From 1e7326cd8c8687666263cf997f6120b64f57251e Mon Sep 17 00:00:00 2001
From: devteamaegis <devteam.aegis@gmail.com>
Date: Wed, 3 Jun 2026 17:05:43 -0400
Subject: [PATCH] fix(rerankers/mrr): raise ValueError on empty vector_results
 list (#3469)

## What's broken

`MRRReranker.rerank_multivector([])` raises `IndexError: list index out
of range`. The crash happens on line 128 (the `all()` type-homogeneity
check passes vacuously on an empty iterable) and on line 134 which
accesses `vector_results[0]` unconditionally, with no prior guard for an
empty list.

## Why it happens

`all()` over an empty iterable returns `True`, so the type check
silently passes and execution falls through to `vector_results[0]` which
crashes.

## Fix

Added a two-line guard at the top of `rerank_multivector` that raises a
clear `ValueError("vector_results must not be empty")` before any
indexing occurs.

## Test

Added `test_mrr_reranker_empty_input` in `test_rerankers.py` which calls
`rerank_multivector([])` and asserts that a `ValueError` with the
message "must not be empty" is raised.

Fixes #3468

Co-authored-by: Aegis Dev <aegis@devteamaegis.com>
---
 python/python/lancedb/rerankers/mrr.py | 3 +++
 python/python/tests/test_rerankers.py  | 6 ++++++
 2 files changed, 9 insertions(+)

diff --git a/python/python/lancedb/rerankers/mrr.py b/python/python/lancedb/rerankers/mrr.py
index e2d5d1a97..af5ce778d 100644
--- a/python/python/lancedb/rerankers/mrr.py
+++ b/python/python/lancedb/rerankers/mrr.py
@@ -125,6 +125,9 @@ class MRRReranker(Reranker):
         This cannot reuse rerank_hybrid because MRR semantics require treating
         each vector result as a separate ranking system.
         """
+        if not vector_results:
+            raise ValueError("vector_results must not be empty")
+
         if not all(isinstance(v, type(vector_results[0])) for v in vector_results):
             raise ValueError(
                 "All elements in vector_results should be of the same type"
diff --git a/python/python/tests/test_rerankers.py b/python/python/tests/test_rerankers.py
index c886772bb..5ab24be5d 100644
--- a/python/python/tests/test_rerankers.py
+++ b/python/python/tests/test_rerankers.py
@@ -344,6 +344,12 @@ def test_mrr_reranker(tmp_path):
     assert len(result_deduped) == len(result)
 
 
+def test_mrr_reranker_empty_input():
+    reranker = MRRReranker()
+    with pytest.raises(ValueError, match="must not be empty"):
+        reranker.rerank_multivector([])
+
+
 def test_rrf_reranker_distance():
     data = pa.table(
         {