Better error message for missing parameter in the file.

Author: YooSunYoung

src/ess/nmx/mcstas/load.py

@@ -67,12 +67,27 @@ def load_raw_event_data(
 def load_crystal_rotation(
     file_path: FilePath, instrument: McStasInstrument
 ) -> CrystalRotation:
-    """Retrieve crystal rotation from the file."""
+    """Retrieve crystal rotation from the file.
+
+    Raises
+    ------
+    KeyError
+        If the crystal rotation is not found in the file.
+
+    """
     with snx.File(file_path, 'r') as file:
-        return sc.vector(
-            value=[file[f"entry1/simulation/Param/XtalPhi{key}"][...] for key in "XYZ"],
+        param_keys = tuple(f"entry1/simulation/Param/XtalPhi{key}" for key in "XYZ")
+        if not all(key in file for key in param_keys):
+            raise KeyError(
+                f"Crystal rotations [{', '.join(param_keys)}] not found in file."
+            )
+
+    return CrystalRotation(
+        sc.vector(
+            value=[file[param_key][...] for param_key in param_keys],
             unit=instrument.simulation_settings.angle_unit,
         )
+    )
 
 
 def event_weights_from_probability(

tests/loader_test.py

@@ -10,7 +10,7 @@
 import scippnexus as snx
 from ess.nmx import default_parameters
 from ess.nmx.data import small_mcstas_2_sample, small_mcstas_3_sample
-from ess.nmx.mcstas.load import bank_names_to_detector_names
+from ess.nmx.mcstas.load import bank_names_to_detector_names, load_crystal_rotation
 from ess.nmx.mcstas.load import providers as loader_providers
 from ess.nmx.reduction import NMXData
 from ess.nmx.types import (
@@ -192,6 +192,26 @@ def test_file_reader_mcstas_additional_fields(tmp_mcstas_file: pathlib.Path) ->
     assert isinstance(dg, sc.DataGroup)
 
 
+@pytest.fixture()
+def rotation_mission_tmp_file(tmp_mcstas_file: pathlib.Path) -> pathlib.Path:
+    import h5py
+
+    param_keys = tuple(f"entry1/simulation/Param/XtalPhi{key}" for key in "XYZ")
+
+    # Remove the rotation parameters from the file.
+    with h5py.File(tmp_mcstas_file, 'a') as file:
+        for key in param_keys:
+            del file[key]
+
+    return tmp_mcstas_file
+
+
+def test_missing_rotation(rotation_mission_tmp_file: FilePath) -> None:
+    with pytest.raises(KeyError, match="XtalPhiX"):
+        load_crystal_rotation(rotation_mission_tmp_file, None)
+        # McStasInstrument is not used due to error in the file.
+
+
 def test_bank_names_to_detector_names_two_detectors():
     res = bank_names_to_detector_names(two_detectors_two_filenames)
     assert len(res) == 2