Athena "temperature" issue

[matthewturk]

There is something strange going on with athena's temperature field for the case that pressure does not exist on-disk.

Unfortunately, surfacing the specific error requires changing a line of source code. In yt/fields/field_info_container.py change line 47 as follows:

diff --git a/yt/fields/field_info_container.py b/yt/fields/field_info_container.py
index fd87796c1..5d10d1d82 100644
--- a/yt/fields/field_info_container.py
+++ b/yt/fields/field_info_container.py
@@ -44,7 +44,7 @@ class FieldInfoContainer(dict):
     extra_union_fields = ()
 
     def __init__(self, ds, field_list, slice_info = None):
-        self._show_field_errors = []
+        self._show_field_errors = [("gas", "temperature")]
         self.ds = ds
         # Now we start setting things up.
         self.field_list = field_list

Now, this script, using the MHD Blast wave sample dataset:

import yt
ds = yt.load("MHDBlast/id0/Blast.0100.vtk")

produces this strange error:

Traceback (most recent call last):
  File "/home/mturk/yt/unyt/unyt/unit_registry.py", line 72, in __getitem__
    ret = self.lut[str(key)]
KeyError: 'code_pressure'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/mturk/yt/unyt/unyt/unit_registry.py", line 75, in __getitem__
    _lookup_unit_symbol(str(key), self.lut)
  File "/home/mturk/yt/unyt/unyt/unit_registry.py", line 331, in _lookup_unit_symbol
    "Could not find unit symbol '%s' in the provided " "symbols." % symbol_str
unyt.exceptions.UnitParseError: Could not find unit symbol 'code_pressure' in the provided symbols.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/mturk/yt/yt/yt/convenience.py", line 70, in load
    return candidates[0](*args, **kwargs)
  File "/home/mturk/yt/yt/yt/frontends/athena/data_structures.py", line 450, in __init__
    self.index
  File "/home/mturk/yt/yt/yt/data_objects/static_output.py", line 448, in index
    self.create_field_info()
  File "/home/mturk/yt/yt/yt/data_objects/static_output.py", line 523, in create_field_info
    deps, unloaded = self.field_info.check_derived_fields()
  File "/home/mturk/yt/yt/yt/fields/field_info_container.py", line 400, in check_derived_fields
    fd = fi.get_dependencies(ds = self.ds)
  File "/home/mturk/yt/yt/yt/fields/derived_field.py", line 223, in get_dependencies
    e[self.name]
  File "/home/mturk/yt/yt/yt/fields/field_detector.py", line 109, in __missing__
    vv = finfo(self)
  File "/home/mturk/yt/yt/yt/fields/derived_field.py", line 263, in __call__
    dd = self._function(self, data)
  File "/home/mturk/yt/yt/yt/frontends/athena/fields.py", line 104, in _temperature
    return data.ds.mu*mh*data["gas","pressure"]/data["gas","density"]/kboltz
  File "/home/mturk/yt/unyt/unyt/array.py", line 1751, in __array_ufunc__
    mul, unit = unit_operator(u0, u1)
  File "/home/mturk/yt/unyt/unyt/array.py", line 163, in _multiply_units
    ret = (unit1 * unit2).simplify()
  File "/home/mturk/yt/unyt/unyt/unit_object.py", line 732, in simplify
    self.expr = _cancel_mul(expr, self.registry)
  File "/home/mturk/yt/unyt/unyt/unit_object.py", line 767, in _cancel_mul
    u1 = _create_unit_from_factor(pair[0], registry)
  File "/home/mturk/yt/unyt/unyt/unit_object.py", line 756, in _create_unit_from_factor
    f = registry[str(base)]
  File "/home/mturk/yt/unyt/unyt/unit_registry.py", line 79, in __getitem__
    "The symbol '%s' does not exist in this registry." % key
unyt.exceptions.SymbolNotFoundError: The symbol 'code_pressure' does not exist in this registry.

I genuinely can't figure out how to address this. Especially because, from what I can see, the unit registry should have code_pressure -- and in fact, if I just let it go without raising the error, it actually shows up in ds.unit_registry . I'm reluctant to say this is a unyt issue as I fear that there's some order of operations issue I'm not seeing. Is it possible that this is occuring because of a different unit registry during field detection, that doesn't have code_pressure defined?

[matthewturk]

It seems that the issue is that multiplication in unyt is not precisely commutative -- changing the definition of the temperature field from

data.ds.mu*mh*data["gas","pressure"]/data["gas","density"]/kboltz

to

data.ds.mu*data["gas","pressure"]/data["gas","density"]/kboltz*mh

fixes the issue! Only the first unit gets checked for a registry. A hotfix here would be to change to the second definition -- not sure if that's the long-term solution.

[matthewturk]

(Also, thanks to @Xarthisius for figuring this out)

[brittonsmith]

I believe this is the same as what was happening in PR #2454.

[neutrinoceros]

It seems that the issue is that multiplication in unyt is not precisely commutative

I think this is resolved upstream since yt-project/unyt#164 (included in unyt 2.8.0, which is the latest release to date and the minimal required version as of yt 4.0)
Indeed, I cannot reproduce the error now, so let's close this.