One thought comes to mind with regards to selecting particle types as part of the new method: There needs to be a simple and memory efficient way to select multiple types of particles (or all types). For example, for the halo finders, sometimes only dark matter is wanted, and other times stars (or perhaps even only certain types of stars) are wanted as well. In the past we've done things to avoid na.concatenate with particles because it fragments memory. I think Matt has thought of this already, but I'd like to extend this discussion to how to allow users to select particle types without having to do na.concatenate manually (or have it happen behind the scenes).
With regards to obj['particle_position_x'] or obj.particles['position_x'], I think the argument for changing it is not a bad one, but that if we do this, I think it would make the most sense to extend it to other things, like obj.grid['Density'], for continuity's sake.
I'm for a user survey if we think we'll get replies. Have users generally been forthcoming with opinions on technical stuff like this before?
I'll see you in the chat on Monday!
-- Stephen Skory firstname.lastname@example.org http://stephenskory.com/... 510.621.3687 (google voice)