Scope assert_valid_lat_lon for >0D grids (#2058)

* Remove XGrid.lonlat_minmax

This was in v3 of parcels and isn't needed anymore

* Only run XGrid lon/lat assertion on non 0D grids

Adds a `.axes` attribute to help with the check (in line with development - see #2054)

* Add test stubs

* Update varname

Author: VeckoTheGecko

parcels/fieldset.py

@@ -161,15 +161,6 @@ def add_constant_field(self, name: str, value, mesh: Mesh = "flat"):
         da = xr.DataArray(
             data=np.full((1, 1, 1, 1), value),
             dims=["time", "ZG", "YG", "XG"],
-            coords={
-                "ZG": (["ZG"], np.arange(1), {"axis": "Z"}),
-                "YG": (["YG"], np.arange(1), {"axis": "Y"}),
-                "XG": (["XG"], np.arange(1), {"axis": "X"}),
-                "lon": (["XG"], np.arange(1), {"axis": "X"}),
-                "lat": (["YG"], np.arange(1), {"axis": "Y"}),
-                "depth": (["ZG"], np.arange(1), {"axis": "Z"}),
-                "time": (["time"], np.arange(1), {"axis": "T"}),
-            },
         )
         grid = XGrid(xgcm.Grid(da))
         self.add_field(

parcels/xgrid.py

@@ -10,6 +10,9 @@
 from parcels.basegrid import BaseGrid
 from parcels.tools.converters import TimeConverter
 
+_XGRID_AXES_ORDERING = "ZYX"
+_XGRID_AXES = Literal["X", "Y", "Z"]
+
 _XGCM_AXIS_DIRECTION = Literal["X", "Y", "Z", "T"]
 _XGCM_AXIS_POSITION = Literal["center", "left", "right", "inner", "outer"]
 _AXIS_DIRECTION = Literal["X", "Y", "Z"]
@@ -29,6 +32,11 @@ def get_time(axis: xgcm.Axis) -> npt.NDArray:
     return axis._ds[axis.coords["center"]].values
 
 
+def _get_xgrid_axes(grid: xgcm.Grid) -> list[_XGRID_AXES]:
+    spatial_axes = [a for a in grid.axes.keys() if a in ["X", "Y", "Z"]]
+    return sorted(spatial_axes, key=_XGRID_AXES_ORDERING.index)
+
+
 class XGrid(BaseGrid):
     """
     Class to represent a structured grid in Parcels. Wraps a xgcm-like Grid object (we use a trimmed down version of the xgcm.Grid class that is vendored with Parcels).
@@ -44,17 +52,13 @@ def __init__(self, grid: xgcm.Grid, mesh="flat"):
         self.xgcm_grid = grid
         self.mesh = mesh
         ds = grid._ds
-        assert_valid_lat_lon(ds["lat"], ds["lon"], grid.axes)
-
-        # ! Not ideal... Triggers computation on a throwaway item. Keeping for now for v3 compat, will be removed in v4.
-        self.lonlat_minmax = np.array(
-            [
-                np.nanmin(self.xgcm_grid._ds["lon"]),
-                np.nanmax(self.xgcm_grid._ds["lon"]),
-                np.nanmin(self.xgcm_grid._ds["lat"]),
-                np.nanmax(self.xgcm_grid._ds["lat"]),
-            ]
-        )
+
+        if len(set(grid.axes) & {"X", "Y", "Z"}) > 0:  # Only if spatial grid is >0D (see #2054 for further development)
+            assert_valid_lat_lon(ds["lat"], ds["lon"], grid.axes)
+
+    @property
+    def axes(self) -> list[_XGRID_AXES]:
+        return _get_xgrid_axes(self.xgcm_grid)
 
     @property
     def lon(self):

tests/v4/test_xgrid.py

@@ -78,10 +78,15 @@ def test_xgrid_against_old(ds, attr):
 
 
 @pytest.mark.parametrize("ds", [pytest.param(ds, id=key) for key, ds in datasets.items()])
-def test_grid_init_on_generic_datasets(ds):
+def test_xgrid_init_on_generic_datasets(ds):
     XGrid(xgcm.Grid(ds, periodic=False))
 
 
+def test_xgrid_axes():
+    # Tests that the xgrid.axes property correctly identifies the axes and ordering
+    ...
+
+
 def test_invalid_xgrid_field_array():
     """Stress test initialiser by creating incompatible datasets that test the edge cases"""
     ...