code: def _csv_str(self, param, stats, quantiles, index=None):
          
  
          buffer = param
          if not index:
              buffer += ', '
          else:
              buffer += '_' + '_'.join([str(i) for i in index]) + ', '
  
          for stat in ('mean', 'standard deviation', 'mc error'):
              buffer += str(stats[stat][index]) + ', '
  
          # Index to interval label
          iindex = [key.split()[-1] for key in stats.keys()].index('interval')
          interval = list(stats.keys())[iindex]
          buffer += ', '.join(stats[interval].T[index].astype(str))
  
          # Process quantiles
          qvalues = stats['quantiles']
          for q in quantiles:
              buffer += ', ' + str(qvalues[q][index])
  
          return buffer + '\n'
docs: Support function for write_csv