code: def _average_precision(self, rec, prec):
          
          ap = 0.
          for t in np.arange(0., 1.1, 0.1):
              if np.sum(rec >= t) == 0:
                  p = 0
              else:
                  p = np.max(prec[rec >= t])
              ap += p / 11.
          return ap
docs: calculate average precision, override the default one,
          special 11-point metric
  
          Params:
          ----------
          rec : numpy.array
              cumulated recall
          prec : numpy.array
              cumulated precision
          Returns:
          ----------
          ap as float