fix(indexing): treat an empty list as an empty integer selection

changes/4518.bugfix.md

@@ -0,0 +1 @@
+`arr[[]]`, and the same empty-list selection inside a tuple, an `oindex` selection or a `vindex` selection, used to raise `IndexError: unsupported selection item for basic indexing`, even though `IntSequence`, the declared selection type, includes `list[int]` and the same selection written as an empty integer array already worked. An empty list is now treated as an empty integer selection, matching NumPy, so `arr[[]]` returns an empty array instead of raising.

src/zarr/core/indexing.py

@@ -231,8 +231,8 @@ def is_bool(x: Any) -> TypeGuard[bool | np.bool_]:
 
 
 def is_integer_list(x: Any) -> TypeGuard[list[int]]:
-    """True if x is a list of integers."""
-    return isinstance(x, list) and len(x) > 0 and all(is_integer(i) for i in x)
+    """True if x is a list of integers (including an empty list, as in NumPy)."""
+    return isinstance(x, list) and all(is_integer(i) for i in x)
 
 
 def is_bool_list(x: Any) -> TypeGuard[list[bool | np.bool_]]:
@@ -510,8 +510,11 @@ def replace_ellipsis(selection: Any, shape: tuple[int, ...]) -> SelectionNormali
 
 
 def replace_lists(selection: SelectionNormalized) -> SelectionNormalized:
+    # An empty list has no dtype to infer; NumPy indexes with it as an empty integer array.
     return tuple(
-        cast("ArrayOfIntOrBool", np.asarray(dim_sel)) if isinstance(dim_sel, list) else dim_sel
+        cast("ArrayOfIntOrBool", np.asarray(dim_sel, dtype=np.intp if len(dim_sel) == 0 else None))
+        if isinstance(dim_sel, list)
+        else dim_sel
         for dim_sel in selection
     )
 

tests/test_indexing.py

@@ -428,6 +428,47 @@ def test_get_basic_selection_2d_rejects_list_in_tuple(store: StorePath) -> None:
         z.get_basic_selection((slice(None), [0, 1]))
 
 
+@pytest.mark.parametrize("zarr_format", [2, 3])
+def test_empty_list_selection_matches_empty_array_and_numpy(zarr_format: ZarrFormat) -> None:
+    """An empty list is an empty integer selection, as it is in NumPy.
+
+    IntSequence, the declared selection type, includes list[int], so an empty list
+    has to behave like the equivalent empty integer array instead of being rejected
+    by basic indexing.
+    """
+    expected = np.arange(16, dtype="i4").reshape(4, 4)
+    arr = zarr.create_array({}, data=expected, chunks=(2, 2), zarr_format=zarr_format)
+    empty = np.array([], dtype=np.intp)
+
+    # Short form, orthogonal form and vectorized form all agree with NumPy.
+    assert_array_equal(arr[[]], expected[[]])
+    assert_array_equal(arr[[], :], expected[[], :])
+    assert_array_equal(arr[:, []], expected[:, []])
+    assert_array_equal(arr[[], []], expected[[], []])
+    assert_array_equal(arr[[], 0], expected[[], 0])
+    assert_array_equal(arr[0, []], expected[0, []])
+    assert_array_equal(arr.oindex[[]], expected[[]])
+    assert_array_equal(arr.oindex[[], :], expected[[], :])
+    # The equivalent empty array selection was already accepted.
+    assert_array_equal(arr[empty], expected[empty])
+    assert_array_equal(arr[empty, :], expected[empty, :])
+
+    # A bare list on a 1D array routes through vectorized indexing.
+    expected_1d = np.arange(4, dtype="i4")
+    arr_1d = zarr.create_array({}, data=expected_1d, chunks=(2,), zarr_format=zarr_format)
+    assert_array_equal(arr_1d[[]], expected_1d[[]])
+    assert_array_equal(arr_1d.vindex[[]], expected_1d[[]])
+
+    # Non-empty integer lists and boolean lists are unaffected.
+    assert_array_equal(arr[[0]], expected[[0]])
+    assert_array_equal(arr[[True, False, True, False]], expected[[True, False, True, False]])
+
+    # Writing through an empty selection is a no-op.
+    arr[[], :] = 1
+    arr[:, []] = 1
+    assert_array_equal(arr[:], expected)
+
+
 def test_fancy_indexing_fallback_on_get_setitem(store: StorePath) -> None:
     """Paired integer-list indexing falls back to vectorized (fancy) get and set via `__getitem__`/`__setitem__`."""
     z = zarr_array_from_numpy_array(store, np.zeros((20, 20)))