(fix): subsetting with empty list

anndata/_core/index.py

@@ -80,7 +80,16 @@ def name_idx(i):
                 indexer = indexer.toarray()
             indexer = np.ravel(indexer)
         if not isinstance(indexer, (np.ndarray, pd.Index)):
-            indexer = np.array(indexer)
+            dtype = "int"
+            if (
+                all(isinstance(x, str) for x in indexer) and len(indexer) > 0
+            ):  # if not all, but any, then dtype=int will cause an error
+                dtype = "object"
+            elif (
+                any(isinstance(x, str) for x in indexer) and len(indexer) > 0
+            ):  # if not all, but any, there must be more than one type.
+                raise ValueError("Mixed type list indexers not supported.")
+            indexer = np.array(indexer, dtype=dtype)
         if issubclass(indexer.dtype.type, (np.integer, np.floating)):
             return indexer  # Might not work for range indexes
         elif issubclass(indexer.dtype.type, np.bool_):

anndata/tests/test_views.py

@@ -693,3 +693,18 @@ def test_x_none():
     new = view.copy()
     assert new.shape == (2, 0)
     assert new.obs_names.tolist() == ["2", "3"]
+
+
+def test_empty_list_subset():
+    orig = gen_adata((10, 10))
+    subset = orig[:, []]
+    assert subset.X.shape == (10, 0)
+    assert subset.obsm["sparse"].shape == (10, 100)
+    assert subset.varm["sparse"].shape == (0, 100)
+
+
+def test_mixed_subset():
+    orig = gen_adata((10, 10))
+    with pytest.raises(ValueError) as exc:
+        orig[:, [1, "cell_c"]].X
+    assert exc.value.args[0] == "Mixed type list indexers not supported."