code: def write_static_networks(gtfs, output_dir, fmt=None):
      
      if fmt is None:
          fmt = "edg"
      single_layer_networks = stop_to_stop_networks_by_type(gtfs)
      util.makedirs(output_dir)
      for route_type, net in single_layer_networks.items():
          tag = route_types.ROUTE_TYPE_TO_LOWERCASE_TAG[route_type]
          file_name = os.path.join(output_dir, "network_" + tag + "." + fmt)
          if len(net.edges()) > 0:
              _write_stop_to_stop_network_edges(net, file_name, fmt=fmt)
docs: Parameters
      ----------
      gtfs: gtfspy.GTFS
      output_dir: (str, unicode)
          a path where to write
      fmt: None, optional
          defaulting to "edg" and writing results as ".edg" files
           If "csv" csv files are produced instead