks current code execution and returns the exception
back until it is handled.

Example

A try block look like below
try:
    print "Hello World"
except:
    print "This is an error message!"

Exception Errors

Some of the common exception errors are:

IOError
If the file cannot be opened.

ImportError
If python cannot find the module

ValueError
Raised when a built-in operation or function receives an argument that has the
right type but an inappropriate value

KeyboardInterrupt
Raised when the user hits the interrupt key (normally Control-C or Delete)

EOFError
Raised when one of the built-in functions (input() or raw_input()) hits an
end-of-file condition (EOF) without reading any data

Example

Let's have a look at some examples using exceptions. 

except IOError:
    print('An error occured trying to read the file.')
    
except ValueError:
    print('Non-numeric data found in the file.')

except ImportError:
    print "NO module found"
    
except EOFError:
    print('Why did you do an EOF on me?')

except KeyboardInterrupt:
    print('You cancelled the operation.')

except:
    print('An error occured.')

There are a number of built-in exceptions in Python. 

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