Fix empty assignment-index backend handling

src/pyrecest/_backend/__init__.py

@@ -349,6 +349,30 @@ def mean(a, axis=None, dtype=None, out=None, keepdims=False):
     return mean
 
 
+def _is_empty_assignment_index(indices):
+    """Return whether ``indices`` selects no elements for assignment helpers."""
+    if isinstance(indices, list):
+        return len(indices) == 0
+    if isinstance(indices, tuple):
+        return False
+
+    ndim = getattr(indices, "ndim", None)
+    shape = getattr(indices, "shape", None)
+    return ndim is not None and ndim > 0 and shape is not None and shape[0] == 0
+
+
+def _assignment_with_empty_indices_noop(assignment_func, copy_func):
+    """Return an assignment wrapper that treats empty indices as a no-op."""
+
+    @wraps(assignment_func)
+    def assignment(x, values, indices, axis=0):
+        if _is_empty_assignment_index(indices):
+            return copy_func(x)
+        return assignment_func(x, values, indices, axis=axis)
+
+    return assignment
+
+
 class BackendImporter(importlib.abc.MetaPathFinder, importlib.abc.Loader):
     """
     Meta path finder and loader for dynamically creating backend modules.
@@ -442,6 +466,14 @@ def _create_backend_module(self, backend_name: str):
                             attribute,
                             getattr(backend, "asarray"),
                         )
+                    if (
+                        module_name == ""
+                        and attribute_name in {"assignment", "assignment_by_sum"}
+                    ):
+                        attribute = _assignment_with_empty_indices_noop(
+                            attribute,
+                            getattr(backend, "copy"),
+                        )
                     setattr(new_submodule, attribute_name, attribute)
 
             for attribute_name in OPTIONAL_BACKEND_ATTRIBUTES.get(module_name, []):

tests/test_backend_contract.py

@@ -27,6 +27,15 @@ def test_convert_to_wider_dtype_preserves_matching_boolean_dtype(self):
         self.assertEqual(to_numpy(first).dtype, np.dtype("bool"))
         self.assertEqual(to_numpy(second).dtype, np.dtype("bool"))
 
+    def test_assignment_with_empty_indices_is_a_noop(self):
+        original = array([1.0, 2.0, 3.0])
+
+        assigned = backend.assignment(original, 99.0, [])
+        added = backend.assignment_by_sum(original, 99.0, [])
+
+        npt.assert_allclose(to_numpy(assigned), [1.0, 2.0, 3.0])
+        npt.assert_allclose(to_numpy(added), [1.0, 2.0, 3.0])
+
     def test_choice_supports_numpy_like_size_replace_and_probabilities(self):
         values = array([0, 1, 2, 3])
         weights = array([0.1, 0.2, 0.3, 0.4])