ENH: [ARMVAC] implement variable He abundance (#4937)

yt/frontends/amrvac/data_structures.py

@@ -203,20 +203,8 @@ def __init__(
         # note: geometry_override and parfiles are specific to this frontend
 
         self._geometry_override = geometry_override
-        super().__init__(
-            filename,
-            dataset_type,
-            units_override=units_override,
-            unit_system=unit_system,
-            default_species_fields=default_species_fields,
-        )
+        self._parfiles = []
 
-        self._parfiles = parfiles
-
-        namelist = None
-        namelist_gamma = None
-        c_adiab = None
-        e_is_internal = None
         if parfiles is not None:
             parfiles = list(always_iterable(parfiles))
             ppf = Path(parfiles[0])
@@ -228,7 +216,22 @@ def __init__(
                     filename,
                 )
                 parfiles = [Path(ytcfg["yt", "test_data_dir"]) / pf for pf in parfiles]
+            self._parfiles = parfiles
+
+        super().__init__(
+            filename,
+            dataset_type,
+            units_override=units_override,
+            unit_system=unit_system,
+            default_species_fields=default_species_fields,
+        )
 
+        namelist = None
+        namelist_gamma = None
+        c_adiab = None
+        e_is_internal = None
+
+        if parfiles is not None:
             namelist = read_amrvac_namelist(parfiles)
             if "hd_list" in namelist:
                 c_adiab = namelist["hd_list"].get("hd_adiab", 1.0)
@@ -373,10 +376,11 @@ def _set_code_unit_attributes(self):
 
         # note: yt sets hydrogen mass equal to proton mass, amrvac doesn't.
         mp_cgs = self.quan(1.672621898e-24, "g")  # This value is taken from AstroPy
-        He_abundance = 0.1  # hardcoded parameter in AMRVAC
 
         # get self.length_unit if overrides are supplied, otherwise use default
         length_unit = getattr(self, "length_unit", self.quan(1, "cm"))
+        namelist = read_amrvac_namelist(self._parfiles)
+        He_abundance = namelist.get("mhd_list", {}).get("he_abundance", 0.1)
 
         # 1. calculations for mass, density, numberdensity
         if "mass_unit" in self.units_override:

yt/frontends/amrvac/io.py

@@ -42,6 +42,9 @@ def read_amrvac_namelist(parfiles):
     for nml in namelists:
         unified_namelist.patch(nml)
 
+    if "filelist" not in unified_namelist:
+        return unified_namelist
+
     # accumulate `&filelist:base_filename`
     base_filename = "".join(
         nml.get("filelist", {}).get("base_filename", "") for nml in namelists