Use special unit conversion for derived variables (#2471)

esmvalcore/preprocessor/_derive/__init__.py

@@ -7,6 +7,8 @@
 
 import iris
 
+from esmvalcore.preprocessor._units import convert_units
+
 logger = logging.getLogger(__name__)
 
 
@@ -18,7 +20,6 @@ def _get_all_derived_variables():
     dict
         All derived variables with `short_name` (keys) and the associated
         python classes (values).
-
     """
     derivers = {}
     for path in Path(__file__).parent.glob('[a-z]*.py'):
@@ -50,7 +51,6 @@ def get_required(short_name, project):
     -------
     list
         List of dictionaries (including at least the key `short_name`).
-
     """
     if short_name.lower() not in ALL_DERIVED_VARIABLES:
         raise NotImplementedError(
@@ -82,7 +82,6 @@ def derive(cubes, short_name, long_name, units, standard_name=None):
     -------
     iris.cube.Cube
         The new derived variable.
-
     """
     if short_name == cubes[0].var_name:
         return cubes[0]
@@ -116,11 +115,13 @@ def derive(cubes, short_name, long_name, units, standard_name=None):
             "'%s', automatically setting them to '%s'. This might lead to "
             "incorrect data", short_name, cube.units, units)
         cube.units = units
-    elif cube.units.is_convertible(units):
-        cube.convert_units(units)
     else:
-        raise ValueError(
-            f"Units '{cube.units}' after executing derivation script of "
-            f"'{short_name}' cannot be converted to target units '{units}'")
+        try:
+            convert_units(cube, units)
+        except ValueError as exc:
+            raise ValueError(
+                f"Units '{cube.units}' after executing derivation script of "
+                f"'{short_name}' cannot be converted to target units '{units}'"
+            ) from exc
 
     return cube

tests/integration/preprocessor/_derive/test_interface.py

@@ -27,10 +27,12 @@ def patched_derive(mocker):
     mocker.patch.object(_derive, 'logger', autospec=True)
 
 
-def mock_all_derived_variables(returned_units):
+def mock_all_derived_variables(returned_units, fail_unit_conversion=False):
     """Mock the :obj:`dict` containing all derived variables accordingly."""
     cube = mock.create_autospec(Cube, instance=True)
     cube.units = returned_units
+    if fail_unit_conversion:
+        cube.convert_units.side_effect = [ValueError]
     calculate_function = mock.Mock(return_value=cube)
     derived_var = mock.Mock(name='DerivedVariable')
     derived_var.return_value.calculate = calculate_function
@@ -127,7 +129,7 @@ def test_check_units_convertible(mock_cubes):
 @pytest.mark.usefixtures("patched_derive")
 def test_check_units_fail(mock_cubes):
     """Test units after derivation if derivation scripts returns None."""
-    mock_all_derived_variables(Unit('kg'))
+    mock_all_derived_variables(Unit('kg'), fail_unit_conversion=True)
     with pytest.raises(ValueError) as err:
         derive(mock_cubes, SHORT_NAME, mock.sentinel.long_name, 'm',
                standard_name=mock.sentinel.standard_name)