Log and emit a warning when merged workflow outputs have name collisions

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Log and emit a warning when merged workflow outputs have name collisions.
+
 ## [5.1.0rc4] - 2026-09-14
 
 ### Added

src/ewokscore/graph/graph_io.py

@@ -1,9 +1,11 @@
 import logging
+import warnings
 from typing import Dict
 from typing import Iterator
 from typing import List
 from typing import Mapping
 from typing import Optional
+from typing import Set
 from typing import Union
 
 import networkx
@@ -234,6 +236,18 @@ def add_output_values(
     task_output_values = extract_output_values(node_id, task_or_outputs, outputs)
     if task_output_values is not None:
         if merge_outputs:
+            collisions = output_values.keys() & task_output_values.keys()
+            if collisions:
+                _warn_output_collisions(node_id, collisions)
             output_values.update(task_output_values)
         else:
             output_values[node_id] = task_output_values
+
+
+def _warn_output_collisions(node_id: NodeIdType, names: Set[Union[str, int]]) -> None:
+    message = (
+        f"Merged workflow outputs {sorted(names, key=str)} are overwritten by node {node_id!r}. "
+        "Rename them with 'new_name' in the 'outputs' items or use 'merge_outputs=False'."
+    )
+    logger.warning(message)
+    warnings.warn(message, UserWarning, stacklevel=3)

src/ewokscore/tests/test_execute_graph.py

@@ -1,3 +1,4 @@
+import pytest
 from ewoksutils.import_utils import qualname
 
 from ..bindings import execute_graph
@@ -123,7 +124,8 @@ def test_execute_graph_outputs():
     assert results == expected
 
     # Merge the results of all tasks
-    results = execute_graph(create_graph(), outputs=[{"all": True}])
+    with pytest.warns(UserWarning, match="are overwritten by node"):
+        results = execute_graph(create_graph(), outputs=[{"all": True}])
     expected = {"inputs": {"a": 10, "b": 6}, "result": 16, "label": "task6"}
     assert results == expected
 

src/ewokscore/tests/test_graph_io.py

@@ -189,3 +189,27 @@ def test_parse_outputs(graph):
     expected = [{"id": "task4"}]
     outputs = graph_io.parse_outputs(graph, outputs)
     assert outputs == expected
+
+
+def test_add_output_values_collision(caplog):
+    outputs = [{"id": "task1"}, {"id": "task2"}]
+    output_values = dict()
+    graph_io.add_output_values(output_values, "task1", {"a": 1, "b": 2}, outputs)
+
+    with pytest.warns(UserWarning, match=r"\['a'\] are overwritten by node 'task2'"):
+        graph_io.add_output_values(output_values, "task2", {"a": 3, "c": 4}, outputs)
+
+    assert output_values == {"a": 3, "b": 2, "c": 4}
+    assert "are overwritten by node 'task2'" in caplog.text
+
+
+def test_add_output_values_no_collision(recwarn):
+    outputs = [{"id": "task1"}, {"id": "task2"}]
+    output_values = dict()
+    graph_io.add_output_values(output_values, "task1", {"a": 1}, outputs)
+    graph_io.add_output_values(
+        output_values, "task2", {"a": 3}, outputs, merge_outputs=False
+    )
+    graph_io.add_output_values(output_values, "task3", {"a": 5}, outputs)
+
+    assert not recwarn.list