Python in a Nutshell, 2nd Edition

The raise Statement

You can use the raise statement to raise an exception explicitly. raise is a simple statement with the following syntax:

raise [expression1[, expression2]]

Only an exception handler (or a function that a handler calls, directly or indirectly) can use raise without any expressions. A plain raise statement re-raises the same exception object that the handler received. The handler terminates, and the exception propagation mechanism keeps searching for other applicable handlers. Using raise without expressions is useful when a handler discovers that it is unable to handle an exception it receives, or can handle the exception only partially, so the exception should keep propagating to allow handlers up the call stack to perform handling and clean-up.

When only expression1 is present, it can be a legacy-style instance object or class object (Python 2.5 changes this rule in order to allow new-style classes, as long as they inherit from the new built-in new-style class BaseException and instances of BaseException). In this case, if expression1 is an instance object, Python raises that instance. When expression1 is a class object, raise instantiates the class without arguments and raises the resulting instance. When both expressions are present, expression1 must be a legacy-style class object. raise instantiates the class, with expression2 as the argument (or multiple arguments if expression2 is a tuple), and raises the resulting instance. Note that the raise statement is ...