fix: AnswerJoiner no longer crashes when sorting answers with a None score (#11701)

Author: hoangsonww

haystack/components/joiners/answer_joiner.py

@@ -130,7 +130,9 @@ def run(self, answers: Variadic[list[AnswerType]], top_k: int | None = None) ->
 
         if self.sort_by_score:
             output_answers = sorted(
-                output_answers, key=lambda answer: answer.score if hasattr(answer, "score") else -inf, reverse=True
+                output_answers,
+                key=lambda answer: score if (score := getattr(answer, "score", None)) is not None else -inf,
+                reverse=True,
             )
 
         top_k = top_k or self.top_k

releasenotes/notes/fix-answerjoiner-sort-by-score-none-30d33e2bc93fd668.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed a ``TypeError`` in ``AnswerJoiner`` when running with ``sort_by_score=True`` and at least one
+    answer had a ``score`` of ``None``. As documented, an answer without a score is now treated as if its
+    score were negative infinity and sorted last, instead of raising
+    ``TypeError: '<' not supported between instances of 'float' and 'NoneType'``.

test/components/joiners/test_answer_joiner.py

@@ -100,3 +100,31 @@ def test_unsupported_join_mode(self):
         unsupported_mode = "unsupported_mode"
         with pytest.raises(ValueError):
             AnswerJoiner(join_mode=unsupported_mode)
+
+    def test_sort_by_score(self):
+        joiner = AnswerJoiner(sort_by_score=True)
+        answers1 = [ExtractedAnswer(query="a", score=0.3, meta={}, document=Document(content="a"))]
+        answers2 = [ExtractedAnswer(query="b", score=0.9, meta={}, document=Document(content="b"))]
+        result = joiner.run([answers1, answers2])
+        scores = [answer.score for answer in result["answers"]]
+        assert scores == [0.9, 0.3]
+
+    def test_sort_by_score_with_none_score(self):
+        # The docstring promises that an answer with no score is handled as if its score is -infinity.
+        # ExtractedAnswer with score=None must not raise a TypeError during sorting and must be sorted last.
+        joiner = AnswerJoiner(sort_by_score=True)
+        answers1 = [ExtractedAnswer(query="a", score=0.5, meta={}, document=Document(content="a"))]
+        answers2 = [ExtractedAnswer(query="b", score=None, meta={}, document=Document(content="b"))]  # type: ignore[arg-type]
+        result = joiner.run([answers1, answers2])
+        assert [answer.data for answer in result["answers"]] == [None, None]
+        assert [answer.score for answer in result["answers"]] == [0.5, None]
+
+    def test_sort_by_score_with_answers_missing_score_attribute(self):
+        # GeneratedAnswer has no score attribute at all; it must be handled as -infinity and sorted last.
+        joiner = AnswerJoiner(sort_by_score=True)
+        answers1 = [GeneratedAnswer(query="a", data="a", meta={}, documents=[Document(content="a")])]
+        answers2 = [ExtractedAnswer(query="b", score=0.9, meta={}, document=Document(content="b"))]
+        result = joiner.run([answers1, answers2])
+        # The ExtractedAnswer (score 0.9) comes first, the GeneratedAnswer (no score) comes last.
+        assert isinstance(result["answers"][0], ExtractedAnswer)
+        assert isinstance(result["answers"][1], GeneratedAnswer)