Reject explicit empty objectives in batch scoring

pyrit/score/scorer.py

@@ -413,11 +413,10 @@ async def score_prompts_batch_async(
             ValueError: If objectives is empty or if the number of objectives doesn't match
                 the number of messages.
         """
-        if not objectives:
+        if objectives is None:
             objectives = [""] * len(messages)
-
         elif len(objectives) != len(messages):
-            raise ValueError("The number of tasks must match the number of messages.")
+            raise ValueError("The number of objectives must match the number of messages.")
 
         if len(messages) == 0:
             return []
@@ -456,7 +455,7 @@ async def score_image_batch_async(
         Raises:
             ValueError: If the number of objectives does not match the number of image_paths.
         """
-        if objectives and len(objectives) != len(image_paths):
+        if objectives is not None and len(objectives) != len(image_paths):
             raise ValueError("The number of objectives must match the number of image_paths.")
 
         if len(image_paths) == 0:
@@ -465,10 +464,10 @@ async def score_image_batch_async(
         prompt_target = getattr(self, "_prompt_target", None)
         results = await batch_task_async(
             task_func=self.score_image_async,
-            task_arguments=["image_path", "objective"] if objectives else ["image_path"],
+            task_arguments=["image_path", "objective"] if objectives is not None else ["image_path"],
             prompt_target=prompt_target,
             batch_size=batch_size,
-            items_to_batch=[image_paths, objectives] if objectives else [image_paths],
+            items_to_batch=[image_paths, objectives] if objectives is not None else [image_paths],
         )
 
         return [score for sublist in results for score in sublist]

tests/unit/score/test_scorer.py

@@ -491,6 +491,25 @@ async def test_scorer_score_responses_batch_async(patch_central_database):
         assert len(fake_scores) == 2
 
 
+@pytest.mark.asyncio
+async def test_score_prompts_batch_async_rejects_explicit_empty_objectives():
+    """Test explicit empty objectives are rejected for non-empty message batches."""
+    scorer = MockScorer()
+    message = MessagePiece(role="user", original_value="Hello user", sequence=1).to_message()
+
+    with pytest.raises(ValueError, match="objectives"):
+        await scorer.score_prompts_batch_async(messages=[message], objectives=[])
+
+
+@pytest.mark.asyncio
+async def test_score_image_batch_async_rejects_explicit_empty_objectives():
+    """Test explicit empty objectives are rejected for non-empty image batches."""
+    scorer = MockScorer()
+
+    with pytest.raises(ValueError, match="objectives"):
+        await scorer.score_image_batch_async(image_paths=["test_image.png"], objectives=[])
+
+
 @pytest.mark.asyncio
 async def test_score_response_async_empty_scorers():
     """Test that score_response_async returns empty list when no scorers provided."""