Use zarr to validate attrs when writing to zarr (#6636)

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

doc/whats-new.rst

@@ -96,6 +96,8 @@ Deprecations
 Bug fixes
 ~~~~~~~~~
 
+- :py:meth:`Dataset.to_zarr` now allows to write all attribute types supported by `zarr-python`.
+  By `Mattia Almansi <https://github.com/malmans2>`_.
 - Set ``skipna=None`` for all ``quantile`` methods (e.g. :py:meth:`Dataset.quantile`) and
   ensure it skips missing values for float dtypes (consistent with other methods). This should
   not change the behavior (:pull:`6303`).

xarray/backends/api.py

@@ -1561,9 +1561,8 @@ def to_zarr(
             f"'w-', 'a' and 'r+', but mode={mode!r}"
         )
 
-    # validate Dataset keys, DataArray names, and attr keys/values
+    # validate Dataset keys, DataArray names
     _validate_dataset_names(dataset)
-    _validate_attrs(dataset)
 
     if region is not None:
         _validate_region(dataset, region)

xarray/backends/zarr.py

@@ -320,6 +320,15 @@ def _validate_existing_dims(var_name, new_var, existing_var, region, append_dim)
         )
 
 
+def _put_attrs(zarr_obj, attrs):
+    """Raise a more informative error message for invalid attrs."""
+    try:
+        zarr_obj.attrs.put(attrs)
+    except TypeError as e:
+        raise TypeError("Invalid attribute in Dataset.attrs.") from e
+    return zarr_obj
+
+
 class ZarrStore(AbstractWritableDataStore):
     """Store for reading and writing data via zarr"""
 
@@ -479,7 +488,7 @@ def set_dimensions(self, variables, unlimited_dims=None):
             )
 
     def set_attributes(self, attributes):
-        self.zarr_group.attrs.put(attributes)
+        _put_attrs(self.zarr_group, attributes)
 
     def encode_variable(self, variable):
         variable = encode_zarr_variable(variable)
@@ -618,7 +627,7 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
                 zarr_array = self.zarr_group.create(
                     name, shape=shape, dtype=dtype, fill_value=fill_value, **encoding
                 )
-                zarr_array.attrs.put(encoded_attrs)
+                zarr_array = _put_attrs(zarr_array, encoded_attrs)
 
             write_region = self._write_region if self._write_region is not None else {}
             write_region = {dim: write_region.get(dim, slice(None)) for dim in dims}

xarray/tests/test_backends.py

@@ -2443,6 +2443,22 @@ def test_write_read_select_write(self):
         with self.create_zarr_target() as final_store:
             ds_sel.to_zarr(final_store, mode="w")
 
+    @pytest.mark.parametrize("obj", [Dataset(), DataArray(name="foo")])
+    def test_attributes(self, obj):
+        obj = obj.copy()
+
+        obj.attrs["good"] = {"key": "value"}
+        ds = obj if isinstance(obj, Dataset) else obj.to_dataset()
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target)
+            assert_identical(ds, xr.open_zarr(store_target))
+
+        obj.attrs["bad"] = DataArray()
+        ds = obj if isinstance(obj, Dataset) else obj.to_dataset()
+        with self.create_zarr_target() as store_target:
+            with pytest.raises(TypeError, match=r"Invalid attribute in Dataset.attrs."):
+                ds.to_zarr(store_target)
+
 
 @requires_zarr
 class TestZarrDictStore(ZarrBase):