working dict serialization for _ImplicitGroupMetadata

Author: d-v-b

src/zarr/core/group.py

@@ -3258,12 +3258,12 @@ def _persist_metadata(
 
 class _ImplicitGroupMetadata(GroupMetadata):
     """
-    This class represents the metadata document of a group that should created at some
+    This class represents the metadata document of a group that should be created at a
     location in storage if and only if there is not already a group at that location.
 
     This class is used to fill group-shaped "holes" in a dict specification of a Zarr hierarchy.
 
-    When attempting to write this class to disk, the writer should first check if a Zarr group
+    When attempting to write this class to storage, the writer should first check if a Zarr group
     already exists at the desired location. If such a group does exist, the writer should do nothing.
     If not, the writer should write this metadata document to storage.
 
@@ -3283,6 +3283,9 @@ def __init__(
 
         super().__init__(attributes, zarr_format, consolidated_metadata)
 
+    def to_dict(self) -> dict[str, JSON]:
+        return asdict(self)
+
 
 async def _from_flat(
     store: StoreLike,

tests/test_group.py

@@ -25,6 +25,7 @@
     ConsolidatedMetadata,
     GroupMetadata,
     _from_flat,
+    _ImplicitGroupMetadata,
     create_hierarchy,
     create_nodes,
 )
@@ -1609,6 +1610,28 @@ async def test_group_from_flat(store: Store, zarr_format, root_key: str):
     assert members_observed_meta == members_expected_meta_relative
 
 
+@pytest.mark.parametrize("store", ["memory"], indirect=True)
+@pytest.mark.parametrize("zarr_format", [2, 3])
+async def test_create_hierarchy_implicit_groups(store: Store, zarr_format):
+    """
+    Test that writing a hierarchy with implicit groups does not result in altering an existing group
+    """
+    spath = await make_store_path(store, path="")
+    key = "a"
+    attrs = {"name": key}
+    _ = await _from_flat(
+        store,
+        nodes={
+            key: GroupMetadata(zarr_format=zarr_format, attributes=attrs),
+        },
+    )
+
+    _ = await _collect_aiterator(
+        create_nodes(store_path=spath, nodes={key: _ImplicitGroupMetadata(zarr_format=zarr_format)})
+    )
+    assert zarr.open_group(store, path=key).metadata.attributes == attrs
+
+
 @pytest.mark.parametrize("store", ["memory"], indirect=True)
 def test_group_members_performance(store: Store) -> None:
     """