def set_default_names(data):
    """Sets index names to 'index' for regular, or 'level_x' for Multi"""
    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