docs: def person(self, person_id):
          '''Access a Person object by id'''
          if not self.cache['persons'].get(person_id, None):
              try:
                  person_xml = self.bc.person(person_id)
                  p = Person(person_xml)
                  self.cache['persons'][person_id] = p
              except HTTPError:
                  return None
          return self.cache['persons'][person_id]