code: def get_bar(self, code, time):
          
          try:
              return self.data.loc[(pd.Timestamp(time), code)]
          except:
              raise ValueError(
                  'DATASTRUCT CURRENTLY CANNOT FIND THIS BAR WITH {} {}'.format(
                      code,
                      time
                  )
              )
docs: 获取一个bar的数据
          返回一个series
          如果不存在,raise ValueError