6.9 The raise statement
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If no expressions are present, raise re-raises the last
expression that was active in the current scope. If no exception is active in the current
scope, an exception is raised indicating this error.
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.
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