About Exception

1) rise exception

To raise an exception, you use the raise statement with an argument that is either a class (which should subclass Exception for the "catch clause") or an instance.

>>> raise ExceptionTraceback (most recent call last):File "<stdin>", line 1, in ?Exception>>> raise Exception('hyperdrive overload')Traceback (most recent call last):File "<stdin>", line 1, in ?Exception: hyperdrive overload

2)custom Exception class

be sure to subclass Exception (either directly or indirectly, which means that subclassing any other built-in exception is okay). Thus, writing a custom exception basically amounts to something like this:

class SomeCustomException(Exception): pass

3) catch exception

1 more than one except clause

try:    x = input('Enter the first number: ')    y = input('Enter the second number: ')    print x/yexcept ZeroDivisionError:    print "The second number can't be zero!"except TypeError:    print "That wasn't a number, was it?"

2 catching many Exceptions with one block

try:    x = input('Enter the first number: ')    y = input('Enter the second number: ')    print x/yexcept (ZeroDivisionError, TypeError, NameError):    print 'Your numbers were bogus...'

3 catch the object

If you want access to the exception object itself in an except clause, you can use two arguments instead of one. (Note that even when you are catching multiple exceptions, you are supplying except with only one argument—a tuple.) Because you explicitly catch the object itself, you can print it out so the user can see what happened.

try:    x = input('Enter the first number: ')    y = input('Enter the second number: ')    print x/yexcept (ZeroDivisionError, TypeError), e:    print e

4) catch all vs catch most

If you do want to catch all exceptions in a piece of code, you can simply omit the exception class from the except clause:

try:    x = input('Enter the first number: ')    y = input('Enter the second number: ')    print x/yexcept:    print 'Something wrong happened...'

Catching all exceptions like this is risky business because it will hide errors you haven’t thought of as well as those you're prepared for.In most cases, it would be better to use except Exception, e and perhaps do some checking on the exception object, e.That is so called "catch most" because the code in your try/except statement may be naughty and use the old-fashioned string exceptions, or perhaps create a custom exception that doesn't subclass Exception which don't match the class Exception. You can use it like:

try:    x = input('Enter the first number: ')    y = input('Enter the second number: ')    value = x/y    print 'x/y is', valueexcept Exception, e:    print 'Invalid input:', e

5) "else" and "finally" statement 

In some cases, it can be useful to have a block of code that is executed unless something bad happens; as with conditionals and loops, you can add an else clause to the try/except statement:

 while True:       try:            x = input('Enter the first number: ')            y = input('Enter the second number: ')            value = x/y            print 'x/y is', value       except:            print 'Invalid input. Please try again.'       else:            break

Finally, there is the finally clause. the finally clause will be executed, no matter what exceptions occur in the try clause. You can use it to do housekeeping after a possible exception. It is combined with a try clause:

x = Nonetry:    x = 1/0finally:    print 'Cleaning up...'    del x