Do not write an empty "coordinates" attribute to a netCDF file (#5121)

Co-authored-by: keewis <[email protected]>

Author: alexamici

xarray/conventions.py

@@ -756,11 +756,13 @@ def _encode_coordinates(variables, attributes, non_dim_coord_names):
         # we get support for attrs["coordinates"] for free.
         coords_str = pop_to(encoding, attrs, "coordinates")
         if not coords_str and variable_coordinates[name]:
-            attrs["coordinates"] = " ".join(
+            coordinates_text = " ".join(
                 str(coord_name)
                 for coord_name in variable_coordinates[name]
                 if coord_name not in not_technically_coordinates
             )
+            if coordinates_text:
+                attrs["coordinates"] = coordinates_text
         if "coordinates" in attrs:
             written_coords.update(attrs["coordinates"].split())
 

xarray/tests/test_backends.py

@@ -920,7 +920,7 @@ def test_grid_mapping_and_bounds_are_not_coordinates_in_file(self):
             with open_dataset(tmp_file, decode_coords=False) as ds:
                 assert ds.coords["latitude"].attrs["bounds"] == "latitude_bnds"
                 assert ds.coords["longitude"].attrs["bounds"] == "longitude_bnds"
-                assert "latlon" not in ds["variable"].attrs["coordinates"]
+                assert "coordinates" not in ds["variable"].attrs
                 assert "coordinates" not in ds.attrs
 
     def test_coordinate_variables_after_dataset_roundtrip(self):