Fix zarr append with groups

doc/whats-new.rst

@@ -70,6 +70,8 @@ Bug fixes
   By `Justus Magin <https://github.com/keewis>`_.
 - :py:meth:`Dataset.rename`, :py:meth:`DataArray.rename` now check for conflicts with
   MultiIndex level names.
+- Fix :py:meth:`xarray.core.dataset.Dataset.to_zarr` when using `append_dim` and `group`
+  simultaneously. (:issue:`3170`). By `Matthias Meyer <https://github.com/niowniow>`_.
 
 Documentation
 ~~~~~~~~~~~~~

xarray/backends/zarr.py

@@ -372,7 +372,7 @@ def store(
         if len(existing_variables) > 0:
             # there are variables to append
             # their encoding must be the same as in the store
-            ds = open_zarr(self.ds.store, chunks=None)
+            ds = open_zarr(self.ds.store, group=self.ds.path, chunks=None)
             variables_with_encoding = {}
             for vn in existing_variables:
                 variables_with_encoding[vn] = variables[vn].copy(deep=False)
@@ -486,7 +486,7 @@ def open_zarr(
         directory in file system where a Zarr DirectoryStore has been stored.
     synchronizer : object, optional
         Array synchronizer provided to zarr
-    group : str, obtional
+    group : str, optional
         Group path. (a.k.a. `path` in zarr terminology.)
     chunks : int or dict or tuple or {None, 'auto'}, optional
         Chunk sizes along each dimension, e.g., ``5`` or

xarray/tests/test_backends.py

@@ -1723,39 +1723,52 @@ def test_hidden_zarr_keys(self):
                 with xr.decode_cf(store):
                     pass
 
-    def test_write_persistence_modes(self):
+    @pytest.mark.parametrize("group", [None, "group1"])
+    def test_write_persistence_modes(self, group):
         original = create_test_data()
 
         # overwrite mode
-        with self.roundtrip(original, save_kwargs={"mode": "w"}) as actual:
+        with self.roundtrip(
+            original,
+            save_kwargs={"mode": "w", "group": group},
+            open_kwargs={"group": group},
+        ) as actual:
             assert_identical(original, actual)
 
         # don't overwrite mode
-        with self.roundtrip(original, save_kwargs={"mode": "w-"}) as actual:
+        with self.roundtrip(
+            original,
+            save_kwargs={"mode": "w-", "group": group},
+            open_kwargs={"group": group},
+        ) as actual:
             assert_identical(original, actual)
 
         # make sure overwriting works as expected
         with self.create_zarr_target() as store:
             self.save(original, store)
             # should overwrite with no error
-            self.save(original, store, mode="w")
-            with self.open(store) as actual:
+            self.save(original, store, mode="w", group=group)
+            with self.open(store, group=group) as actual:
                 assert_identical(original, actual)
                 with pytest.raises(ValueError):
                     self.save(original, store, mode="w-")
 
         # check append mode for normal write
-        with self.roundtrip(original, save_kwargs={"mode": "a"}) as actual:
+        with self.roundtrip(
+            original,
+            save_kwargs={"mode": "a", "group": group},
+            open_kwargs={"group": group},
+        ) as actual:
             assert_identical(original, actual)
 
-        ds, ds_to_append, _ = create_append_test_data()
-
         # check append mode for append write
+        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")
+            ds.to_zarr(store_target, mode="w", group=group)
+            ds_to_append.to_zarr(store_target, append_dim="time", group=group)
             original = xr.concat([ds, ds_to_append], dim="time")
-            assert_identical(original, xr.open_zarr(store_target))
+            actual = xr.open_zarr(store_target, group=group)
+            assert_identical(original, actual)
 
     def test_compressor_encoding(self):
         original = create_test_data()