Writing Exception Classes

I (LionKimbro) don't know much about writing exception classes; Here's hoping someone rewrites this better.

Exception classes are not special, you just derive them from Exception:

   1 class HostNotFound(Exception):
   2     def __init__( self, host ):
   3         self.host = host
   4         Exception.__init__(self)

You may later write:

   1 try:
   2     raise HostNotFound( "taoriver.net" )
   3 except HostNotFound, X:
   4     print "Host Not Found:", X.host

Overloading __str__

You can overload __str__ to get the exception to explain itself:

   1 class HostNotFound(Exception):
   2     def __init__( self, host ):
   3         self.host = host
   4     def __str__( self ):
   5         return "Host Not Found exception: missing %s" % self.host

That way, you only need print the exception instance:

   1 try:
   2     raise HostNotFound( "taoriver.net" )
   3 except HostNotFound, X:
   4     print X

I don't know if this is a good idea or not.

In this instance, there's no need to overload __str__, as the standard Exception already has a __str__ method:

   1 class HostNotFound(Exception):
   2     def __init__(self, host):
   3         self.host = host
   4         Exception.__init__(self, 'Host Not Found exception: missing %s' % host)

Questions

• How do you relay the traceback information? Relay the traceback information? Moving it higher up the call-stack? Could you try to explain your question?

• What better exception-foo is out there?