Load data only once for ESMPy regridders (#2418)

esmvalcore/preprocessor/_regrid_esmpy.py

@@ -67,6 +67,9 @@ def __init__(
         mask_threshold: float = 0.99,
     ):
         """Initialize class instance."""
+        # These regridders are not lazy, so load source and target data once.
+        src_cube.data  # pylint: disable=pointless-statement
+        tgt_cube.data  # pylint: disable=pointless-statement
         self.src_cube = src_cube
         self.tgt_cube = tgt_cube
         self.method = method
@@ -86,6 +89,8 @@ def __call__(self, cube: Cube) -> Cube:
             Regridded cube.
 
         """
+        # These regridders are not lazy, so load source data once.
+        cube.data  # pylint: disable=pointless-statement
         src_rep, dst_rep = get_grid_representants(cube, self.tgt_cube)
         regridder = build_regridder(
             src_rep, dst_rep, self.method, mask_threshold=self.mask_threshold

tests/unit/preprocessor/_regrid_esmpy/test_regrid_esmpy.py

@@ -713,6 +713,31 @@ def test_regrid_area_weighted(self, mock_build_regridder, mock_map_slices):
                                                 mock.sentinel.regridder,
                                                 self.cube_3d, self.cube)
 
+    @mock.patch('esmvalcore.preprocessor._regrid_esmpy.map_slices')
+    @mock.patch('esmvalcore.preprocessor._regrid_esmpy.build_regridder')
+    @mock.patch('esmvalcore.preprocessor._regrid_esmpy.get_grid_representants',
+                mock.Mock(side_effect=identity))
+    def test_data_realized_once(self, mock_build_regridder, mock_map_slices):
+        """Test that the regridder realizes the data only once."""
+        src_cube = mock.MagicMock()
+        src_data = mock.PropertyMock()
+        type(src_cube).data = src_data
+        tgt_cube1 = mock.MagicMock()
+        tgt_data1 = mock.PropertyMock()
+        type(tgt_cube1).data = tgt_data1
+        # Check that constructing the regridder realizes the source and
+        # target data.
+        regridder = ESMPyAreaWeighted().regridder(src_cube, tgt_cube1)
+        src_data.assert_called_with()
+        tgt_data1.assert_called_with()
+        tgt_cube2 = mock.MagicMock()
+        tgt_data2 = mock.PropertyMock()
+        # Check that calling the regridder with another cube also realizes
+        # target data.
+        type(tgt_cube2).data = tgt_data2
+        regridder(tgt_cube2)
+        tgt_data2.assert_called_with()
+
 
 @pytest.mark.parametrize(
     'scheme,output',