fix: handle empty completions in parse_answer

tests/test_parser.py

@@ -45,6 +45,17 @@ def test_parse_answer_with_completion(self, basic_parser):
         result = basic_parser.parse_answer(completion)
         assert result == "The answer is 4"
 
+    def test_parse_answer_with_empty_completion(self, basic_parser):
+        """Test parse_answer with empty completion list."""
+        result = basic_parser.parse_answer([])
+        assert result is None
+
+    def test_parse_answer_with_no_assistant_messages(self, basic_parser):
+        """Test parse_answer with completion containing no assistant messages."""
+        completion = [{"role": "user", "content": "Hello"}]
+        result = basic_parser.parse_answer(completion)
+        assert result is None
+
     def test_get_format_reward_func(self, basic_parser):
         """Test that format reward function returns 1.0 by default."""
         reward_func = basic_parser.get_format_reward_func()

verifiers/parsers/parser.py

@@ -42,7 +42,10 @@ def parse_answer(self, completion: Messages) -> str | None:
         if isinstance(completion, str):
             return self.parse(completion)
         else:
-            ans = str(self.get_assistant_messages(completion)[-1].get("content", ""))
+            assistant_messages = self.get_assistant_messages(completion)
+            if not assistant_messages:
+                return None
+            ans = str(assistant_messages[-1].get("content", ""))
             return self.parse(ans)
 
     def get_format_reward_func(self) -> Callable: