Use zarr v3 dimension_names (#9669)

* use zarr v3 dimension_names

* Update xarray/backends/zarr.py

Co-authored-by: Stephan Hoyer <shoyer@google.com>

* Update xarray/backends/zarr.py

Co-authored-by: Joe Hamman <jhamman1@gmail.com>

---------

Co-authored-by: Stephan Hoyer <shoyer@google.com>
Co-authored-by: Joe Hamman <joe@earthmover.io>
Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>
Co-authored-by: Joe Hamman <jhamman1@gmail.com>

xarray/backends/zarr.py

@@ -308,6 +308,18 @@ def _determine_zarr_chunks(
 
 
 def _get_zarr_dims_and_attrs(zarr_obj, dimension_key, try_nczarr):
+    # Zarr V3 explicitly stores the dimension names in the metadata
+    try:
+        # if this exists, we are looking at a Zarr V3 array
+        # convert None to empty tuple
+        dimensions = zarr_obj.metadata.dimension_names or ()
+    except AttributeError:
+        # continue to old code path
+        pass
+    else:
+        attributes = dict(zarr_obj.attrs)
+        return dimensions, attributes
+
     # Zarr arrays do not have dimensions. To get around this problem, we add
     # an attribute that specifies the dimension. We have to hide this attribute
     # when we send the attributes to the user.
@@ -919,6 +931,7 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
         import zarr
 
         existing_keys = tuple(self.zarr_group.array_keys())
+        is_zarr_v3_format = _zarr_v3() and self.zarr_group.metadata.zarr_format == 3
 
         for vn, v in variables.items():
             name = _encode_variable_name(vn)
@@ -1022,7 +1035,10 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
                 # new variable
                 encoded_attrs = {}
                 # the magic for storing the hidden dimension data
-                encoded_attrs[DIMENSION_KEY] = dims
+                if is_zarr_v3_format:
+                    encoding["dimension_names"] = dims
+                else:
+                    encoded_attrs[DIMENSION_KEY] = dims
                 for k2, v2 in attrs.items():
                     encoded_attrs[k2] = self.encode_attribute(v2)
 

xarray/tests/test_backends.py

@@ -2559,6 +2559,8 @@ def test_drop_encoding(self):
                 ds.to_zarr(store, encoding=encodings)
 
     def test_hidden_zarr_keys(self) -> None:
+        skip_if_zarr_format_3("This test is unnecessary; no hidden Zarr keys")
+
         expected = create_test_data()
         with self.create_store() as store:
             expected.dump_to_store(store)
@@ -2586,6 +2588,16 @@ def test_hidden_zarr_keys(self) -> None:
                 with xr.decode_cf(store):
                     pass
 
+    def test_dimension_names(self) -> None:
+        skip_if_zarr_format_2("No dimension names in V2")
+
+        expected = create_test_data()
+        with self.create_store() as store:
+            expected.dump_to_store(store)
+            zarr_group = store.ds
+            for var in zarr_group:
+                assert expected[var].dims == zarr_group[var].metadata.dimension_names
+
     @pytest.mark.parametrize("group", [None, "group1"])
     def test_write_persistence_modes(self, group) -> None:
         original = create_test_data()