'existing_entry will have the who, when, and other properties of the existing entry'

def __init__(self, course_id, existing_entry):

    super(DuplicateCourseError, self).__init__(u'Cannot    create    course    {},    which    duplicates    {}'.format(course_id, existing_entry))
    self.course_id = course_id
    self.existing_entry = existing_entry
