code: def sum(self, axis=0, *args, **kwargs):
          
          nv.validate_sum(args, kwargs)
          valid_vals = self._valid_sp_values
          sp_sum = valid_vals.sum()
          if self._null_fill_value:
              return sp_sum
          else:
              nsparse = self.sp_index.ngaps
              return sp_sum + self.fill_value * nsparse
docs: Sum of non-NA/null values
  
          Returns
          -------
          sum : float