Raise helpful errors when opening a group with wrong node type. (#3444)

* Raise helpful errors when opening a group with wrong node type.

* Add changelog

* Use custom messages

---------

Co-authored-by: Davis Bennett <davis.v.bennett@gmail.com>

Author: dstansby

changes/3444.feature.rst

@@ -0,0 +1 @@
+Trying to open a group at a path were a array already exists now raises a helpful error.

src/zarr/core/group.py

@@ -53,6 +53,7 @@
 from zarr.errors import (
     ContainsArrayError,
     ContainsGroupError,
+    GroupNotFoundError,
     MetadataValidationError,
     ZarrDeprecationWarning,
     ZarrUserWarning,
@@ -673,6 +674,13 @@ def from_dict(
         store_path: StorePath,
         data: dict[str, Any],
     ) -> AsyncGroup:
+        node_type = data.pop("node_type", None)
+        if node_type == "array":
+            msg = f"An array already exists in store {store_path.store} at path {store_path.path}."
+            raise ContainsArrayError(msg)
+        elif node_type not in ("group", None):
+            msg = f"Node type in metadata ({node_type}) is not 'group'"
+            raise GroupNotFoundError(msg)
         return cls(
             metadata=GroupMetadata.from_dict(data),
             store_path=store_path,

tests/test_group.py

@@ -2234,3 +2234,9 @@ def test_get_roots(roots: tuple[str, ...]):
     }
     data = root_nodes | child_nodes
     assert set(_get_roots(data)) == set(roots)
+
+
+def test_open_array_as_group():
+    z = zarr.create_array(shape=(40, 50), chunks=(10, 10), dtype="f8", store={})
+    with pytest.raises(ContainsArrayError):
+        zarr.open_group(z.store)