zarr.open should fall back to opening a group

src/zarr/api/asynchronous.py

@@ -14,6 +14,7 @@
 from zarr.core.group import AsyncGroup
 from zarr.core.metadata.v2 import ArrayV2Metadata
 from zarr.core.metadata.v3 import ArrayV3Metadata
+from zarr.errors import NodeTypeValidationError
 from zarr.storage import (
     StoreLike,
     StorePath,
@@ -247,7 +248,10 @@ async def open(
 
     try:
         return await open_array(store=store_path, zarr_format=zarr_format, **kwargs)
-    except KeyError:
+    except (KeyError, NodeTypeValidationError):
+        # KeyError for a missing key
+        # NodeTypeValidationError for failing to parse node metadata as an array when it's
+        # actually a group
         return await open_group(store=store_path, zarr_format=zarr_format, **kwargs)
 
 
@@ -580,6 +584,8 @@ async def open_group(
     meta_array : array-like, optional
         An array instance to use for determining arrays to create and return
         to users. Use `numpy.empty(())` by default.
+    attributes : dict
+        A dictionary of JSON-serializable values with user-defined attributes.
 
     Returns
     -------

src/zarr/api/synchronous.py

@@ -207,6 +207,7 @@ def open_group(
     zarr_version: ZarrFormat | None = None,  # deprecated
     zarr_format: ZarrFormat | None = None,
     meta_array: Any | None = None,  # not used in async api
+    attributes: dict[str, JSON] | None = None,
 ) -> Group:
     return Group(
         sync(
@@ -221,6 +222,7 @@ def open_group(
                 zarr_version=zarr_version,
                 zarr_format=zarr_format,
                 meta_array=meta_array,
+                attributes=attributes,
             )
         )
     )

src/zarr/core/metadata/v3.py

@@ -29,6 +29,7 @@
 from zarr.core.config import config
 from zarr.core.metadata.common import ArrayMetadata, parse_attributes
 from zarr.core.strings import _STRING_DTYPE as STRING_NP_DTYPE
+from zarr.errors import MetadataValidationError, NodeTypeValidationError
 from zarr.registry import get_codec_class
 
 DEFAULT_DTYPE = "float64"
@@ -37,13 +38,13 @@
 def parse_zarr_format(data: object) -> Literal[3]:
     if data == 3:
         return 3
-    raise ValueError(f"Invalid value. Expected 3. Got {data}.")
+    raise MetadataValidationError(3, data)
 
 
 def parse_node_type_array(data: object) -> Literal["array"]:
     if data == "array":
         return "array"
-    raise ValueError(f"Invalid value. Expected 'array'. Got {data}.")
+    raise NodeTypeValidationError("array", data)
 
 
 def parse_codecs(data: object) -> tuple[Codec, ...]:

src/zarr/errors.py

@@ -25,6 +25,23 @@ class ContainsArrayAndGroupError(_BaseZarrError):
     )
 
 
+class MetadataValidationError(_BaseZarrError):
+    """An exception raised when the Zarr metadata is invalid in some way"""
+
+    _msg = "Invalid value. Expected '{}'. Got '{}'."
+
+
+class NodeTypeValidationError(MetadataValidationError):
+    """
+    Specialized exception when the node_type of the metadata document is incorrect..
+
+    This can be raised when the value is invalid or unexpected given the context,
+    for example an 'array' node when we expected a 'group'.
+    """
+
+    _msg = "Invalid value. Expected '{}'. Got '{}'."
+
+
 __all__ = [
     "ContainsArrayAndGroupError",
     "ContainsArrayError",

tests/v3/test_api.py

@@ -6,6 +6,8 @@
 from numpy.testing import assert_array_equal
 
 import zarr
+import zarr.api.asynchronous
+import zarr.core.group
 from zarr import Array, Group
 from zarr.abc.store import Store
 from zarr.api.synchronous import create, group, load, open, open_group, save, save_array, save_group
@@ -921,3 +923,23 @@ def test_open_group_positional_args_deprecated() -> None:
     store = MemoryStore({}, mode="w")
     with pytest.warns(FutureWarning, match="pass"):
         open_group(store, "w")
+
+
+def test_open_falls_back_to_open_group() -> None:
+    # https://github.com/zarr-developers/zarr-python/issues/2309
+    store = MemoryStore(mode="w")
+    zarr.open_group(store, attributes={"key": "value"})
+
+    group = zarr.open(store)
+    assert isinstance(group, Group)
+    assert group.attrs == {"key": "value"}
+
+
+async def test_open_falls_back_to_open_group_async() -> None:
+    # https://github.com/zarr-developers/zarr-python/issues/2309
+    store = MemoryStore(mode="w")
+    await zarr.api.asynchronous.open_group(store, attributes={"key": "value"})
+
+    group = await zarr.api.asynchronous.open(store=store)
+    assert isinstance(group, zarr.core.group.AsyncGroup)
+    assert group.attrs == {"key": "value"}

tests/v3/test_metadata/test_v3.py

@@ -24,6 +24,7 @@
     default_fill_value,
     parse_dimension_names,
     parse_fill_value,
+    parse_node_type_array,
     parse_zarr_format,
 )
 
@@ -54,14 +55,24 @@
 
 @pytest.mark.parametrize("data", [None, 1, 2, 4, 5, "3"])
 def test_parse_zarr_format_invalid(data: Any) -> None:
-    with pytest.raises(ValueError, match=f"Invalid value. Expected 3. Got {data}"):
+    with pytest.raises(ValueError, match=f"Invalid value. Expected '3'. Got '{data}'."):
         parse_zarr_format(data)
 
 
 def test_parse_zarr_format_valid() -> None:
     assert parse_zarr_format(3) == 3
 
 
+@pytest.mark.parametrize("data", [None, "group"])
+def test_parse_node_type_arrayinvalid(data: Any) -> None:
+    with pytest.raises(ValueError, match=f"Invalid value. Expected 'array'. Got '{data}'."):
+        parse_node_type_array(data)
+
+
+def test_parse_node_typevalid() -> None:
+    assert parse_node_type_array("array") == "array"
+
+
 @pytest.mark.parametrize("data", [(), [1, 2, "a"], {"foo": 10}])
 def parse_dimension_names_invalid(data: Any) -> None:
     with pytest.raises(TypeError, match="Expected either None or iterable of str,"):