I have a pretty simple question that I ought to know the answer to -  how do I extract the actual values in the field_list or derived_field_list of a ds?

For example, I have a field: ('PartType0', 'Coordinates'),

but printing something like pf[('PartType0','Coordinates')] doesn't return the list of particle type 0 coordinates (or anything for that matter).  Do I have to access this through a pf.all_data() command somehow?