Cannot use particle unions with derived fields

[cphyc]

Bug report

Bug summary

It is not possible to create particle unions made of particle fields from particle filters.

Code for reproduction

import yt
from yt.data_objects.particle_unions import \
    ParticleUnion

ds = yt.load('ramses_new_format/output_00002/info_00002.txt')

u = ParticleUnion("tracer", ["gas_tracer", "star_tracer"])
ds.add_particle_union(u)

ds.r['tracer', 'particle_family']

In this case gas_tracer and star_tracer are particle filters that get dynamically defined by the RAMSES frontend.

Actual outcome

Traceback (most recent call last):
  File "test_part_counts.py", line 11, in <module>
    u = ParticleUnion("tracer", ["gas_tracer", "star_tracer"])
  File "~/Documents/prog/yt/yt/data_objects/region_expression.py", line 42, in __getitem__
    return self.all_data[item]
  File "~/Documents/prog/yt/yt/data_objects/data_containers.py", line 255, in __getitem__
    self.get_data(f)
  File "~/Documents/prog/yt/yt/data_objects/data_containers.py", line 1359, in get_data
    read_particles, gen_particles = self.index._read_particle_fields(
  File "~/Documents/prog/yt/yt/geometry/geometry_handler.py", line 227, in _read_particle_fields
    fields_to_read)
  File "~/Documents/prog/yt/yt/utilities/io_handler.py", line 201, in _read_particle_selection
    psize = self._count_particles_chunks(chunks, ptf, selector)
  File "~/Documents/prog/yt/yt/utilities/io_handler.py", line 170, in _count_particles_chunks
    for ptype, (x, y, z) in self._read_particle_coords(chunks, ptf):
  File "~/Documents/prog/yt/yt/frontends/ramses/io.py", line 146, in _read_particle_coords
    rv = self._read_particle_subset(subset, fields)
  File "~/Documents/prog/yt/yt/frontends/ramses/io.py", line 195, in _read_particle_subset
    raise YTFieldTypeNotFound(ptype)
yt.utilities.exceptions.YTFieldTypeNotFound: Could not find field type 'gas_tracer'.

Version Information

• Operating System: Arch Linux
• Python Version: 3.6.6
• yt version: 3.5dev0 46da297

[ngoldbaum]

It is not possible to create particle unions made of derived fields.

Do you mean particle filters?

[cphyc]

@ngoldbaum yes sorry ; I edited my OP