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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_items(self, assessment_id):
 """Gets the items in sequence from an assessment. arg: assessment_id (osid.id.Id):
 the ``Id`` of the ``Assessment`` return: (osid.assessment.ItemList) - list of items raise: NotFound - ``assessmentid`` not found raise: NullArgument - ``assessment_id`` is ``null`` raise: OperationFailed - unable to complete request raise: PermissionDenied - authorization failure occurred *compliance: mandatory -- This method must be implemented.* """

        if assessment_id.get_identifier_namespace() != 'assessment.Assessment':
            raise errors.InvalidArgument
        return self._part_item_session.get_assessment_part_items(self._get_first_part_id(assessment_id))