code: def pvariance(self):
          '返回DataStruct.price的方差 variance'
          res = self.price.groupby(level=1
                                  ).apply(lambda x: statistics.pvariance(x))
          res.name = 'pvariance'
          return ref pvariance(self):
          '返回DataStruct.price的方差 variance'
          res = self.price.groupby(level=1
                                  ).apply(lambda x: statistics.pvariance(x))
          res.name = 'pvariance'
          return res
docs: 返回DataStruct.price的方差 variance