Add sgrid axes parsing

cf_xarray/__init__.py

@@ -1,3 +1,4 @@
+from . import sgrid  # noqa
 from .accessor import CFAccessor  # noqa
 from .coding import (  # noqa
     decode_compress_to_multi_index,

cf_xarray/accessor.py

@@ -30,6 +30,7 @@
 from xarray.core.rolling import Coarsen, Rolling
 from xarray.core.weighted import Weighted
 
+from . import sgrid
 from .criteria import cf_role_criteria, coordinate_criteria, regex
 from .helpers import _guess_bounds_1d, _guess_bounds_2d, bounds_to_vertices
 from .options import OPTIONS
@@ -300,6 +301,11 @@ def _get_axis_coord(obj: DataArray | Dataset, key: str) -> list[str]:
                     units = getattr(var.data, "units", None)
                     if units in expected:
                         results.update((coord,))
+
+    if key in _AXIS_NAMES and "grid_topology" in obj.cf.cf_roles:
+        sgrid_axes = sgrid.parse_axes(obj)
+        results.update((search_in | set(obj.dims)) & sgrid_axes[key])
+
     return list(results)
 
 
@@ -424,7 +430,7 @@ def _get_coords(obj: DataArray | Dataset, key: Hashable) -> list[Hashable]:
     One or more of ('X', 'Y', 'Z', 'T', 'longitude', 'latitude', 'vertical', 'time',
     'area', 'volume'), or arbitrary measures, or standard names present in .coords
     """
-    return [k for k in _get_all(obj, key) if k in obj.coords]
+    return [k for k in _get_all(obj, key) if k in obj.coords or k in obj.dims]
 
 
 def _variables(func: F) -> F:

cf_xarray/datasets.py

@@ -604,3 +604,23 @@ def _create_inexact_bounds():
         "trajectory": ("trajectory", [0, 1], {"cf_role": "trajectory_id"}),
     },
 )
+
+
+roms_sgrid = xr.Dataset()
+roms_sgrid["grid"] = xr.DataArray(
+    0,
+    attrs=dict(
+        cf_role="grid_topology",
+        topology_dimension=2,
+        node_dimensions="xi_psi eta_psi",
+        face_dimensions="xi_rho: xi_psi (padding: both) eta_rho: eta_psi (padding: both)",
+        edge1_dimensions="xi_u: xi_psi eta_u: eta_psi (padding: both)",
+        edge2_dimensions="xi_v: xi_psi (padding: both) eta_v: eta_psi",
+        node_coordinates="lon_psi lat_psi",
+        face_coordinates="lon_rho lat_rho",
+        edge1_coordinates="lon_u lat_u",
+        edge2_coordinates="lon_v lat_v",
+        vertical_dimensions="s_rho: s_w (padding: none)",
+    ),
+)
+roms_sgrid["u"] = (("xi_u", "eta_u"), np.ones((2, 2)))

cf_xarray/sgrid.py

@@ -0,0 +1,27 @@
+def parse_axes(ds):
+    import re
+
+    (gridvar,) = ds.cf.cf_roles["grid_topology"]
+    grid = ds[gridvar]
+    pattern = re.compile("\\s?(.*?):\\s*(.*?)\\s+(?:\\(padding:(.+?)\\))?")
+    ndim = grid.attrs["topology_dimension"]
+    axes_names = ["X", "Y", "Z"][:ndim]
+    axes = dict(
+        zip(
+            axes_names,
+            ({k} for k in grid.attrs["node_dimensions"].split(" ")),
+        )
+    )
+    for attr in ["face_dimensions", "edge1_dimensions", "edge2_dimensions", "foo"]:
+        if attr in grid.attrs:
+            matches = re.findall(pattern, grid.attrs[attr] + "\n")
+            assert len(matches) == ndim, matches
+            for ax, match in zip(axes_names, matches):
+                axes[ax].update(set(match[:2]))
+
+    if ndim == 2 and "vertical_dimensions" in grid.attrs:
+        matches = re.findall(pattern, grid.attrs["vertical_dimensions"] + "\n")
+        assert len(matches) == 1
+        axes["Z"] = set(matches[0][:2])
+
+    return axes

cf_xarray/tests/test_accessor.py

@@ -30,6 +30,7 @@
     multiple,
     pomds,
     popds,
+    roms_sgrid,
     romsds,
     rotds,
     vert,
@@ -1799,3 +1800,29 @@ def plane(coords, slopex, slopey):
         coords=["latitude", "longitude"], func=plane
     )
     assert_identical(expected, actual)
+
+
+def test_sgrid():
+    from ..sgrid import parse_axes
+
+    positions = ["psi", "u", "v", "rho"]
+    expected = {
+        "X": {f"xi_{pos}" for pos in positions},
+        "Y": {f"eta_{pos}" for pos in positions},
+        "Z": {"s_rho", "s_w"},
+    }
+    assert parse_axes(roms_sgrid) == expected
+    assert roms_sgrid.cf.axes == {"X": ["xi_u"], "Y": ["eta_u"]}
+
+    assert_identical(roms_sgrid.cf["X"], roms_sgrid.xi_u)
+    assert_identical(roms_sgrid.cf["Y"], roms_sgrid.eta_u)
+
+    with pytest.raises(KeyError):
+        roms_sgrid.u.cf["X"]
+    with pytest.raises(KeyError):
+        roms_sgrid.u.cf["Y"]
+
+    roms_ = roms_sgrid.set_coords("grid")
+    assert roms_.u.cf.axes == {"X": ["xi_u"], "Y": ["eta_u"]}
+    assert_identical(roms_.u.cf["X"], roms_sgrid.xi_u)
+    assert_identical(roms_.u.cf["Y"], roms_sgrid.eta_u)