refactor(core): use pytest style in TestGetBufferString (#32786)

Co-authored-by: Mason Daugherty <mason@langchain.dev>

Author: cbornet

libs/core/tests/unit_tests/test_messages.py

@@ -1,4 +1,3 @@
-import unittest
 import uuid
 from typing import Optional, Union
 
@@ -335,54 +334,45 @@ def test_ai_message_chunks() -> None:
         )
 
 
-class TestGetBufferString(unittest.TestCase):
-    def setUp(self) -> None:
-        self.human_msg = HumanMessage(content="human")
-        self.ai_msg = AIMessage(content="ai")
-        self.sys_msg = SystemMessage(content="system")
-        self.func_msg = FunctionMessage(name="func", content="function")
-        self.tool_msg = ToolMessage(tool_call_id="tool_id", content="tool")
-        self.chat_msg = ChatMessage(role="Chat", content="chat")
-        self.tool_calls_msg = AIMessage(content="tool")
+class TestGetBufferString:
+    _HUMAN_MSG = HumanMessage(content="human")
+    _AI_MSG = AIMessage(content="ai")
 
     def test_empty_input(self) -> None:
         assert get_buffer_string([]) == ""
 
     def test_valid_single_message(self) -> None:
-        expected_output = f"Human: {self.human_msg.content}"
-        assert get_buffer_string([self.human_msg]) == expected_output
+        expected_output = "Human: human"
+        assert get_buffer_string([self._HUMAN_MSG]) == expected_output
 
     def test_custom_human_prefix(self) -> None:
-        prefix = "H"
-        expected_output = f"{prefix}: {self.human_msg.content}"
-        assert get_buffer_string([self.human_msg], human_prefix="H") == expected_output
+        expected_output = "H: human"
+        assert get_buffer_string([self._HUMAN_MSG], human_prefix="H") == expected_output
 
     def test_custom_ai_prefix(self) -> None:
-        prefix = "A"
-        expected_output = f"{prefix}: {self.ai_msg.content}"
-        assert get_buffer_string([self.ai_msg], ai_prefix="A") == expected_output
+        expected_output = "A: ai"
+        assert get_buffer_string([self._AI_MSG], ai_prefix="A") == expected_output
 
     def test_multiple_msg(self) -> None:
         msgs = [
-            self.human_msg,
-            self.ai_msg,
-            self.sys_msg,
-            self.func_msg,
-            self.tool_msg,
-            self.chat_msg,
-            self.tool_calls_msg,
+            self._HUMAN_MSG,
+            self._AI_MSG,
+            SystemMessage(content="system"),
+            FunctionMessage(name="func", content="function"),
+            ToolMessage(tool_call_id="tool_id", content="tool"),
+            ChatMessage(role="Chat", content="chat"),
+            AIMessage(content="tool"),
         ]
-        expected_output = "\n".join(  # noqa: FLY002
-            [
-                "Human: human",
-                "AI: ai",
-                "System: system",
-                "Function: function",
-                "Tool: tool",
-                "Chat: chat",
-                "AI: tool",
-            ]
+        expected_output = (
+            "Human: human\n"
+            "AI: ai\n"
+            "System: system\n"
+            "Function: function\n"
+            "Tool: tool\n"
+            "Chat: chat\n"
+            "AI: tool"
         )
+
         assert get_buffer_string(msgs) == expected_output