raise_stmt: "raise" condition ["," condition ["," condition]]
raise evaluates its first condition, which must yield
a string, class, or instance object. If there is a second condition,
this is evaluated, else None is substituted. If the first
condition is a class object, then the second condition must be an
instance of that class or one of its derivatives. If the first
condition is an instance object, the second condition must be
None.
If the first object is a class or string, it then raises the exception
identified by the first object, with the second one (or None)
as its parameter. If the first object is an instance, it raises the
exception identified by the class of the object, with the instance as
its parameter (and there should be no second object, or the second
object should be None).
If a third object is present, and it it not None, it should be
a traceback object (see section 
), and it is
substituted instead of the current location as the place where the
exception occurred. This is useful to re-raise an exception
transparently in an except clause.