code: def _to_str(self, value, field_begin, field_len):
          
          value_str = '{0:0{1}d}'.format(value, field_len)
          if self.precision is not None and \
                  self.precision < field_begin + field_len:
              # field is partly or completely affected by precision
              # -> replace insignificant digits with asterisks
              precision_index = max(0, self.precision - field_begin)
              value_str = value_str[:precision_index] + \
                  '*' * (field_len - precision_index)
          return value_str
docs: Convert value (int) to a field string, considering precision.