code: def distance_between(self, place_1, place_2, unit='km'):
          
          pickled_place_1 = self._pickle(place_1)
          pickled_place_2 = self._pickle(place_2)
          try:
              return self.redis.geodist(
                  self.key, pickled_place_1, pickled_place_2, unit=unit
              )
          except TypeError:
              return None
docs: Return the great-circle distance between *place_1* and *place_2*,
          in the *unit* specified.
  
          The default unit is ``'km'``, but ``'m'``, ``'mi'``, and ``'ft'`` can
          also be specified.