Adding `BaseChatMessageHistory.__str__` (#14311)

Adding __str__ to base chat message history to make it easier to debug

libs/core/langchain_core/chat_history.py

@@ -3,7 +3,12 @@ from __future__ import annotations
 from abc import ABC, abstractmethod
 from typing import List
 
-from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
+from langchain_core.messages import (
+    AIMessage,
+    BaseMessage,
+    HumanMessage,
+    get_buffer_string,
+)
 
 
 class BaseChatMessageHistory(ABC):
@@ -65,3 +70,6 @@ class BaseChatMessageHistory(ABC):
     @abstractmethod
     def clear(self) -> None:
         """Remove all messages from the store"""
+
+    def __str__(self) -> str:
+        return get_buffer_string(self.messages)

libs/core/tests/unit_tests/runnables/test_history.py

@@ -1,6 +1,6 @@
 from typing import Any, Callable, Sequence, Union
 
-from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
+from langchain_core.messages import AIMessage, BaseMessage, HumanMessage, SystemMessage
 from langchain_core.pydantic_v1 import BaseModel
 from langchain_core.runnables.base import RunnableLambda
 from langchain_core.runnables.config import RunnableConfig
@@ -8,6 +8,15 @@ from langchain_core.runnables.history import RunnableWithMessageHistory
 from tests.unit_tests.fake.memory import ChatMessageHistory
 
 
+def test_interfaces() -> None:
+    history = ChatMessageHistory()
+    history.add_message(SystemMessage(content="system"))
+    history.add_user_message("human 1")
+    history.add_ai_message("ai")
+    history.add_message(HumanMessage(content="human 2"))
+    assert str(history) == "System: system\nHuman: human 1\nAI: ai\nHuman: human 2"
+
+
 def _get_get_session_history() -> Callable[..., ChatMessageHistory]:
     chat_history_store = {}