@drdavella and I were just talking about this as it came up yesterday.

There could be a use-case to have multiple sub-regions in a SpectralRegion. This could be for calculating an RMS over the left and right ends of a spectrum (for example) so one would need to define a SpectralRegion with one sub-region on the left and one sub-region on the right.

The main variable would be a list of 2-tuples (rather than the current single 2-tuple). Each 2-tuple would be a (lower, upper) bound which could also have units on both (as currently is implemented).

Some possible useful usage:

• SpectralRegion([(1, 2), (8, 20), (40, 50)]) (__init__ could take a list of 2-tuples, units allowed too)
• spectral_region_out = spectral_region1 + spectral_region2 (effectively a list extend)
• spectral_region1[1] (__getitem__)
• del spectral_region[1] (__delitem__)

Considerations

• What happens with overlapping sub-regions?
  • One could say that sub-regions are allowed to overlap (so no real checking needs to happen) but then the spectral_region.extract(spectrum) would have to be thought through. I think it would make the most sense to not duplicate points in the extracted spectrum (i.e., sub-regions don't duplicate on extraction). One could also have a flag in there.
  • Actually, I think more thought needs to happen with the extract spectrum of a multiple sub-region SpectralRegion. Does it make sense to extract multiple sub-regions into one Spectrum1D? The dispersion axis could be very disjoint, which might be fine, just might have to be careful with the understanding/interpretation of it.
• Currently spectral_region.lower returns a single value, with the new implementation it should probably return a list of the lower bounds (same for upper)
• It might be useful to implement __iter__ and __next__ so one can iterate over the spectral_region (to get each sub-region). Not 100% sure of the use-case on this, but it could be interesting/useful.

[This was in discussion with @drdavella]

@eteq @stscicrawford @nmearl @drdavella