code: def set_default_names(data):
      
      if com._all_not_none(*data.index.names):
          nms = data.index.names
          if len(nms) == 1 and data.index.name == 'index':
              warnings.warn("Index name of 'index' is not round-trippable")
          elif len(nms) > 1 and any(x.startswith('level_') for x in nms):
              warnings.warn("Index names beginning with 'level_' are not "
                            "round-trippable")
          return data
  
      data = data.copy()
      if data.index.nlevels > 1:
          names = [name if name is not None else 'level_{}'.format(i)
                   for i, name in enumerate(data.index.names)]
          data.index.names = names
      else:
          data.index.name = data.index.name or 'index'
      return data
docs: Sets index names to 'index' for regular, or 'level_x' for Multi