docs: def _decade_ranges_in_date_range(self, begin_date, end_date):
          """Return a list of decades which is covered by date range."""
          begin_dated = begin_date.year / 10
          end_dated = end_date.year / 10
          decades = []
          for d in range(begin_dated, end_dated + 1):
              decades.append('{}-{}'.format(d * 10, d * 10 + 9))
          return decades