Default zarr.open to open_group if shape is not provided

src/zarr/api/asynchronous.py

@@ -225,6 +225,11 @@ async def open(
     if path is not None:
         store_path = store_path / path
 
+    if "shape" not in kwargs and mode in {"a", "w", "w-"}:
+        try:
+            return await open_group(store=store_path, zarr_format=zarr_format, mode=mode, **kwargs)
+        except AssertionError:
+            return await open_array(store=store_path, zarr_format=zarr_format, mode=mode, **kwargs)
     try:
         return await open_array(store=store_path, zarr_format=zarr_format, mode=mode, **kwargs)
     except KeyError:

tests/v3/test_api.py

@@ -116,7 +116,12 @@ def test_open_with_mode_r_plus(tmp_path: pathlib.Path) -> None:
     z2[:] = 3
 
 
-def test_open_with_mode_a(tmp_path: pathlib.Path) -> None:
+async def test_open_with_mode_a(tmp_path: pathlib.Path) -> None:
+    # Open without shape argument should default to group
+    g = zarr.open(store=tmp_path, mode="a")
+    assert isinstance(g, Group)
+    await g.store_path.delete()
+
     # 'a' means read/write (create if doesn't exist)
     arr = zarr.open(store=tmp_path, mode="a", shape=(3, 3))
     assert isinstance(arr, Array)