From discussions in #6348, the question here is about the opportunities and implications of allowing SkyCoord to be mutable by adding a __setitem__ method. An implementation of __setitem__ for Time can be seen in #6028, and this addresses some of the complications that arise in setting items in an object with non-trivial attributes (e.g. scale for Time or frame). This issue is not mostly about implementation, we can assume that a suitable implementation can be found.

Opportunities include:

• Support table vstack and join
• Support table insert and add rows
• Making SkyCoord be more like normal arrays. In general mutable arrays are obviously rather useful!
• Allowing resize
• Others?

Implications:

• Astropy code needs to be scrubbed to be sure that __setitem__ clears all caches.
• Code that really depends on a SkyCoord being immutable? Where can this introduce bugs in code?

Discuss!