code: def swap_priority(self, key1, key2):
          
          heap = self._heap
          position = self._position
          if key1 not in self or key2 not in self:
              raise KeyError
          pos1, pos2 = position[key1], position[key2]
          heap[pos1].key, heap[pos2].key = key2, key1
          position[key1], position[key2] = pos2, pos1
docs: Fast way to swap the priority level of two items in the pqdict. Raises
          ``KeyError`` if either key does not exist.