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
Return a list of decades which is covered by date range.