Exception Errors

Below is some common exceptions errors in Python:

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

Exception Errors Examples

Now, when we know what some of the exception errors means, let's see some examples:

except IOError:

print('An error occurred 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 occurred.')

Set up exception handling blocks To use exception handling in Python, you first need to have a catch-all except clause.

The words "try" and "except" are Python keywords and are used to catch exceptions.

try-except [exception-name] (see above for examples) blocks

The code within the try clause will be executed statement by statement.

If an exception occurs, the rest of the try block will be skipped and the except clause will be executed.

try:
    some statements here
except:
    exception handling

Let's see a short example on how to do this:

try:
    print 1/0

except ZeroDivisionError:
    print "You can't divide by zero, you're silly."

How does it work? The error handling is done through the use of exceptions that are caught in try blocks and handled in except blocks. 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.

EXAMPLE:

import sys

print "Lets fix the previous code with exception handling"

try:
    number = int(raw_input("Enter a number between 1 - 10 \n"))

except ValueError:
    print "Err.. numbers only"
    sys.exit()

print "you entered number \n", number

Try ... except ... else clause

The else clause in a try , except statement must follow all except clauses, and is useful for code that must be executed if the try clause does not raise an exception.

try:
    data = something_that_can_go_wrong

except IOError:
    handle_the_exception_error

else:
    doing_different_exception_handling

Exceptions in the else clause are not handled by the preceding except clauses.

Make sure that the else clause is run before the finally block.

Try ... finally clause

The finally clause is optional. It is intended to define clean-up actions that must be executed under all circumstances

try:
    raise KeyboardInterrupt
finally:
    print 'Goodbye, world!'
...
Goodbye, world!
KeyboardInterrupt

A finally clause is always executed before leaving the try statement, whether an exception has occurred or not.

Remember that if you don't specify an exception type on the except line, it will catch all exceptions, which is a bad idea, since it means your program will ignore unexpected errors as well as ones which the except block is actually prepared to handle.