When is "raising a class instance" useful?

Question

If you have an exception instance, why do you need to specify its class?

What is the advantage over just raise instance ?

Answer 1

There is no advantage. raise Class, instance is an obsolete expression and is fully equivalent to raise instance . In particular, the first notation was removed in Python 3 and replaced by the last .

Answer 2

There is no reason to actually use it. This archaic usage from the back before the exceptions was supposed to be class instances, and it went away in Python 3.

Answer 3

You can pass one exception to another:

 exc = StopIteration('Iterator has already exhaused!') try: raise ValueError, exc except ValueError, e: print e.args[0] 

A good practical application of this still eludes me, although there must be some.