Search for coord_names in separate_coords (#191)

* find coord_names in vars

* resolve merge conflict

* add 2d coords test

* add kerchunk dep and add 1d coord test

---------

Co-authored-by: Tom Nicholas <tom@cworthy.org>

Author: ayushnag

conftest.py

@@ -35,6 +35,15 @@ def netcdf4_file(tmpdir):
     return filepath
 
 
+@pytest.fixture
+def netcdf4_file_with_2d_coords(tmpdir):
+    ds = xr.tutorial.open_dataset("ROMS_example")
+    filepath = f"{tmpdir}/ROMS_example.nc"
+    ds.to_netcdf(filepath, format="NETCDF4")
+    ds.close()
+    return filepath
+
+
 @pytest.fixture
 def netcdf4_virtual_dataset(netcdf4_file):
     from virtualizarr import open_virtual_dataset

virtualizarr/readers/common.py

@@ -144,8 +144,13 @@ def separate_coords(
     coord_vars: dict[
         str, tuple[Hashable, Any, dict[Any, Any], dict[Any, Any]] | Variable
     ] = {}
+    found_coord_names: set[str] = set()
+    # Search through variable attributes for coordinate names
+    for var in vars.values():
+        if "coordinates" in var.attrs:
+            found_coord_names.update(var.attrs["coordinates"].split(" "))
     for name, var in vars.items():
-        if name in coord_names or var.dims == (name,):
+        if name in coord_names or var.dims == (name,) or name in found_coord_names:
             # use workaround to avoid creating IndexVariables described here https://github.com/pydata/xarray/pull/8107#discussion_r1311214263
             if len(var.dims) == 1:
                 dim1d, *_ = var.dims

virtualizarr/tests/test_backend.py

@@ -156,6 +156,28 @@ def test_coordinate_variable_attrs_preserved(self, netcdf4_file):
         }
 
 
+@requires_kerchunk
+class TestDetermineCoords:
+    def test_infer_one_dimensional_coords(self, netcdf4_file):
+        vds = open_virtual_dataset(netcdf4_file, indexes={})
+        assert set(vds.coords) == {"time", "lat", "lon"}
+
+    def test_var_attr_coords(self, netcdf4_file_with_2d_coords):
+        vds = open_virtual_dataset(netcdf4_file_with_2d_coords, indexes={})
+
+        expected_dimension_coords = ["ocean_time", "s_rho"]
+        expected_2d_coords = ["lon_rho", "lat_rho", "h"]
+        expected_1d_non_dimension_coords = ["Cs_r"]
+        expected_scalar_coords = ["hc", "Vtransform"]
+        expected_coords = (
+            expected_dimension_coords
+            + expected_2d_coords
+            + expected_1d_non_dimension_coords
+            + expected_scalar_coords
+        )
+        assert set(vds.coords) == set(expected_coords)
+
+
 @network
 @requires_s3fs
 class TestReadFromS3: