Allow user to explicitly disable coordinates attribute

doc/whats-new.rst

@@ -48,6 +48,9 @@ New Features
   By `Justus Magin <https://github.com/keewis>`_.
 - Allow plotting categorical data (:pull:`5464`).
   By `Jimmy Westling <https://github.com/illviljan>`_.
+- Allow removal of the coordinate attribute ``coordinates`` on variables by setting ``.attrs['coordinates']= None``
+  (:issue:`5510`).
+  By `Elle Smith <https://github.com/ellesmith88>`_.
 
 Breaking changes
 ~~~~~~~~~~~~~~~~

xarray/conventions.py

@@ -751,6 +751,10 @@ def _encode_coordinates(variables, attributes, non_dim_coord_names):
                 f"'coordinates' found in both attrs and encoding for variable {name!r}."
             )
 
+        # if coordinates set to None, don't write coordinates attribute
+        if "coordinates" in attrs and attrs.get("coordinates") is None:
+            continue
+
         # this will copy coordinates from encoding to attrs if "coordinates" in attrs
         # after the next line, "coordinates" is never in encoding
         # we get support for attrs["coordinates"] for free.
@@ -772,6 +776,7 @@ def _encode_coordinates(variables, attributes, non_dim_coord_names):
     # conventions, only do it if necessary.
     # Reference discussion:
     # http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/007571.html
+
     global_coordinates.difference_update(written_coords)
     if global_coordinates:
         attributes = dict(attributes)

xarray/tests/test_conventions.py

@@ -142,6 +142,31 @@ def test_do_not_overwrite_user_coordinates(self):
         with pytest.raises(ValueError, match=r"'coordinates' found in both attrs"):
             conventions.encode_dataset_coordinates(orig)
 
+    def test_emit_coordinates_attribute(self):
+        orig = Dataset(
+            {
+                "foo": (("x", "y"), np.random.randn(2, 3)),
+                "x": [10, 20],
+                "bar": ("x", [1, 2]),
+            }
+        )
+        orig = orig.assign_coords(
+            {"t": np.array("2004-11-01T00:00:00", dtype=np.datetime64)}
+        )
+        orig = orig.assign({"a": 1})
+        orig = orig.assign({"b": 1})
+
+        orig["a"].attrs["coordinates"] = None
+        enc, _ = conventions.encode_dataset_coordinates(orig)
+
+        # check coordiante attribute emitted for 'a'
+        assert enc["a"].attrs.get("coordinates") is None
+        assert enc["a"].encoding.get("coordinates") is None
+
+        # check coordinate attribute not emitted for 'b'
+        assert enc["b"].attrs.get("coordinates") == "t"
+        assert enc["b"].encoding.get("coordinates") is None
+
     @requires_dask
     def test_string_object_warning(self):
         original = Variable(("x",), np.array(["foo", "bar"], dtype=object)).chunk()