Adding a new UnitConverter Unity()

docs/user_guide/examples/tutorial_unitconverters.ipynb

@@ -146,7 +146,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "So the U field has a `GeographicPolar` UnitConverter object, the V field has a `Geographic` UnitConverter and the `temp` field has a `UnitConverter` object.\n",
+    "So the U field has a `GeographicPolar` UnitConverter object, the V field has a `Geographic` UnitConverter and the `temp` field has a `Unity` object.\n",
     "\n",
     "Indeed, if we multiply the value of the V field with 1852 \\* 60 (the number of meters in 1 degree of latitude), we get the expected 1 m/s.\n"
    ]
@@ -248,7 +248,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Indeed, in this case all Fields have the same default `UnitConverter` object.\n"
+    "Indeed, in this case all Fields have the same default `Unity` object.\n"
    ]
   },
   {
@@ -296,7 +296,7 @@
     "kh_meridional_field = parcels.Field(\n",
     "    \"Kh_meridional\",\n",
     "    ds[\"Kh_meridional\"],\n",
-    "    grid=grid,\n",
+    "    grid=fieldset.U.grid,\n",
     "    interp_method=parcels.interpolators.XLinear,\n",
     ")\n",
     "\n",
@@ -348,7 +348,7 @@
     "| `\"V\"`              | `Geographic`            | $1852 \\cdot 60$                                           | 1                            |\n",
     "| `\"Kh_zonal\"`       | `GeographicPolarSquare` | $(1852 \\cdot 60 \\cdot \\cos(lat \\cdot \\frac{\\pi}{180}))^2$ | 1                            |\n",
     "| `\"Kh_meridional\"`  | `GeographicSquare`      | $(1852 \\cdot 60)^2$                                       | 1                            |\n",
-    "| All other fields   | `UnitConverter`         | 1                                                         | 1                            |\n",
+    "| All other fields   | `Unity`                 | 1                                                         | 1                            |\n",
     "\n",
     "Only four Field names are recognised and assigned an automatic UnitConverter object. This means that things might go very wrong when e.g. a velocity field is not called `U` or `V`.\n",
     "\n",

docs/user_guide/v4-migration.md

@@ -43,3 +43,7 @@ Version 4 of Parcels is unreleased at the moment. The information in this migrat
 ## GridSet
 
 - `GridSet` is now a list, so change `fieldset.gridset.grids[0]` to `fieldset.gridset[0]`.
+
+## UnitConverters
+
+- The default `UnitConverter` is now called `Unity()`

src/parcels/__init__.py

@@ -14,7 +14,7 @@
     GeographicPolar,
     GeographicPolarSquare,
     GeographicSquare,
-    UnitConverter,
+    Unity,
 )
 from parcels._core.field import Field, VectorField
 from parcels._core.fieldset import FieldSet
@@ -66,7 +66,7 @@
     "GeographicPolar",
     "GeographicPolarSquare",
     "GeographicSquare",
-    "UnitConverter",
+    "Unity",
     # Status codes and errors
     "AllParcelsErrorCodes",
     "FieldInterpolationError",

src/parcels/_core/converters.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from abc import ABC, abstractmethod
 from math import pi
 
 import numpy as np
@@ -11,6 +12,7 @@
     "GeographicPolarSquare",
     "GeographicSquare",
     "UnitConverter",
+    "Unity",
     "_convert_to_flat_array",
     "_unitconverters_map",
 ]
@@ -27,12 +29,23 @@ def _convert_to_flat_array(var: npt.ArrayLike) -> npt.NDArray:
     return np.array(var).flatten()
 
 
-class UnitConverter:
-    """Interface class for spatial unit conversion during field sampling that performs no conversion."""
-
+class UnitConverter(ABC):
     source_unit: str | None = None
     target_unit: str | None = None
 
+    @abstractmethod
+    def to_target(self, value, z, y, x): ...
+
+    @abstractmethod
+    def to_source(self, value, z, y, x): ...
+
+
+class Unity(UnitConverter):
+    """Interface class for spatial unit conversion during field sampling that performs no conversion."""
+
+    source_unit: None
+    target_unit: None
+
     def to_target(self, value, z, y, x):
         return value
 

src/parcels/_core/field.py

@@ -10,6 +10,7 @@
 
 from parcels._core.converters import (
     UnitConverter,
+    Unity,
     _unitconverters_map,
 )
 from parcels._core.index_search import GRID_SEARCH_ERROR, LEFT_OUT_OF_BOUNDS, RIGHT_OUT_OF_BOUNDS, _search_time_index
@@ -135,7 +136,7 @@ def __init__(
         self.igrid = -1  # Default the grid index to -1
 
         if self.grid._mesh == "flat" or (self.name not in _unitconverters_map.keys()):
-            self.units = UnitConverter()
+            self.units = Unity()
         elif self.grid._mesh == "spherical":
             self.units = _unitconverters_map[self.name]
 

tests-v3/test_fieldset.py

@@ -13,7 +13,7 @@
     Variable,
 )
 from parcels.field import VectorField
-from parcels.tools.converters import GeographicPolar, UnitConverter
+from parcels.tools.converters import GeographicPolar, Unity
 from tests.utils import TEST_DATA
 
 
@@ -119,7 +119,7 @@ def test_field_from_netcdf_fieldtypes():
 
     # first try without setting fieldtype
     fset = FieldSet.from_nemo(filenames, variables, dimensions)
-    assert isinstance(fset.varU.units, UnitConverter)
+    assert isinstance(fset.varU.units, Unity)
 
     # now try with setting fieldtype
     fset = FieldSet.from_nemo(filenames, variables, dimensions, fieldtype={"varU": "U", "varV": "V"})

tests/test_diffusion.py

@@ -4,7 +4,18 @@
 import pytest
 from scipy import stats
 
-from parcels import Field, FieldSet, Particle, ParticleSet, Variable, VectorField, XGrid
+from parcels import (
+    Field,
+    FieldSet,
+    GeographicPolarSquare,
+    GeographicSquare,
+    Particle,
+    ParticleSet,
+    Unity,
+    Variable,
+    VectorField,
+    XGrid,
+)
 from parcels._datasets.structured.generated import simple_UV_dataset
 from parcels.interpolators import XLinear
 from parcels.kernels import AdvectionDiffusionEM, AdvectionDiffusionM1, DiffusionUniformKh
@@ -28,6 +39,13 @@ def test_fieldKh_Brownian(mesh):
     fieldset.add_constant_field("Kh_zonal", kh_zonal, mesh=mesh)
     fieldset.add_constant_field("Kh_meridional", kh_meridional, mesh=mesh)
 
+    if mesh == "spherical":
+        assert isinstance(fieldset.Kh_zonal.units, GeographicPolarSquare)
+        assert isinstance(fieldset.Kh_meridional.units, GeographicSquare)
+    else:
+        assert isinstance(fieldset.Kh_zonal.units, Unity)
+        assert isinstance(fieldset.Kh_meridional.units, Unity)
+
     npart = 100
     runtime = np.timedelta64(2, "h")