Fix open(..., mode='w') to create a group (#2629)

* Add test for #2490

* Add 'w' to list of valid modes in open to create a group

---------

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

Author: jni

src/zarr/api/asynchronous.py

@@ -312,7 +312,7 @@ async def open(
     store_path = await make_store_path(store, mode=mode, path=path, storage_options=storage_options)
 
     # TODO: the mode check below seems wrong!
-    if "shape" not in kwargs and mode in {"a", "r", "r+"}:
+    if "shape" not in kwargs and mode in {"a", "r", "r+", "w"}:
         try:
             metadata_dict = await get_array_metadata(store_path, zarr_format=zarr_format)
             # TODO: remove this cast when we fix typing for array metadata dicts

tests/test_api.py

@@ -1086,6 +1086,13 @@ async def test_open_falls_back_to_open_group_async() -> None:
     assert group.attrs == {"key": "value"}
 
 
+def test_open_mode_write_creates_group(tmp_path: pathlib.Path) -> None:
+    # https://github.com/zarr-developers/zarr-python/issues/2490
+    zarr_dir = tmp_path / "test.zarr"
+    group = zarr.open(zarr_dir, mode="w")
+    assert isinstance(group, Group)
+
+
 async def test_metadata_validation_error() -> None:
     with pytest.raises(
         MetadataValidationError,