fix group overwrite -> mode conversion, and add locally-passing-but-globally-failing tests

Author: d-v-b

src/zarr/api/asynchronous.py

@@ -677,7 +677,7 @@ async def group(
     if overwrite:
         mode = "w"
     else:
-        mode = "a"
+        mode = "w-"
     store_path = await make_store_path(store, path=path, mode=mode, storage_options=storage_options)
 
     if chunk_store is not None:

tests/test_api/test_asynchronous.py

@@ -116,3 +116,15 @@ async def test_open_group_new_path(tmp_path: Path) -> None:
     # tmp_path exists, but tmp_path / "test.zarr" will not, which is important for this test
     store = tmp_path / "test.zarr"
     await group(store=store)
+
+
+async def test_open_group_old_path(tmp_path: Path) -> None:
+    """
+    Test that zarr.api.asynchronous.group will not overwrite an existing file.
+
+    See https://github.com/zarr-developers/zarr-python/issues/3406
+    """
+    store = tmp_path
+    await group(store=store, overwrite=True, attributes={"existing_group": True})
+    with pytest.raises(FileExistsError):
+        await group(store=store)