6.9 The raise statement
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If no expressions are present, raise re-raises the last
exception that was active in the current scope. If no exception is
active in the current scope, a TypeError exception is
raised indicating that this is an error (if running under IDLE, a
Queue.Empty exception is raised instead).
Otherwise, raise evaluates the expressions to get three
objects, using None as the value of omitted expressions. The
first two objects are used to determine the type and
value of the exception.
If the first object is an instance, the type of the exception is the
class of the instance, the instance itself is the value, and the
second object must be None.
If the first object is a class, it becomes the type of the exception.
The second object is used to determine the exception value: If it is
an instance of the class, the instance becomes the exception value.
If the second object is a tuple, it is used as the argument list for
the class constructor; if it is None, an empty argument list is
used, and any other object is treated as a single argument to the
constructor. The instance so created by calling the constructor is
used as the exception value.
If a third object is present and not None, it must be a
traceback object (see section 3.2), and
it is substituted instead of the current location as the place where
the exception occurred. If the third object is present and not a
traceback object or None, a TypeError exception is
raised. The three-expression form of raise is useful to
re-raise an exception transparently in an except clause, but
raise with no expressions should be preferred if the
exception to be re-raised was the most recently active exception in
the current scope.
Additional information on exceptions can be found in
section 4.2, and information about handling exceptions is
in section 7.4.
Release 2.4.5, documentation updated on 18 October 2006.
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