From 8ea1502b5bfe6ad93bd8fc18d1250587267aa782 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 16 Nov 2022 16:37:42 +0100
Subject: [PATCH 01/19] Add iris-esmf-regrid to env file

---
 environment.yml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/environment.yml b/environment.yml
index 5873831080..7db41f52c1 100644
--- a/environment.yml
+++ b/environment.yml
@@ -18,6 +18,7 @@ dependencies:
   - humanfriendly
   - importlib_resources
   - iris>=3.2.1
+  - iris-esmf-regrid
   - isodate
   - jinja2
   - nc-time-axis

From f97985dc2ee07c1cb49cedb39ba1f2a3181616d4 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Thu, 17 Nov 2022 16:55:46 +0100
Subject: [PATCH 02/19] Allow to use regrid_rectilinear_to_rectilinear scheme

---
 esmvalcore/preprocessor/_regrid.py | 35 +++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index 3cf0e057de..c323dcbf84 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -1,6 +1,7 @@
 """Horizontal and vertical regridding module."""
 
 import importlib
+import inspect
 import logging
 import os
 import re
@@ -14,7 +15,7 @@
 from dask import array as da
 from geopy.geocoders import Nominatim
 from iris.analysis import AreaWeighted, Linear, Nearest, UnstructuredNearest
-from iris.util import broadcast_to_shape
+from iris.util import broadcast_to_shape, squeeze
 
 from ..cmor._fixes.shared import add_altitude_from_plev, add_plev_from_altitude
 from ..cmor.fix import fix_file, fix_metadata
@@ -24,6 +25,8 @@
 from ._regrid_esmpy import ESMF_REGRID_METHODS
 from ._regrid_esmpy import regrid as esmpy_regrid
 
+from esmf_regrid.schemes import regrid_rectilinear_to_rectilinear
+
 logger = logging.getLogger(__name__)
 
 # Regular expression to parse a "MxN" cell-specification.
@@ -451,7 +454,7 @@ def extract_point(cube, latitude, longitude, scheme):
     return cube
 
 
-def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
+def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True, mdtol=0):
     """Perform horizontal regridding.
 
     Note that the target grid can be a cube (:py:class:`~iris.cube.Cube`),
@@ -536,8 +539,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
               extrapolation_mode: nanmask
 
     To use the area weighted regridder available in
-    :class:`esmf_regrid.schemes.ESMFAreaWeighted`, make sure that
-    :doc:`iris-esmf-regrid:index` is installed and use
+    :class:`esmf_regrid.schemes.ESMFAreaWeighted` use
 
     .. code-block:: yaml
 
@@ -547,10 +549,8 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             scheme:
               reference: esmf_regrid.schemes:ESMFAreaWeighted
 
-    .. note::
-
-        Note that :doc:`iris-esmf-regrid:index` is still experimental.
     """
+    scheme_args = None
     if isinstance(scheme, dict):
         try:
             object_ref = scheme.pop("reference")
@@ -568,12 +568,12 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
         if separator:
             for attr in scheme_name.split('.'):
                 obj = getattr(obj, attr)
-        loaded_scheme = obj(**scheme)
+        scheme_args = inspect.getfullargspec(obj).args
     else:
         loaded_scheme = HORIZONTAL_SCHEMES.get(scheme.lower())
-    if loaded_scheme is None:
-        emsg = 'Unknown regridding scheme, got {!r}.'
-        raise ValueError(emsg.format(scheme))
+        if loaded_scheme is None:
+            emsg = 'Unknown regridding scheme, got {!r}.'
+            raise ValueError(emsg.format(scheme))
 
     if isinstance(target_grid, str):
         if os.path.isfile(target_grid):
@@ -599,6 +599,12 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
 
     if not isinstance(target_grid, iris.cube.Cube):
         raise ValueError('Expecting a cube, got {}.'.format(target_grid))
+    
+    if isinstance(scheme_args, list):
+        if 'src_cube' in scheme_args:
+            scheme['src_cube'] = cube
+        if 'grid_cube' in scheme_args:
+            scheme['grid_cube'] = target_grid
 
     # Unstructured regridding requires x2 2d spatial coordinates,
     # so ensure to purge any 1d native spatial dimension coordinates
@@ -625,11 +631,15 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             else:
                 fill_value = GLOBAL_FILL_VALUE
             da.ma.set_fill_value(cube.core_data(), fill_value)
-
+        a = regrid_rectilinear_to_rectilinear(cube, target_grid)
         # Perform the horizontal regridding
         if _attempt_irregular_regridding(cube, scheme):
             cube = esmpy_regrid(cube, target_grid, scheme)
         else:
+            loaded_scheme = obj(**scheme)
+            if isinstance(loaded_scheme, iris.cube.Cube):
+                return loaded_scheme
+
             cube = cube.regrid(target_grid, loaded_scheme)
 
         # Preserve dtype and use masked arrays for 'unstructured_nearest'
@@ -652,7 +662,6 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
 
     return cube
 
-
 def _horizontal_grid_is_close(cube1, cube2):
     """Check if two cubes have the same horizontal grid definition.
 

From 263f4b4e929ec437bc319352f535dc07d7ce187b Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Thu, 17 Nov 2022 16:56:02 +0100
Subject: [PATCH 03/19] Add tests

---
 .../preprocessor/_regrid/test_regrid.py       | 29 +++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/tests/integration/preprocessor/_regrid/test_regrid.py b/tests/integration/preprocessor/_regrid/test_regrid.py
index a6ba8b95b5..d954891235 100644
--- a/tests/integration/preprocessor/_regrid/test_regrid.py
+++ b/tests/integration/preprocessor/_regrid/test_regrid.py
@@ -14,6 +14,7 @@
 from tests.unit.preprocessor._regrid import _make_cube
 
 
+
 class Test(tests.Test):
     def setUp(self):
         """Prepare tests."""
@@ -91,6 +92,15 @@ def test_regrid__linear(self):
         result = regrid(self.cube, self.grid_for_linear, 'linear')
         expected = np.array([[[1.5]], [[5.5]], [[9.5]]])
         self.assert_array_equal(result.data, expected)
+    
+    def test_regrid__esmf_rectilinear(self):
+        result = regrid(
+            self.cube,
+            self.grid_for_linear,
+            {'reference': 
+             'esmf_regrid.schemes:regrid_rectilinear_to_rectilinear',})
+        expected = np.array([[[1.5]], [[5.5]], [[9.5]]])
+        np.testing.assert_array_almost_equal(result.data, expected, decimal=1)
 
     def test_regrid__regular_coordinates(self):
         data = np.ones((1, 1))
@@ -214,6 +224,25 @@ def test_regrid__area_weighted(self):
         expected = np.array([1.499886, 5.499886, 9.499886])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=6)
 
+    def test_regrid__esmf_area_weighted(self):
+        data = np.empty((1, 1))
+        lons = iris.coords.DimCoord([1.6],
+                                    standard_name='longitude',
+                                    bounds=[[1, 2]],
+                                    units='degrees_east',
+                                    coord_system=self.cs)
+        lats = iris.coords.DimCoord([1.6],
+                                    standard_name='latitude',
+                                    bounds=[[1, 2]],
+                                    units='degrees_north',
+                                    coord_system=self.cs)
+        coords_spec = [(lats, 0), (lons, 1)]
+        grid = iris.cube.Cube(data, dim_coords_and_dims=coords_spec)
+        result = regrid(self.cube, grid, {'reference': 
+             'esmf_regrid.schemes:ESMFAreaWeighted'})
+        expected = np.array([1.499886, 5.499886, 9.499886])
+        np.testing.assert_array_almost_equal(result.data, expected, decimal=6)
+
     def test_regrid__unstructured_nearest_float(self):
         """Test unstructured_nearest regridding with cube of floats."""
         result = regrid(self.unstructured_grid_cube,

From b5dda6fb0b976a2fe4cf3d5453f2f31b64151d59 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Thu, 17 Nov 2022 17:25:11 +0100
Subject: [PATCH 04/19] Clean

---
 esmvalcore/preprocessor/_regrid.py                    | 10 ++++------
 tests/integration/preprocessor/_regrid/test_regrid.py |  9 ++++-----
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index c323dcbf84..fb27495878 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -15,7 +15,7 @@
 from dask import array as da
 from geopy.geocoders import Nominatim
 from iris.analysis import AreaWeighted, Linear, Nearest, UnstructuredNearest
-from iris.util import broadcast_to_shape, squeeze
+from iris.util import broadcast_to_shape
 
 from ..cmor._fixes.shared import add_altitude_from_plev, add_plev_from_altitude
 from ..cmor.fix import fix_file, fix_metadata
@@ -25,8 +25,6 @@
 from ._regrid_esmpy import ESMF_REGRID_METHODS
 from ._regrid_esmpy import regrid as esmpy_regrid
 
-from esmf_regrid.schemes import regrid_rectilinear_to_rectilinear
-
 logger = logging.getLogger(__name__)
 
 # Regular expression to parse a "MxN" cell-specification.
@@ -454,7 +452,7 @@ def extract_point(cube, latitude, longitude, scheme):
     return cube
 
 
-def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True, mdtol=0):
+def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
     """Perform horizontal regridding.
 
     Note that the target grid can be a cube (:py:class:`~iris.cube.Cube`),
@@ -599,7 +597,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True, mdtol=0)
 
     if not isinstance(target_grid, iris.cube.Cube):
         raise ValueError('Expecting a cube, got {}.'.format(target_grid))
-    
+
     if isinstance(scheme_args, list):
         if 'src_cube' in scheme_args:
             scheme['src_cube'] = cube
@@ -631,7 +629,6 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True, mdtol=0)
             else:
                 fill_value = GLOBAL_FILL_VALUE
             da.ma.set_fill_value(cube.core_data(), fill_value)
-        a = regrid_rectilinear_to_rectilinear(cube, target_grid)
         # Perform the horizontal regridding
         if _attempt_irregular_regridding(cube, scheme):
             cube = esmpy_regrid(cube, target_grid, scheme)
@@ -662,6 +659,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True, mdtol=0)
 
     return cube
 
+
 def _horizontal_grid_is_close(cube1, cube2):
     """Check if two cubes have the same horizontal grid definition.
 
diff --git a/tests/integration/preprocessor/_regrid/test_regrid.py b/tests/integration/preprocessor/_regrid/test_regrid.py
index d954891235..a601fcba3a 100644
--- a/tests/integration/preprocessor/_regrid/test_regrid.py
+++ b/tests/integration/preprocessor/_regrid/test_regrid.py
@@ -14,7 +14,6 @@
 from tests.unit.preprocessor._regrid import _make_cube
 
 
-
 class Test(tests.Test):
     def setUp(self):
         """Prepare tests."""
@@ -92,13 +91,13 @@ def test_regrid__linear(self):
         result = regrid(self.cube, self.grid_for_linear, 'linear')
         expected = np.array([[[1.5]], [[5.5]], [[9.5]]])
         self.assert_array_equal(result.data, expected)
-    
+
     def test_regrid__esmf_rectilinear(self):
         result = regrid(
             self.cube,
             self.grid_for_linear,
-            {'reference': 
-             'esmf_regrid.schemes:regrid_rectilinear_to_rectilinear',})
+            {'reference':
+                 'esmf_regrid.schemes:regrid_rectilinear_to_rectilinear'})
         expected = np.array([[[1.5]], [[5.5]], [[9.5]]])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=1)
 
@@ -239,7 +238,7 @@ def test_regrid__esmf_area_weighted(self):
         coords_spec = [(lats, 0), (lons, 1)]
         grid = iris.cube.Cube(data, dim_coords_and_dims=coords_spec)
         result = regrid(self.cube, grid, {'reference': 
-             'esmf_regrid.schemes:ESMFAreaWeighted'})
+            'esmf_regrid.schemes:ESMFAreaWeighted'})
         expected = np.array([1.499886, 5.499886, 9.499886])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=6)
 

From 5d4a34890f7920d445605971bb8256cfb959e881 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Thu, 17 Nov 2022 17:35:19 +0100
Subject: [PATCH 05/19] Fix flake

---
 .../integration/preprocessor/_regrid/test_regrid.py | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/tests/integration/preprocessor/_regrid/test_regrid.py b/tests/integration/preprocessor/_regrid/test_regrid.py
index a601fcba3a..78a167aa91 100644
--- a/tests/integration/preprocessor/_regrid/test_regrid.py
+++ b/tests/integration/preprocessor/_regrid/test_regrid.py
@@ -93,11 +93,14 @@ def test_regrid__linear(self):
         self.assert_array_equal(result.data, expected)
 
     def test_regrid__esmf_rectilinear(self):
+        scheme_name = 'esmf_regrid.schemes:regrid_rectilinear_to_rectilinear'
+        scheme = {
+            'reference': scheme_name
+        }
         result = regrid(
             self.cube,
             self.grid_for_linear,
-            {'reference':
-                 'esmf_regrid.schemes:regrid_rectilinear_to_rectilinear'})
+            scheme)
         expected = np.array([[[1.5]], [[5.5]], [[9.5]]])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=1)
 
@@ -237,8 +240,10 @@ def test_regrid__esmf_area_weighted(self):
                                     coord_system=self.cs)
         coords_spec = [(lats, 0), (lons, 1)]
         grid = iris.cube.Cube(data, dim_coords_and_dims=coords_spec)
-        result = regrid(self.cube, grid, {'reference': 
-            'esmf_regrid.schemes:ESMFAreaWeighted'})
+        scheme = {
+            'reference': 'esmf_regrid.schemes:ESMFAreaWeighted'
+        }
+        result = regrid(self.cube, grid, scheme)
         expected = np.array([1.499886, 5.499886, 9.499886])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=6)
 

From 473af2309c1b2800dc601647a12fcc19d4885d4c Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Fri, 18 Nov 2022 12:22:12 +0100
Subject: [PATCH 06/19] Add package to setup

---
 setup.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/setup.py b/setup.py
index 98401f48b7..74893cf186 100755
--- a/setup.py
+++ b/setup.py
@@ -39,6 +39,7 @@
         'humanfriendly',
         "importlib_resources;python_version<'3.9'",
         'isodate',
+        'iris-esmf-regrid',
         'jinja2',
         'nc-time-axis',  # needed by iris.plot
         'nested-lookup',

From 54b3acf852a384298a139530e80f1cc5d57c20cd Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Fri, 18 Nov 2022 12:26:56 +0100
Subject: [PATCH 07/19] Fix some tests

---
 esmvalcore/preprocessor/_regrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index fb27495878..3d6c90299e 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -603,6 +603,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             scheme['src_cube'] = cube
         if 'grid_cube' in scheme_args:
             scheme['grid_cube'] = target_grid
+        loaded_scheme = obj(**scheme)
 
     # Unstructured regridding requires x2 2d spatial coordinates,
     # so ensure to purge any 1d native spatial dimension coordinates
@@ -633,7 +634,6 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
         if _attempt_irregular_regridding(cube, scheme):
             cube = esmpy_regrid(cube, target_grid, scheme)
         else:
-            loaded_scheme = obj(**scheme)
             if isinstance(loaded_scheme, iris.cube.Cube):
                 return loaded_scheme
 

From 41ff490665c96901199b83aa4a5cfd176289657e Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Fri, 18 Nov 2022 13:23:41 +0100
Subject: [PATCH 08/19] Correct package name

---
 setup.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/setup.py b/setup.py
index 74893cf186..c5e0caf870 100755
--- a/setup.py
+++ b/setup.py
@@ -32,6 +32,7 @@
         'cf-units',
         'dask[array]',
         'esgf-pyclient>=0.3.1',
+        'esmf-regrid',
         'esmpy!=8.1.0',  # see github.com/ESMValGroup/ESMValCore/issues/1208
         'fiona',
         'fire',
@@ -39,7 +40,6 @@
         'humanfriendly',
         "importlib_resources;python_version<'3.9'",
         'isodate',
-        'iris-esmf-regrid',
         'jinja2',
         'nc-time-axis',  # needed by iris.plot
         'nested-lookup',

From 448f1c4896b4b43e236533e9f6849449c1b25c5d Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Fri, 18 Nov 2022 13:37:53 +0100
Subject: [PATCH 09/19] Fix shape in tests

---
 tests/integration/preprocessor/_regrid/test_regrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/integration/preprocessor/_regrid/test_regrid.py b/tests/integration/preprocessor/_regrid/test_regrid.py
index 78a167aa91..8a720d461f 100644
--- a/tests/integration/preprocessor/_regrid/test_regrid.py
+++ b/tests/integration/preprocessor/_regrid/test_regrid.py
@@ -244,7 +244,7 @@ def test_regrid__esmf_area_weighted(self):
             'reference': 'esmf_regrid.schemes:ESMFAreaWeighted'
         }
         result = regrid(self.cube, grid, scheme)
-        expected = np.array([1.499886, 5.499886, 9.499886])
+        expected = np.array([[[1.499886]], [[5.499886]], [[9.499886]]])
         np.testing.assert_array_almost_equal(result.data, expected, decimal=6)
 
     def test_regrid__unstructured_nearest_float(self):

From fcf77a245b57d36a5e3629fa4185aa3d53ac9193 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Fri, 18 Nov 2022 16:40:36 +0100
Subject: [PATCH 10/19] Remove warning from docs

---
 doc/recipe/preprocessor.rst | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/doc/recipe/preprocessor.rst b/doc/recipe/preprocessor.rst
index 6b372b9143..9f80d556a0 100644
--- a/doc/recipe/preprocessor.rst
+++ b/doc/recipe/preprocessor.rst
@@ -926,10 +926,6 @@ example of its usage in an ESMValTool preprocessor is:
             reference: esmf_regrid.schemes:ESMFAreaWeighted
             mdtol: 0.7
 
-.. TODO: Remove the following warning once things have settled a bit.
-.. warning::
-   Just as the mesh support in Iris itself, this new regridding package is
-   still considered experimental.
 
 .. _ensemble statistics:
 

From 1de2ccd44adeb911e4bdd02ade7ade7057e9f207 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 11:41:47 +0100
Subject: [PATCH 11/19] Reorder code to make it cleaner

---
 esmvalcore/preprocessor/_regrid.py            | 55 +++++++++----------
 .../unit/preprocessor/_regrid/test_regrid.py  |  3 +-
 2 files changed, 27 insertions(+), 31 deletions(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index 3d6c90299e..6444253c25 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -548,31 +548,6 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
               reference: esmf_regrid.schemes:ESMFAreaWeighted
 
     """
-    scheme_args = None
-    if isinstance(scheme, dict):
-        try:
-            object_ref = scheme.pop("reference")
-        except KeyError as key_err:
-            raise ValueError(
-                "No reference specified for generic regridding.") from key_err
-        module_name, separator, scheme_name = object_ref.partition(":")
-        try:
-            obj = importlib.import_module(module_name)
-        except ImportError as import_err:
-            raise ValueError(
-                "Could not import specified generic regridding module. "
-                "Please double check spelling and that the required module is "
-                "installed.") from import_err
-        if separator:
-            for attr in scheme_name.split('.'):
-                obj = getattr(obj, attr)
-        scheme_args = inspect.getfullargspec(obj).args
-    else:
-        loaded_scheme = HORIZONTAL_SCHEMES.get(scheme.lower())
-        if loaded_scheme is None:
-            emsg = 'Unknown regridding scheme, got {!r}.'
-            raise ValueError(emsg.format(scheme))
-
     if isinstance(target_grid, str):
         if os.path.isfile(target_grid):
             target_grid = iris.load_cube(target_grid)
@@ -590,20 +565,43 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             ycoord = target_grid.coord(axis='y', dim_coords=True)
             xcoord.coord_system = src_cs
             ycoord.coord_system = src_cs
-
     elif isinstance(target_grid, dict):
         # Generate a target grid from the provided specification,
         target_grid = _regional_stock_cube(target_grid)
-
+    
     if not isinstance(target_grid, iris.cube.Cube):
         raise ValueError('Expecting a cube, got {}.'.format(target_grid))
 
-    if isinstance(scheme_args, list):
+    if isinstance(scheme, dict):
+        try:
+            object_ref = scheme.pop("reference")
+        except KeyError as key_err:
+            raise ValueError(
+                "No reference specified for generic regridding.") from key_err
+        module_name, separator, scheme_name = object_ref.partition(":")
+        try:
+            obj = importlib.import_module(module_name)
+        except ImportError as import_err:
+            raise ValueError(
+                "Could not import specified generic regridding module. "
+                "Please double check spelling and that the required module is "
+                "installed.") from import_err
+        if separator:
+            for attr in scheme_name.split('.'):
+                obj = getattr(obj, attr)
+
+        scheme_args = inspect.getfullargspec(obj).args
         if 'src_cube' in scheme_args:
             scheme['src_cube'] = cube
         if 'grid_cube' in scheme_args:
             scheme['grid_cube'] = target_grid
+
         loaded_scheme = obj(**scheme)
+    else:
+        loaded_scheme = HORIZONTAL_SCHEMES.get(scheme.lower())
+        if loaded_scheme is None:
+            emsg = 'Unknown regridding scheme, got {!r}.'
+            raise ValueError(emsg.format(scheme))
 
     # Unstructured regridding requires x2 2d spatial coordinates,
     # so ensure to purge any 1d native spatial dimension coordinates
@@ -636,7 +634,6 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
         else:
             if isinstance(loaded_scheme, iris.cube.Cube):
                 return loaded_scheme
-
             cube = cube.regrid(target_grid, loaded_scheme)
 
         # Preserve dtype and use masked arrays for 'unstructured_nearest'
diff --git a/tests/unit/preprocessor/_regrid/test_regrid.py b/tests/unit/preprocessor/_regrid/test_regrid.py
index 3846ccac81..06001589b1 100644
--- a/tests/unit/preprocessor/_regrid/test_regrid.py
+++ b/tests/unit/preprocessor/_regrid/test_regrid.py
@@ -103,10 +103,9 @@ def test_invalid_tgt_grid__unknown(self):
             regrid(self.src_cube, dummy, scheme)
 
     def test_invalid_scheme__unknown(self):
-        dummy = mock.sentinel.dummy
         emsg = 'Unknown regridding scheme'
         with self.assertRaisesRegex(ValueError, emsg):
-            regrid(dummy, dummy, 'wibble')
+            regrid(self.src_cube, self.src_cube, 'wibble')
 
     def test_horizontal_schemes(self):
         self.assertEqual(

From aaa0cae36f695ed8739101df63d642b67957ae63 Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 12:12:13 +0100
Subject: [PATCH 12/19] Improve documentation

---
 doc/recipe/preprocessor.rst        | 16 +++++++++++++++-
 esmvalcore/preprocessor/_regrid.py |  8 ++++++--
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/doc/recipe/preprocessor.rst b/doc/recipe/preprocessor.rst
index 9f80d556a0..26c984c72d 100644
--- a/doc/recipe/preprocessor.rst
+++ b/doc/recipe/preprocessor.rst
@@ -892,7 +892,7 @@ necessary information. That includes a ``reference`` to the desired scheme
 itself, as well as any arguments that should be passed through to the
 scheme. For example, the following shows the use of the built-in scheme
 :class:`iris.analysis.AreaWeighted` with a custom threshold for missing data
-tolerance.
+tolerance. 
 
 .. code-block:: yaml
 
@@ -926,6 +926,20 @@ example of its usage in an ESMValTool preprocessor is:
             reference: esmf_regrid.schemes:ESMFAreaWeighted
             mdtol: 0.7
 
+Additionally, the use of generic schemes that take source and target grid cubes as 
+arguments is also supported. The `regrid` module will automatically pass 
+the cubes as inputs of the scheme. An example of this usage is
+the `regrid_rectilinear_to_rectilinear` scheme available in `iris-esmf-regrid`:
+
+.. code-block:: yaml
+
+    preprocessors:
+      regrid_preprocessor:
+        regrid:
+          target_grid: 2.5x2.5
+          scheme:
+            reference: esmf_regrid.schemes:regrid_rectilinear_to_rectilinear
+            mdtol: 0.7
 
 .. _ensemble statistics:
 
diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index 6444253c25..bc69ed7023 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -591,6 +591,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
                 obj = getattr(obj, attr)
 
         scheme_args = inspect.getfullargspec(obj).args
+        # Add source and target cubes as arguments if required
         if 'src_cube' in scheme_args:
             scheme['src_cube'] = cube
         if 'grid_cube' in scheme_args:
@@ -628,12 +629,15 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             else:
                 fill_value = GLOBAL_FILL_VALUE
             da.ma.set_fill_value(cube.core_data(), fill_value)
+        
         # Perform the horizontal regridding
         if _attempt_irregular_regridding(cube, scheme):
             cube = esmpy_regrid(cube, target_grid, scheme)
+        elif isinstance(loaded_scheme, iris.cube.Cube):
+            # Return regridded cube in cases in which the 
+            # scheme is a function f(src_cube, grid_cube) -> Cube.
+            return loaded_scheme
         else:
-            if isinstance(loaded_scheme, iris.cube.Cube):
-                return loaded_scheme
             cube = cube.regrid(target_grid, loaded_scheme)
 
         # Preserve dtype and use masked arrays for 'unstructured_nearest'

From 176f4ce1d4750e42674f45a0df342ee678a22f5d Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 12:21:27 +0100
Subject: [PATCH 13/19] Fix flake

---
 esmvalcore/preprocessor/_regrid.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index b665cfc074..a31c417286 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -568,7 +568,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
     elif isinstance(target_grid, dict):
         # Generate a target grid from the provided specification,
         target_grid = _regional_stock_cube(target_grid)
-    
+
     if not isinstance(target_grid, iris.cube.Cube):
         raise ValueError(f'Expecting a cube, got {target_grid}.')
 
@@ -629,13 +629,13 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
             else:
                 fill_value = GLOBAL_FILL_VALUE
             da.ma.set_fill_value(cube.core_data(), fill_value)
-        
+
         # Perform the horizontal regridding
         if _attempt_irregular_regridding(cube, scheme):
             cube = esmpy_regrid(cube, target_grid, scheme)
         elif isinstance(loaded_scheme, iris.cube.Cube):
-            # Return regridded cube in cases in which the 
-            # scheme is a function f(src_cube, grid_cube) -> Cube.
+            # Return regridded cube in cases in which the
+            # scheme is a function f(src_cube, grid_cube) -> Cube
             return loaded_scheme
         else:
             cube = cube.regrid(target_grid, loaded_scheme)

From 56b85aa1c789d5b7e855033abb37dedeca9e8deb Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 12:25:32 +0100
Subject: [PATCH 14/19] Restore condition

---
 esmvalcore/preprocessor/_regrid.py | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index a31c417286..df47523f77 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -600,9 +600,10 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
         loaded_scheme = obj(**scheme)
     else:
         loaded_scheme = HORIZONTAL_SCHEMES.get(scheme.lower())
-        if loaded_scheme is None:
-            emsg = 'Unknown regridding scheme, got {!r}.'
-            raise ValueError(emsg.format(scheme))
+    
+    if loaded_scheme is None:
+        emsg = 'Unknown regridding scheme, got {!r}.'
+        raise ValueError(emsg.format(scheme))
 
     # Unstructured regridding requires x2 2d spatial coordinates,
     # so ensure to purge any 1d native spatial dimension coordinates

From 47005ea54bfc8bb565f19ba5a1a7d62a388d4bba Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 12:29:18 +0100
Subject: [PATCH 15/19] Fix flake

---
 esmvalcore/preprocessor/_regrid.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/esmvalcore/preprocessor/_regrid.py b/esmvalcore/preprocessor/_regrid.py
index df47523f77..eabbfd853e 100644
--- a/esmvalcore/preprocessor/_regrid.py
+++ b/esmvalcore/preprocessor/_regrid.py
@@ -600,7 +600,7 @@ def regrid(cube, target_grid, scheme, lat_offset=True, lon_offset=True):
         loaded_scheme = obj(**scheme)
     else:
         loaded_scheme = HORIZONTAL_SCHEMES.get(scheme.lower())
-    
+
     if loaded_scheme is None:
         emsg = 'Unknown regridding scheme, got {!r}.'
         raise ValueError(emsg.format(scheme))

From 6305e7dfdd5fd4d918929a28d1b2e69445ef7e03 Mon Sep 17 00:00:00 2001
From: sloosvel <45196700+sloosvel@users.noreply.github.com>
Date: Wed, 23 Nov 2022 15:13:14 +0100
Subject: [PATCH 16/19] Apply suggestions from code review

Co-authored-by: Manuel Schlund <32543114+schlunma@users.noreply.github.com>
---
 doc/recipe/preprocessor.rst | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/recipe/preprocessor.rst b/doc/recipe/preprocessor.rst
index 26c984c72d..bd0f53fb2c 100644
--- a/doc/recipe/preprocessor.rst
+++ b/doc/recipe/preprocessor.rst
@@ -892,7 +892,7 @@ necessary information. That includes a ``reference`` to the desired scheme
 itself, as well as any arguments that should be passed through to the
 scheme. For example, the following shows the use of the built-in scheme
 :class:`iris.analysis.AreaWeighted` with a custom threshold for missing data
-tolerance. 
+tolerance.
 
 .. code-block:: yaml
 
@@ -929,7 +929,7 @@ example of its usage in an ESMValTool preprocessor is:
 Additionally, the use of generic schemes that take source and target grid cubes as 
 arguments is also supported. The `regrid` module will automatically pass 
 the cubes as inputs of the scheme. An example of this usage is
-the `regrid_rectilinear_to_rectilinear` scheme available in `iris-esmf-regrid`:
+the :func:`~esmf_regrid.schemes.regrid_rectilinear_to_rectilinear` scheme available in :doc:`iris-esmf-regrid:index`:
 
 .. code-block:: yaml
 

From d81ca55fb456dbd072ff95728fe8867bbcfe811c Mon Sep 17 00:00:00 2001
From: sloosvel <saskia.loosveldt@bsc.es>
Date: Wed, 23 Nov 2022 15:45:45 +0100
Subject: [PATCH 17/19] Improve documentation

---
 doc/recipe/preprocessor.rst | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/doc/recipe/preprocessor.rst b/doc/recipe/preprocessor.rst
index bd0f53fb2c..fecf8f7007 100644
--- a/doc/recipe/preprocessor.rst
+++ b/doc/recipe/preprocessor.rst
@@ -926,10 +926,13 @@ example of its usage in an ESMValTool preprocessor is:
             reference: esmf_regrid.schemes:ESMFAreaWeighted
             mdtol: 0.7
 
-Additionally, the use of generic schemes that take source and target grid cubes as 
-arguments is also supported. The `regrid` module will automatically pass 
-the cubes as inputs of the scheme. An example of this usage is
-the :func:`~esmf_regrid.schemes.regrid_rectilinear_to_rectilinear` scheme available in :doc:`iris-esmf-regrid:index`:
+Additionally, the use of generic schemes that take source and target grid cubes as
+arguments is also supported. The call function for such schemes must be defined as
+`(src_cube, grid_cube, **kwargs)` and they must return `iris.cube.Cube` objects.
+The `regrid` module will automatically pass the source and grid cubes as inputs
+of the scheme. An example of this usage is
+the :func:`~esmf_regrid.schemes.regrid_rectilinear_to_rectilinear`
+scheme available in :doc:`iris-esmf-regrid:index`:git 
 
 .. code-block:: yaml
 

From aff8b2a8707ba89af87b98676dac3eac0410fc2a Mon Sep 17 00:00:00 2001
From: Valeriu Predoi <valeriu.predoi@gmail.com>
Date: Wed, 23 Nov 2022 16:30:52 +0000
Subject: [PATCH 18/19] run GA tests

---
 .github/workflows/run-tests.yml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index a9322fe9e8..4fbdeb49a3 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -21,6 +21,7 @@ on:
   push:
     branches:
       - main
+      - dev_install_iris_esfm
   # run the test only if the PR is to main
   # turn it on if required
   #pull_request:

From c7ec2ccb0deee95b0815df0836e2e6275b902657 Mon Sep 17 00:00:00 2001
From: Valeriu Predoi <valeriu.predoi@gmail.com>
Date: Wed, 23 Nov 2022 16:48:36 +0000
Subject: [PATCH 19/19] GA tests run fine turning them off

---
 .github/workflows/run-tests.yml | 1 -
 1 file changed, 1 deletion(-)

diff --git a/.github/workflows/run-tests.yml b/.github/workflows/run-tests.yml
index 4fbdeb49a3..a9322fe9e8 100644
--- a/.github/workflows/run-tests.yml
+++ b/.github/workflows/run-tests.yml
@@ -21,7 +21,6 @@ on:
   push:
     branches:
       - main
-      - dev_install_iris_esfm
   # run the test only if the PR is to main
   # turn it on if required
   #pull_request: