code: def validate_grid_districts(self):
          #TODO: check docstring
          
  
          msg_invalidity = []
          invalid_mv_grid_districts = []
  
          for grid_district in self.mv_grid_districts():
  
              # there's only one node (MV station) => grid is empty
              if len(grid_district.mv_grid._graph.nodes()) == 1:
                  invalid_mv_grid_districts.append(grid_district)
                  msg_invalidity.append('MV Grid District {} seems to be empty ' \
                                        'and ' \
                                        'was removed'.format(grid_district))
              # there're only aggregated load areas
              elif all([lvla.is_aggregated for lvla in
                        grid_district.lv_load_areas()]):
                  invalid_mv_grid_districts.append(grid_district)
                  msg_invalidity.append("MV Grid District {} contains only " \
                                   "aggregated Load Areas and was removed" \
                                   "".format(grid_district))
  
          for grid_district in invalid_mv_grid_districts:
              self._mv_grid_districts.remove(grid_district)
  
          logger.warning("\n".join(msg_invalidity))
          logger.info('=====> MV Grids validated')
          return msg_invalidity
docs: Tests MV grid districts for validity concerning imported data such as:
              
              i) Uno            
              ii) Dos
          
          Invalid MV grid districts are subsequently deleted from Network.