code: def union(self, other, sort=None):
          self._assert_can_do_setop(other)
          if len(other) == 0 or self.equals(other) or len(self) == 0:
              return super().union(other, sort=sort)
          if isinstance(other, RangeIndex) and sort is None:
              start_s, step_s = self._start, self._step
              end_s = self._start + self._step * (len(self) - 1)
              start_o, step_o = other._start, other._step
              end_o = other._start + other._step * (len(other) - 1)
              if self._step < 0:
                  start_s, step_s, end_s = end_s, -step_s, start_s
              if other._step < 0:
                  start_o, step_o, end_o = end_o, -step_o, start_o
              if len(self) == 1 and len(other) == 1:
                  step_s = step_o = abs(self._start - other._start)
              elif len(self) == 1:
                  step_s = step_o
              elif len(other) == 1:
                  step_o = step_s
              start_r = min(start_s, start_o)
              end_r = max(end_s, end_o)
              if step_o == step_s:
                  if ((start_s - start_o) % step_s == 0 and
                          (start_s - end_o) <= step_s and
                          (start_o - end_s) <= step_s):
                      return RangeIndex(start_r, end_r + step_s, step_s)
                  if ((step_s % 2 == 0) and
                          (abs(start_s - start_o) <= step_s / 2) and
                          (abs(end_s - end_o) <= step_s / 2)):
                      return RangeIndex(start_r, end_r + step_s / 2, step_s / 2)
              elif step_o % step_s == 0:
                  if ((start_o - start_s) % step_s == 0 and
                          (start_o + step_s >= start_s) and
                          (end_o - step_s <= end_s)):
                      return RangeIndex(start_r, end_r + step_s, step_s)
              elif step_s % step_o == 0:
                  if ((start_s - start_o) % step_o == 0 and
                          (start_s + step_o >= start_o) and
                          (end_s - step_o <= end_o)):
                      return RangeIndex(start_r, end_r + step_o, step_o)
          return self._int64index.union(other, sort=sort)