Add mode='a-': Do not overwrite coordinates when appending to Zarr with append_dim

xarray/backends/zarr.py

@@ -628,8 +628,21 @@ def store(
             self.set_attributes(attributes)
             self.set_dimensions(variables_encoded, unlimited_dims=unlimited_dims)
 
+        # if we are appending to an append_dim, only write either
+        # - new variables not already present, OR
+        # - variables with the append_dim in their dimensions
+        # We do NOT overwrite other variables.
+        if self._append_dim is not None:
+            variables_to_set = {
+                k: v
+                for k, v in variables_encoded.items()
+                if (k not in existing_variable_names) or (self._append_dim in v.dims)
+            }
+        else:
+            variables_to_set = variables_encoded
+
         self.set_variables(
-            variables_encoded, check_encoding_set, writer, unlimited_dims=unlimited_dims
+            variables_to_set, check_encoding_set, writer, unlimited_dims=unlimited_dims
         )
         if self._consolidate_on_close:
             zarr.consolidate_metadata(self.zarr_group.store)

xarray/tests/test_backends.py

@@ -2382,6 +2382,19 @@ def test_append_with_new_variable(self) -> None:
                 xr.open_dataset(store_target, engine="zarr", **self.version_kwargs),
             )
 
+    def test_append_with_append_dim_no_overwrite(self) -> None:
+        ds, ds_to_append, _ = create_append_test_data()
+        with self.create_zarr_target() as store_target:
+            ds.to_zarr(store_target, mode="w", **self.version_kwargs)
+            original = xr.concat([ds, ds_to_append], dim="time")
+
+            # overwrite a coordinate; this will not get written to the store
+            # because it does not have the append_dim as a dim
+            ds_to_append.lon.data[:] = -999
+            ds_to_append.to_zarr(store_target, append_dim="time", **self.version_kwargs)
+            actual = xr.open_dataset(store_target, engine="zarr", **self.version_kwargs)
+            assert_identical(original, actual)
+
     @requires_dask
     def test_to_zarr_compute_false_roundtrip(self) -> None:
         from dask.delayed import Delayed