code: def mean_harmonic(self):
          '返回DataStruct.price的调和平均数'
          res = self.price.groupby(level=1
                                  ).apply(lambda x: statistics.harmonic_mean(x))
          res.name = 'mean_harmonic'
          return ref mean_harmonic(self):
          '返回DataStruct.price的调和平均数'
          res = self.price.groupby(level=1
                                  ).apply(lambda x: statistics.harmonic_mean(x))
          res.name = 'mean_harmonic'
          return res
docs: 返回DataStruct.price的调和平均数