code: def select_day(self, day):
          
  
          def _select_day(day):
              return self.data.loc[day, slice(None)]
  
          try:
              return self.new(_select_day(day), self.type, self.if_fq)
          except:
              raise ValueError('QA CANNOT GET THIS Day {} '.format(day))
docs: 选取日期(一般用于分钟线)
  
          Arguments:
              day {[type]} -- [description]
  
          Raises:
              ValueError -- [description]
  
          Returns:
              [type] -- [description]