Filtering when reading a Grid file, avoid time leaking into the Grid

test/test_subset.py

@@ -5,6 +5,7 @@
 from uxarray.grid.slice import _remap_dense, _remap_kernel, _remap_searchsorted
 
 import pytest
+import xarray as xr
 
 
 def test_repr(gridpath, datasetpath):
@@ -331,3 +332,63 @@ def open_with_edges(**kwargs):
                 eager_subset._ds[name].values,
                 err_msg=f"force_sparse={force_sparse}: {name}",
             )
+
+
+def test_bounding_box_with_stray_grid_time(gridpath):
+    """Subsetting must work when the grid was built from a source that carried a stray
+    scalar ``time`` coordinate. Ensures issue #1444 has been fixed.
+    """
+    # grid built from a source carrying a stray scalar `time`
+    grid_ds = xr.open_dataset(gridpath("ugrid", "quad-hexagon", "grid.nc"))
+    uxgrid = ux.open_grid(grid_ds.assign_coords(time=np.datetime64("2016-01-01")))
+
+    # the stray coordinate is dropped during grid construction
+    assert "time" not in uxgrid._ds.coords
+
+    # a face-centered variable carrying a *different* time coordinate
+    times = np.array(["2018-01-01", "2020-01-01"], dtype="datetime64[ns]")
+    uxda = ux.UxDataArray(
+        data=np.ones((times.size, uxgrid.n_face)),
+        dims=("time", "n_face"),
+        coords={"time": times},
+        uxgrid=uxgrid,
+    )
+
+    # previously raised: "IndexError: dimension coordinate 'time' conflicts ..."
+    res = uxda.subset.bounding_box(lon_bounds=(-10, 10), lat_bounds=(-10, 10))
+
+    assert isinstance(res, ux.UxDataArray)
+    assert res.sizes["time"] == times.size
+    assert "n_face" in res.dims
+
+
+def test_bounding_box_with_stray_grid_time_dimension(gridpath):
+    """Subsetting must also work when the grid source carried a full ``time`` *dimension*
+    (a variable along ``time``) rather than the scalar coordinate of issue #1444.
+
+    A dimensional coordinate has dims ``(time,)``, which is not a subset of the
+    ``(n_face,)`` ``subgrid_face_indices`` indexer, so — unlike a 0-d coordinate — it
+    never attaches to that indexer and cannot collide during ``.isel``. The stray
+    dimension harmlessly rides along in ``Grid._ds`` but never reaches the indexing path.
+    """
+    # grid built from a source carrying a variable along a stray `time` dimension
+    grid_ds = xr.open_dataset(gridpath("ugrid", "quad-hexagon", "grid.nc"))
+    uxgrid = ux.open_grid(grid_ds.assign(stray=("time", [0.0, 1.0])))
+
+    # the bare dimension is not a coordinate, so it is not dropped; that is harmless
+    assert "time" in uxgrid._ds.dims
+
+    # a face-centered variable carrying its own time coordinate
+    times = np.array(["2018-01-01", "2020-01-01"], dtype="datetime64[ns]")
+    uxda = ux.UxDataArray(
+        data=np.ones((times.size, uxgrid.n_face)),
+        dims=("time", "n_face"),
+        coords={"time": times},
+        uxgrid=uxgrid,
+    )
+
+    res = uxda.subset.bounding_box(lon_bounds=(-10, 10), lat_bounds=(-10, 10))
+
+    assert isinstance(res, ux.UxDataArray)
+    assert res.sizes["time"] == times.size
+    assert "n_face" in res.dims

uxarray/grid/grid.py

@@ -66,7 +66,7 @@
     _populate_edge_node_distances,
 )
 from uxarray.grid.point_in_face import _point_in_face_query
-from uxarray.grid.utils import make_setter
+from uxarray.grid.utils import _drop_non_grid_coords, make_setter
 from uxarray.grid.validation import (
     _check_area,
     _check_connectivity,
@@ -191,8 +191,10 @@ def __init__(
         # source grid specification (i.e. UGRID, MPAS, SCRIP, etc.)
         self.source_grid_spec = source_grid_spec
 
-        # internal xarray dataset for storing grid variables
-        self._ds = grid_ds
+        # internal xarray dataset for storing grid variables.
+        # drop stray scalar coordinates (e.g. a `time` carried in from the source file)
+        # so they can't leak onto the grid and collide during subsetting (see #1444).
+        self._ds = _drop_non_grid_coords(grid_ds)
 
         # source grid specification (i.e. UGRID, MPAS, SCRIP, etc.)
         self.source_grid_spec = source_grid_spec

uxarray/grid/utils.py

@@ -3,6 +3,7 @@
 from numba import njit, prange
 
 from uxarray.constants import INT_FILL_VALUE
+from uxarray.conventions import ugrid
 from uxarray.utils.numba_math import (
     _numba_add3,
     _numba_mul3_scalar,
@@ -11,6 +12,27 @@
 )
 
 
+def _drop_non_grid_coords(ds):
+    """Drop scalar (0-d) coordinates that aren't recognized grid coordinates
+    (e.g. a stray ``time`` carried in from the source file).
+
+    Only 0-d strays can cause the subset collision behind issue #1444: a scalar
+    coordinate attaches to every variable, including the ``(n_face,)``
+    ``subgrid_*_indices`` indexer, where it can clash with a like-named dimension
+    on the data being indexed. A coordinate with its own dimension (e.g. ICON's
+    1-d ``clon``/``clat`` or FESOM2's ``lon``/``lat``) never attaches to that
+    indexer, so it is left in place and those grids round-trip unchanged.
+
+    Coordinate-only, so grid data variables — connectivity, descriptors, and the
+    subset's ``subgrid_*_indices`` — are always left intact.
+    """
+    grid_coords = set(ugrid.SPHERICAL_COORD_NAMES) | set(ugrid.CARTESIAN_COORD_NAMES)
+    stray = [
+        coord for coord in ds.coords if coord not in grid_coords and ds[coord].ndim == 0
+    ]
+    return ds.drop_vars(stray, errors="ignore")
+
+
 @njit(cache=True)
 def _small_angle_of_2_vectors(u, v):
     """