Hi Stephanie,

Thanks for including the script to reproduce the issue you're having. That made it much easier for me to see what was going on.

I think just adding the else statement is fine. I suspect what happened is that the original author of the OWLS/EAGLE frontends didn't consider that those functions might be called when a user defines a particle filter. In that case, ftype should be the name of the particle filter ('gas_cold' for your example). Just adding the else statement as you've done should be sufficient to fix the issue.

Would you like to make a pull request to the main yt repository making that fix? You could also add a test to the owls frontend (I'd do it in yt/frontends/owls/tests/test_outputs.py) that just makes sure your example script does something sensible.

If you don't want to make the pull request I'm happy to do so, just let me know.

-Nathan

On Tue, Mar 13, 2018 at 3:36 PM, Stephanie Ho <stephaniehkho@gmail.com> wrote:

Hi,

I tried to set a particle filter with EAGLE and got an error. Here is a simple example, which uses the EAGLE dataset from yt sample data.

import yt
from yt.data_objects.particle_filters import add_particle_filter
eagle_fpath = './data/snapshot_028_z000p000/snap_028_z000p000.0.hdf5'
unit_base = {'UnitLength_in_cm' : 3.085678e+24,
'UnitMass_in_g' : 1.989e+43,
'UnitVelocity_in_cm_per_s' : 100000.0}
ds = yt.load(eagle_fpath,unit_base=unit_base)
ad = ds.all_data()

def cold_gas(pfilter, data):
temperature = data[pfilter.filtered_type, "Temperature"]
filter = (temperature.in_units('K') <= 1e5)
return filter

add_particle_filter("gas_cold", function=cold_gas, filtered_type='PartType0', requires=["Temperature"])
ds.add_particle_filter('gas_cold')

Below shows the error.

UnboundLocalError Traceback (most recent call last)
/Users/stephaniehho/EAGLE_sim/sho/test_with_yt/test_z000p000/particle_filter_simple.py in <module>()
20
21 add_particle_filter("gas_cold", function=cold_gas, filtered_type='PartType0', requires=["Temperature"])
---> 22 ds.add_particle_filter('gas_cold')

/Users/stephaniehho/yt-git/yt/data_objects/static_output.pyc in add_particle_filter(self, filter)
673 if f is None:
674 return False
--> 675 used = self._setup_filtered_type(f)
676 if used:
677 filter = f

/Users/stephaniehho/yt-git/yt/data_objects/static_output.pyc in _setup_filtered_type(self, filter)
714 if filter.name not in self.filtered_particle_types:
715 self.filtered_particle_types.append(filter.name)
--> 716 new_fields = self._setup_particle_types([filter.name])
717 deps, _ = self.field_info.check_derived_fields(new_fields)
718 self.field_dependencies.update(deps)

/Users/stephaniehho/yt-git/yt/data_objects/static_output.pyc in _setup_particle_types(self, ptypes)
723 if ptypes is None: ptypes = self.ds.particle_types_raw
724 for ptype in set(ptypes):
--> 725 df += self._setup_particle_type(ptype)
726 return df
727

/Users/stephaniehho/yt-git/yt/data_objects/static_output.pyc in _setup_particle_type(self, ptype)
889 def _setup_particle_type(self, ptype):
890 orig = set(self.field_info.items())
--> 891 self.field_info.setup_particle_fields(ptype)
892 return [n for n, v in set(self.field_info.items()).difference(orig)]
893

/Users/stephaniehho/yt-git/yt/frontends/owls/fields.pyc in setup_particle_fields(self, ptype)
111
112 super(OWLSFieldInfo,self).setup_particle_fields(
--> 113 ptype, num_neighbors=self._num_neighbors, ftype=ftype)
114
115

UnboundLocalError: local variable 'ftype' referenced before assignment

What I find is that, this problem seems to be temporarily solved if I add en else case at line 111 of the code: https://github.com/yt-project/yt/blob/master/yt/frontends/owls/fields.py,
else:
ftype=ptype

But this will raise the questions of, what are ftype and ptype designed for at the first place, or will this temporary fix cause other errors that I have not yet encountered.

Thanks,
Stephanie
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