test: add unit test for unitless PSU conversion

tests/unit/test_dimensionless_units.py

@@ -0,0 +1,33 @@
+import numpy as np
+import xarray as xr
+
+from pymorize.data_request import DataRequestVariable
+from pymorize.rule import Rule
+from pymorize.units import handle_unit_conversion
+
+
+def test_units_with_psu():
+    da = xr.DataArray(10, name="sos", attrs={"units": "psu"})
+    rule = Rule(
+        cmor_variable="sos",
+        dimensionless_unit_mappings={"sos": {"0.001": "g/kg"}},
+    )
+    drv = DataRequestVariable(
+        variable_id="sos",
+        unit="0.001",
+        description="Salinity of the ocean",
+        time_method="MEAN",
+        table="Omon",
+        frequency="Monthly",
+        realms="Ocean",
+        standard_name="salinity_ocean",
+        cell_methods="area: mean where sea",
+        cell_measures="area: areacello",
+    )
+    rule.data_request_variable = drv
+    new_da = handle_unit_conversion(da, rule)
+    assert new_da.attrs.get("units") == "0.001"
+    # Check the magnitude of the data
+    # 1 g/kg is approximately 1 psu, so the values should be 10
+    # after conversion:
+    assert np.allclose(new_da.values, 10)