fix: avoid in-place mutation of ExtractedAnswer in ExtractiveReader

haystack/components/readers/extractive.py

@@ -323,7 +323,7 @@ def _postprocess(
 
     def _add_answer_page_number(self, answer: ExtractedAnswer) -> ExtractedAnswer:
         if answer.meta is None:
-            answer.meta = {}
+            answer = replace(answer, meta={})
 
         if answer.document_offset is None:
             return answer

releasenotes/notes/fix-extractive-reader-inplace-mutation-53436ea625324943.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed in-place mutation of ``ExtractedAnswer.meta`` in
+    ``ExtractiveReader._add_answer_page_number`` when the answer's ``meta``
+    was ``None``. Now uses ``dataclasses.replace`` to avoid triggering the
+    dataclass mutation warning.

test/components/readers/test_extractive.py

@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 import logging
+import warnings
 from math import ceil, exp
 from unittest.mock import Mock, patch
 
@@ -486,6 +487,25 @@ def test_add_answer_page_number_returns_same_answer(mock_reader: ExtractiveReade
         assert "page_number must be int" in caplog.text
 
 
+def test_add_answer_page_number_with_none_meta(mock_reader: ExtractiveReader):
+    # when answer.meta is None, it should be initialized to {} without triggering a mutation warning
+    document = Document(content="I thought a lot about this. The answer is 42.", meta={"page_number": 5})
+    answer = ExtractedAnswer(
+        data="42",
+        query="What is the answer?",
+        document=document,
+        score=1.0,
+        document_offset=ExtractedAnswer.Span(42, 44),
+        meta=None,
+    )
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        result = mock_reader._add_answer_page_number(answer=answer)
+    assert result.meta is not None
+    assert "answer_page_number" in result.meta
+
+
 def test_add_answer_page_number_with_form_feed(mock_reader: ExtractiveReader):
     document = Document(
         content="I thought a lot about this. \f And this document is long. \f The answer is 42.",