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6.9 The `raise` statement

  raise_stmt | ::= | "raise" [expression ["," expression ["," expression]]]

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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.