Adding TypeError when FieldSet.from_c_grid_datatset called with interp_method

Also added unit test.

Author: erikvansebille

parcels/fieldset.py

@@ -587,6 +587,8 @@ def from_c_grid_dataset(cls, filenames, variables, dimensions, indices=None, mes
         if 'U' in dimensions and 'W' in dimensions and dimensions['U'] != dimensions['W']:
             raise ValueError("On a C-grid, the dimensions of velocities should be the corners (f-points) of the cells, so the same for U, V and W. "
                              "See also https://nbviewer.jupyter.org/github/OceanParcels/parcels/blob/master/parcels/examples/documentation_indexing.ipynb")
+        if 'interp_method' in kwargs.keys():
+            raise TypeError("On a C-grid, the interpolation method for velocities should not be overridden")
 
         interp_method = {}
         for v in variables:

tests/test_fieldset.py

@@ -172,6 +172,23 @@ def test_field_from_netcdf_fieldtypes():
     assert isinstance(fset.varU.units, GeographicPolar)
 
 
+def test_fieldset_from_cgrid_interpmethod():
+    data_path = path.join(path.dirname(__file__), 'test_data/')
+
+    filenames = {'lon': data_path + 'mask_nemo_cross_180lon.nc',
+                 'lat': data_path + 'mask_nemo_cross_180lon.nc',
+                 'data': data_path + 'Uu_eastward_nemo_cross_180lon.nc'}
+    variable = 'U'
+    dimensions = {'lon': 'glamf', 'lat': 'gphif'}
+    failed = False
+    try:
+        # should fail because FieldSet.from_c_grid_dataset does not support interp_method
+        FieldSet.from_c_grid_dataset(filenames, variable, dimensions, interp_method='partialslip')
+    except TypeError:
+        failed = True
+    assert failed
+
+
 @pytest.mark.parametrize('indslon', [range(10, 20), [1]])
 @pytest.mark.parametrize('indslat', [range(30, 60), [22]])
 def test_fieldset_from_file_subsets(indslon, indslat, tmpdir, filename='test_subsets'):