Ensure backwards compatibility (#2695)

* Ensure backwards compatibility

* Add test that detects backwards compatibility break

* Add test description

Co-authored-by: Davis Bennett <[email protected]>

* Add fix to release notes

* Minor improvement to release note

---------

Co-authored-by: Davis Bennett <[email protected]>
Co-authored-by: David Stansby <[email protected]>

Author: 3 people

docs/release-notes.rst

@@ -11,6 +11,8 @@ Bug fixes
 ~~~~~~~~~
 * Fixes ``order`` argument for Zarr format 2 arrays (:issue:`2679`).
 
+* Fixes a bug that prevented reading Zarr format 2 data with consolidated metadata written using ``zarr-python`` version 2 (:issue:`2694`).
+
 Behaviour changes
 ~~~~~~~~~~~~~~~~~
 

src/zarr/core/group.py

@@ -573,8 +573,8 @@ def _from_bytes_v2(
             v2_consolidated_metadata = json.loads(consolidated_metadata_bytes.to_bytes())
             v2_consolidated_metadata = v2_consolidated_metadata["metadata"]
             # We already read zattrs and zgroup. Should we ignore these?
-            v2_consolidated_metadata.pop(".zattrs")
-            v2_consolidated_metadata.pop(".zgroup")
+            v2_consolidated_metadata.pop(".zattrs", None)
+            v2_consolidated_metadata.pop(".zgroup", None)
 
             consolidated_metadata: defaultdict[str, dict[str, Any]] = defaultdict(dict)
 

tests/test_metadata/test_consolidated.py

@@ -17,7 +17,7 @@
     open,
     open_consolidated,
 )
-from zarr.core.buffer import default_buffer_prototype
+from zarr.core.buffer import cpu, default_buffer_prototype
 from zarr.core.group import ConsolidatedMetadata, GroupMetadata
 from zarr.core.metadata import ArrayV3Metadata
 from zarr.core.metadata.v2 import ArrayV2Metadata
@@ -476,6 +476,30 @@ async def test_open_consolidated_raises_async(self, zarr_format: ZarrFormat):
         with pytest.raises(ValueError):
             await zarr.api.asynchronous.open_consolidated(store, zarr_format=None)
 
+    @pytest.fixture
+    async def v2_consolidated_metadata_empty_dataset(
+        self, memory_store: zarr.storage.MemoryStore
+    ) -> AsyncGroup:
+        zgroup_bytes = cpu.Buffer.from_bytes(json.dumps({"zarr_format": 2}).encode())
+        zmetadata_bytes = cpu.Buffer.from_bytes(
+            b'{"metadata":{".zgroup":{"zarr_format":2}},"zarr_consolidated_format":1}'
+        )
+        return AsyncGroup._from_bytes_v2(
+            None, zgroup_bytes, zattrs_bytes=None, consolidated_metadata_bytes=zmetadata_bytes
+        )
+
+    async def test_consolidated_metadata_backwards_compatibility(
+        self, v2_consolidated_metadata_empty_dataset
+    ):
+        """
+        Test that consolidated metadata handles a missing .zattrs key. This is necessary for backwards compatibility  with zarr-python 2.x. See https://github.com/zarr-developers/zarr-python/issues/2694
+        """
+        store = zarr.storage.MemoryStore()
+        await zarr.api.asynchronous.open(store=store, zarr_format=2)
+        await zarr.api.asynchronous.consolidate_metadata(store)
+        result = await zarr.api.asynchronous.open_consolidated(store, zarr_format=2)
+        assert result.metadata == v2_consolidated_metadata_empty_dataset.metadata
+
     async def test_consolidated_metadata_v2(self):
         store = zarr.storage.MemoryStore()
         g = await AsyncGroup.from_store(store, attributes={"key": "root"}, zarr_format=2)