Slicing and subscript operator for `skyfield.api.Star`

[bryan-lunt]

When we load a Star object from a pandas dataframe, it ends up acting like an array or container for multiple values. It behaves as a list of stars.

It would be nice to be able to subscript it, slice it, etc. the same way we can a Time object that contains multiple times.

For now, I have a function:

def get_one_star(st,i):
            return skyfield.api.Star(ra_hours = st.ra.hours[i],
                                    dec_degrees = st.dec.degrees[i],
                                    ra_mas_per_year=st.ra_mas_per_year[i],
                                    parallax_mas=st.parallax_mas[i],
                                    epoch=st.epoch[i]
                                )

But it would feel much more natural to do something like:

one_star = stars_from_dataframe[i]

The advantage of adding this to the API and putting the responsibility onto Skyfield is that if you change the contents and fields of Star later, then any code which uses the subscript operator will automagically benefit, without the user needing to read the and understand the inner workings of the objects.

More broadly speaking, there are many objects in Skyfield that can either act as a single thing or act as a numpy-like array of things. It would be nice if any of the ones that act as an array, when acting as an array, could be sliced or subscripted. Otherwise we end up needing to subscript every one of their fields.

[bryan-lunt]

What I'm trying to express is that by asking the client programmer to often know and worry about all of the fields of the objects, it's giving up some of the value of having a layer of abstraction. Much of the benefit of having an object is that the client doesn't necessarily need to know all its inner workings.