fix: raise a clear error for zero-sum weights in DocumentJoiner (#11629)

Co-authored-by: bogdankostic <bogdankostic@web.de>

Author: i-anubhav-anand

haystack/components/joiners/document_joiner.py

@@ -123,7 +123,13 @@ def __init__(
         }
         self.join_mode_function = join_mode_functions[join_mode]
         self.join_mode = join_mode
-        self.weights = [float(i) / sum(weights) for i in weights] if weights else None
+        if weights:
+            weight_sum = sum(weights)
+            if weight_sum == 0:
+                raise ValueError("The provided `weights` must not sum to zero.")
+            self.weights: list[float] | None = [float(i) / weight_sum for i in weights]
+        else:
+            self.weights = None
         self.top_k = top_k
         self.sort_by_score = sort_by_score
 

releasenotes/notes/document-joiner-zero-sum-weights-263b18a84420f1cb.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    ``DocumentJoiner`` now raises a clear ``ValueError`` when the provided ``weights``
+    sum to zero, instead of failing with an opaque ``ZeroDivisionError`` while
+    normalizing them.

test/components/joiners/test_document_joiner.py

@@ -26,6 +26,11 @@ def test_init_with_custom_parameters(self):
         assert joiner.top_k == 5
         assert not joiner.sort_by_score
 
+    def test_init_with_zero_sum_weights_raises(self):
+        # weights that sum to zero would divide by zero during normalization
+        with pytest.raises(ValueError, match="must not sum to zero"):
+            DocumentJoiner(join_mode="merge", weights=[0.0, 0.0, 0.0])
+
     def test_to_dict(self):
         joiner = DocumentJoiner()
         data = joiner.to_dict()