code: def list_files(tag=None, sat_id=None, data_path=None, format_str=None):
      
      
      index = pds.date_range(pysat.datetime(2017,12,1), pysat.datetime(2018,12,1)) 
      # file list is effectively just the date in string format - '%D' works only in Mac. '%x' workins in both Windows and Mac
      names = [ data_path+date.strftime('%Y-%m-%d')+'.nofile' for date in index]
      return pysat.Series(names, index=index)
docs: Produce a fake list of files spanning a year