code: def _calculate(self, field):
          '''
          If the offset is unknown, return 0
          '''
          base_offset = 0
          if self.base_field is not None:
              base_offset = self.base_field.offset
          target_offset = self._field.offset
          if (target_offset is None) or (base_offset is None):
              return 0
          return target_offset - base_offsef _calculate(self, field):
          '''
          If the offset is unknown, return 0
          '''
          base_offset = 0
          if self.base_field is not None:
              base_offset = self.base_field.offset
          target_offset = self._field.offset
          if (target_offset is None) or (base_offset is None):
              return 0
          return target_offset - base_offset
docs: If the offset is unknown, return 0