Support overriding existing variables in to_zarr() without appending

doc/whats-new.rst

@@ -49,7 +49,9 @@ New Features
   By `Todd Jennings <https://github.com/toddrjen>`_
 - More support for unit aware arrays with pint (:pull:`3643`)
   By `Justus Magin <https://github.com/keewis>`_.
-
+- Support overriding existing variables in ``to_zarr()`` with ``mode='a'`` even
+  without ``append_dim``, as long as dimension sizes do not change.
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 - Allow plotting of boolean arrays. (:pull:`3766`)
   By `Marek Jacob <https://github.com/MeraX>`_
 - A ``days_in_month`` accessor for :py:class:`xarray.CFTimeIndex`, analogous to

xarray/backends/api.py

@@ -1279,18 +1279,35 @@ def _validate_append_dim_and_encoding(
         return
     if append_dim:
         if append_dim not in ds.dims:
-            raise ValueError(f"{append_dim} not a valid dimension in the Dataset")
-    for data_var in ds_to_append:
-        if data_var in ds:
-            if append_dim is None:
+            raise ValueError(
+                f"append_dim={append_dim!r} does not match any existing "
+                f"dataset dimensions {ds.dims}"
+            )
+    for var_name in ds_to_append:
+        if var_name in ds:
+            if ds_to_append[var_name].dims != ds[var_name].dims:
+                raise ValueError(
+                    f"variable {var_name!r} already exists with different "
+                    f"dimension names {ds[var_name].dims} != "
+                    f"{ds_to_append[var_name].dims}, but changing variable "
+                    "dimensions is not supported by to_zarr()."
+                )
+            existing_sizes = {
+                k: v for k, v in ds[var_name].sizes.items() if k != append_dim
+            }
+            new_sizes = {
+                k: v for k, v in ds_to_append[var_name].sizes.items() if k != append_dim
+            }
+            if existing_sizes != new_sizes:
                 raise ValueError(
-                    "variable '{}' already exists, but append_dim "
-                    "was not set".format(data_var)
+                    f"variable {var_name!r} already exists with different "
+                    "dimension sizes: {existing_sizes} != {new_sizes}. "
+                    "to_zarr() only supports changing dimension sizes when "
+                    f"explicitly appending, but append_dim={append_dim!r}."
                 )
-            if data_var in encoding.keys():
+            if var_name in encoding.keys():
                 raise ValueError(
-                    "variable '{}' already exists, but encoding was"
-                    "provided".format(data_var)
+                    f"variable {var_name!r} already exists, but encoding was provided"
                 )
 
 

xarray/backends/zarr.py

@@ -445,18 +445,23 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
             fill_value = attrs.pop("_FillValue", None)
             if v.encoding == {"_FillValue": None} and fill_value is None:
                 v.encoding = {}
-            if name in self.ds:
+
+            if self.append_dim is not None and self.append_dim in dims:
+                # resize existing variable
                 zarr_array = self.ds[name]
-                if self.append_dim in dims:
-                    # this is the DataArray that has append_dim as a
-                    # dimension
-                    append_axis = dims.index(self.append_dim)
-                    new_shape = list(zarr_array.shape)
-                    new_shape[append_axis] += v.shape[append_axis]
-                    new_region = [slice(None)] * len(new_shape)
-                    new_region[append_axis] = slice(zarr_array.shape[append_axis], None)
-                    zarr_array.resize(new_shape)
-                    writer.add(v.data, zarr_array, region=tuple(new_region))
+                append_axis = dims.index(self.append_dim)
+
+                new_region = [slice(None)] * len(dims)
+                new_region[append_axis] = slice(zarr_array.shape[append_axis], None)
+                region = tuple(new_region)
+
+                new_shape = list(zarr_array.shape)
+                new_shape[append_axis] += v.shape[append_axis]
+                zarr_array.resize(new_shape)
+            elif name in self.ds:
+                # override existing variable
+                zarr_array = self.ds[name]
+                region = None
             else:
                 # new variable
                 encoding = extract_zarr_variable_encoding(
@@ -474,7 +479,9 @@ def set_variables(self, variables, check_encoding_set, writer, unlimited_dims=No
                     name, shape=shape, dtype=dtype, fill_value=fill_value, **encoding
                 )
                 zarr_array.attrs.put(encoded_attrs)
-                writer.add(v.data, zarr_array)
+                region = None
+
+            writer.add(v.data, zarr_array, region=region)
 
     def close(self):
         if self._consolidate_on_close:

xarray/core/dataset.py

@@ -1579,7 +1579,7 @@ def to_zarr(
         mode : {'w', 'w-', 'a', None}
             Persistence mode: 'w' means create (overwrite if exists);
             'w-' means create (fail if exists);
-            'a' means append (create if does not exist).
+            'a' means override existing variables (create if does not exist).
             If ``append_dim`` is set, ``mode`` can be omitted as it is
             internally set to ``'a'``. Otherwise, ``mode`` will default to
             `w-` if not set.
@@ -1598,7 +1598,8 @@ def to_zarr(
             If True, apply zarr's `consolidate_metadata` function to the store
             after writing.
         append_dim: hashable, optional
-            If set, the dimension on which the data will be appended.
+            If set, the dimension along which the data will be appended. All
+            other dimensions on overriden variables must remain the same size.
 
         References
         ----------
@@ -1766,7 +1767,7 @@ def maybe_chunk(name, var, chunks):
         return self._replace(variables)
 
     def _validate_indexers(
-        self, indexers: Mapping[Hashable, Any], missing_dims: str = "raise",
+        self, indexers: Mapping[Hashable, Any], missing_dims: str = "raise"
     ) -> Iterator[Tuple[Hashable, Union[int, slice, np.ndarray, Variable]]]:
         """ Here we make sure
         + indexer has a valid keys
@@ -5933,7 +5934,7 @@ def polyfit(
                             "The number of data points must exceed order to scale the covariance matrix."
                         )
                     fac = residuals / (x.shape[0] - order)
-                covariance = xr.DataArray(Vbase, dims=("cov_i", "cov_j"),) * fac
+                covariance = xr.DataArray(Vbase, dims=("cov_i", "cov_j")) * fac
                 variables[name + "polyfit_covariance"] = covariance
 
         return Dataset(data_vars=variables, attrs=self.attrs.copy())
@@ -6199,7 +6200,7 @@ def idxmin(
                 skipna=skipna,
                 fill_value=fill_value,
                 keep_attrs=keep_attrs,
-            ),
+            )
         )
 
     def idxmax(
@@ -6297,7 +6298,7 @@ def idxmax(
                 skipna=skipna,
                 fill_value=fill_value,
                 keep_attrs=keep_attrs,
-            ),
+            )
         )
 
 

xarray/tests/test_backends.py

@@ -1526,12 +1526,6 @@ def roundtrip(
             with self.open(store_target, **open_kwargs) as ds:
                 yield ds
 
-    @contextlib.contextmanager
-    def roundtrip_append(
-        self, data, save_kwargs=None, open_kwargs=None, allow_cleanup_failure=False
-    ):
-        pytest.skip("zarr backend does not support appending")
-
     def test_roundtrip_consolidated(self):
         pytest.importorskip("zarr", minversion="2.2.1.dev2")
         expected = create_test_data()
@@ -1826,63 +1820,52 @@ def test_encoding_kwarg_fixed_width_string(self):
         # not relevant for zarr, since we don't use EncodedStringCoder
         pass
 
-    # TODO: someone who understand caching figure out whether chaching
+    # TODO: someone who understand caching figure out whether caching
     # makes sense for Zarr backend
     @pytest.mark.xfail(reason="Zarr caching not implemented")
     def test_dataset_caching(self):
         super().test_dataset_caching()
 
     @pytest.mark.skipif(LooseVersion(dask_version) < "2.4", reason="dask GH5334")
     def test_append_write(self):
-        ds, ds_to_append, _ = create_append_test_data()
-        with self.create_zarr_target() as store_target:
-            ds.to_zarr(store_target, mode="w")
-            ds_to_append.to_zarr(store_target, append_dim="time")
-            original = xr.concat([ds, ds_to_append], dim="time")
-            assert_identical(original, xr.open_zarr(store_target))
-
-    @pytest.mark.xfail(reason="Zarr stores can not be appended to")
-    def test_append_overwrite_values(self):
-        super().test_append_overwrite_values()
+        super().test_append_write()
 
     def test_append_with_invalid_dim_raises(self):
-
         ds, ds_to_append, _ = create_append_test_data()
-
-        # check failure when append_dim not valid
-        with pytest.raises(ValueError):
-            with self.create_zarr_target() as store_target:
-                ds.to_zarr(store_target, mode="w")
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w")
+            with pytest.raises(
+                ValueError, match="does not match any existing dataset dimensions"
+            ):
                 ds_to_append.to_zarr(store_target, append_dim="notvalid")
 
-    def test_append_with_append_dim_not_set_raises(self):
+    def test_append_with_no_dims_raises(self):
+        with self.create_zarr_target() as store_target:
+            Dataset({"foo": ("x", [1])}).to_zarr(store_target, mode="w")
+            with pytest.raises(ValueError, match="different dimension names"):
+                Dataset({"foo": ("y", [2])}).to_zarr(store_target, mode="a")
 
+    def test_append_with_append_dim_not_set_raises(self):
         ds, ds_to_append, _ = create_append_test_data()
-
-        # check failure when append_dim not set
-        with pytest.raises(ValueError):
-            with self.create_zarr_target() as store_target:
-                ds.to_zarr(store_target, mode="w")
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w")
+            with pytest.raises(ValueError, match="different dimension sizes"):
                 ds_to_append.to_zarr(store_target, mode="a")
 
     def test_append_with_mode_not_a_raises(self):
-
         ds, ds_to_append, _ = create_append_test_data()
-
-        # check failure when append_dim is set and mode != 'a'
-        with pytest.raises(ValueError):
-            with self.create_zarr_target() as store_target:
-                ds.to_zarr(store_target, mode="w")
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w")
+            with pytest.raises(
+                ValueError, match="append_dim was set along with mode='w'"
+            ):
                 ds_to_append.to_zarr(store_target, mode="w", append_dim="time")
 
     def test_append_with_existing_encoding_raises(self):
-
         ds, ds_to_append, _ = create_append_test_data()
-
-        # check failure when providing encoding to existing variable
-        with pytest.raises(ValueError):
-            with self.create_zarr_target() as store_target:
-                ds.to_zarr(store_target, mode="w")
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w")
+            with pytest.raises(ValueError, match="but encoding was provided"):
                 ds_to_append.to_zarr(
                     store_target,
                     append_dim="time",