revert to changing how unit_system_name is saved

yt/frontends/ytdata/tests/test_data_reload.py

@@ -1,23 +1,27 @@
 import numpy as np
 
-from yt.loaders import load, load_uniform_grid
+from yt.loaders import load_uniform_grid
+from yt.testing import requires_module_pytest
+from yt.utilities.on_demand_imports import _h5py as h5py
 
 
+@requires_module_pytest("h5py")
 def test_save_as_data_unit_system(tmp_path):
 
-    # This test loads in a DataContainer saved with save_as_dataset and checks
-    # for the "code" string in the unit_system name. Added to help
-    # avoid regressions with https://github.com/yt-project/yt/issues/4315 The
-    # test here is not a complete test as that bug manifests when restarting
-    # the python kernel.
+    # This test checks that the file saved with calling save_as_dataset
+    # for a ds with a "code" unit system contains the proper "unit_system_name".
+    # It checks the hdf5 file directly rather than using yt.load(), because
+    # https://github.com/yt-project/yt/issues/4315 only manifested restarting
+    # the python kernel (because the unit registry is state dependent).
 
     fi = tmp_path / "output_data.h5"
     shp = (4, 4, 4)
     data = {"density": np.random.random(shp)}
     ds = load_uniform_grid(data, shp, unit_system="code")
-    assert "code" in ds.unit_system.name
+    assert "code" in ds._unit_system_name
+
     sp = ds.sphere(ds.domain_center, ds.domain_width[0] / 2.0)
     sp.save_as_dataset(fi)
 
-    ds_new = load(fi, unit_system="code")
-    assert "code" in ds_new.unit_system.name
+    with h5py.File(fi, mode="r") as f:
+        assert f.attrs["unit_system_name"] == "code"

yt/frontends/ytdata/utilities.py

@@ -103,7 +103,12 @@ def save_as_dataset(ds, filename, data, field_types=None, extra_attrs=None):
         _yt_array_hdf5_attr(fh, "unit_registry_json", ds.unit_registry.to_json())
 
     if hasattr(ds, "unit_system"):
-        _yt_array_hdf5_attr(fh, "unit_system_name", ds.unit_system.name.split("_")[0])
+        # Note: ds._unit_system_name is written here rather than
+        # ds.unit_system.name because for a 'code' unit system, ds.unit_system.name
+        # is a hash, not a unit system. And on re-load, we want to designate
+        # a unit system not a hash value.
+        # See https://github.com/yt-project/yt/issues/4315 for more background.
+        _yt_array_hdf5_attr(fh, "unit_system_name", ds._unit_system_name)
 
     for attr in base_attrs:
         if isinstance(ds, dict):

yt/units/unit_systems.py

@@ -2,16 +2,8 @@
 
 
 def create_code_unit_system(unit_registry, current_mks_unit=None):
-
-    unit_system_name = "code_{}".format(unit_registry.unit_system_id)
-    # Note: the above unit system name needs both "code_" and the unique
-    # unit registry hash so that the unit system can be uniquely tied to
-    # a dataset. When calling `save_as_dataset`, only the "code" portion is
-    # saved as this name is supplied to Dataset._assign_unit_system().
-    # See https://github.com/yt-project/yt/issues/4315 for more background.
-
     code_unit_system = UnitSystem(
-        name=unit_system_name,
+        name=unit_registry.unit_system_id,
         length_unit="code_length",
         mass_unit="code_mass",
         time_unit="code_time",