Fix for encoding objects for zarr<3

xarray/backends/zarr.py

@@ -1130,6 +1130,20 @@ def _create_new_array(
                 if c in encoding:
                     encoding["config"][c] = encoding.pop(c)
 
+        else:
+            from zarr.util import normalize_dtype
+
+            _, object_codec = normalize_dtype(dtype, None)
+
+            if object_codec is not None:
+                existing_filters = encoding.get("filters")
+                if (
+                    existing_filters is not None
+                    and len(existing_filters) == 1
+                    and existing_filters[0] == object_codec
+                ):
+                    del encoding["filters"]
+
         zarr_array = self.zarr_group.create(
             name,
             shape=shape,

xarray/tests/test_backends.py

@@ -121,7 +121,6 @@
 except ImportError:
     pass
 
-
 if has_zarr:
     import zarr
     import zarr.codecs
@@ -3075,6 +3074,17 @@ def test_zarr_mode_w_overwrites_encoding(self) -> None:
                 zarr.open_group(store, **self.version_kwargs)["foo"], data.foo.data
             )
 
+    def test_object_codec(self) -> None:
+        data = xr.DataArray(
+            data=np.zeros((2, 2)),
+            dims=["x", "y"],
+            coords=dict(y=np.array(["a", "b"], dtype=object)),
+        )
+        with create_tmp_file() as path1:
+            data.to_zarr(path1, mode="w")
+            data = xr.open_zarr(path1)
+            data.to_zarr(path1, mode="w")
+
     def test_encoding_kwarg_fixed_width_string(self) -> None:
         # not relevant for zarr, since we don't use EncodedStringCoder
         pass