Standardize behaviour: error if directory doesn't exist, error if no …

…objects passed

anndata/experimental/merge.py

@@ -527,7 +527,7 @@ def concat_on_disk(
     Name: count, dtype: int64
     """
     if len(in_files) == 0:
-        return
+        raise ValueError("No objects to concatenate.")
 
     # Argument normalization
     if pairwise:
@@ -537,8 +537,11 @@ def concat_on_disk(
 
     merge = resolve_merge_strategy(merge)
     uns_merge = resolve_merge_strategy(uns_merge)
+
     out_file = Path(out_file)
-    out_file.parent.mkdir(parents=True, exist_ok=True)
+    if not out_file.parent.exists():
+        raise FileNotFoundError(f"Parent directory of {out_file} does not exist.")
+
     if isinstance(in_files, Mapping):
         if keys is not None:
             raise TypeError(

anndata/tests/test_concatenate_disk.py

@@ -250,3 +250,18 @@ def gen_index(n):
 
 def test_concatenate_obsm_inner(obsm_adatas, tmp_path, file_format):
     assert_eq_concat_on_disk(obsm_adatas, tmp_path, file_format, join="inner")
+
+
+def test_output_dir_exists(tmp_path):
+    in_pth = tmp_path / "in.h5ad"
+    out_pth = tmp_path / "does_not_exist" / "out.h5ad"
+
+    AnnData(X=np.ones((5, 1))).write_h5ad(in_pth)
+
+    with pytest.raises(FileNotFoundError, match=f"{out_pth}"):
+        concat_on_disk([in_pth], out_pth)
+
+
+def test_failure_w_no_args(tmp_path):
+    with pytest.raises(ValueError, match="No objects to concatenate"):
+        concat_on_disk([], tmp_path / "out.h5ad")