code: def set_preferred(self, preferred_addr, addr_1, addr_2):
          '''Set the preferred address.'''
          if addr_1 > addr_2:
              addr_1, addr_2 = addr_2, addr_1
  
          self._cache[(addr_1, addr_2)] = preferred_addf set_preferred(self, preferred_addr, addr_1, addr_2):
          '''Set the preferred address.'''
          if addr_1 > addr_2:
              addr_1, addr_2 = addr_2, addr_1
  
          self._cache[(addr_1, addr_2)] = preferred_addr
docs: Set the preferred address.