core(minor): Add bulk add messages to BaseChatMessageHistory interface (#15709)

* Add bulk add_messages method to the interface.
* Update documentation for add_ai_message and add_human_message to
denote them as being marked for deprecation. We should stop using them
as they create more incorrect (inefficient) ways of doing things

libs/core/langchain_core/chat_history.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from abc import ABC, abstractmethod
-from typing import List, Union
+from typing import List, Sequence, Union
 
 from langchain_core.messages import (
     AIMessage,
@@ -14,9 +14,18 @@ from langchain_core.messages import (
 class BaseChatMessageHistory(ABC):
     """Abstract base class for storing chat message history.
 
-    See `ChatMessageHistory` for default implementation.
+    Implementations should over-ride the add_messages method to handle bulk addition
+    of messages.
+
+    The default implementation of add_message will correctly call add_messages, so
+    it is not necessary to implement both methods.
+
+    When used for updating history, users should favor usage of `add_messages`
+    over `add_message` or other variants like `add_user_message` and `add_ai_message`
+    to avoid unnecessary round-trips to the underlying persistence layer.
+
+    Example: Shows a default implementation.
 
-    Example:
         .. code-block:: python
 
             class FileChatMessageHistory(BaseChatMessageHistory):
@@ -29,8 +38,13 @@ class BaseChatMessageHistory(ABC):
                        messages = json.loads(f.read())
                     return messages_from_dict(messages)
 
-               def add_message(self, message: BaseMessage) -> None:
+               def add_messages(self, messages: Sequence[BaseMessage]) -> None:
-                   messages = self.messages.append(_message_to_dict(message))
+                   all_messages = list(self.messages) # Existing messages
+                   all_messages.extend(messages) # Add new messages
+
+                   serialized = [message_to_dict(message) for message in all_messages]
+                   # Can be further optimized by only writing new messages
+                   # using append mode.
                    with open(os.path.join(storage_path, session_id), 'w') as f:
                        json.dump(f, messages)
 
@@ -45,6 +59,12 @@ class BaseChatMessageHistory(ABC):
     def add_user_message(self, message: Union[HumanMessage, str]) -> None:
         """Convenience method for adding a human message string to the store.
 
+        Please note that this is a convenience method. Code should favor the
+        bulk add_messages interface instead to save on round-trips to the underlying
+        persistence layer.
+
+        This method may be deprecated in a future release.
+
         Args:
             message: The human message to add
         """
@@ -56,6 +76,12 @@ class BaseChatMessageHistory(ABC):
     def add_ai_message(self, message: Union[AIMessage, str]) -> None:
         """Convenience method for adding an AI message string to the store.
 
+        Please note that this is a convenience method. Code should favor the bulk
+        add_messages interface instead to save on round-trips to the underlying
+        persistence layer.
+
+        This method may be deprecated in a future release.
+
         Args:
             message: The AI message to add.
         """
@@ -64,18 +90,38 @@ class BaseChatMessageHistory(ABC):
         else:
             self.add_message(AIMessage(content=message))
 
-    @abstractmethod
     def add_message(self, message: BaseMessage) -> None:
         """Add a Message object to the store.
 
         Args:
             message: A BaseMessage object to store.
         """
-        raise NotImplementedError()
+        if type(self).add_messages != BaseChatMessageHistory.add_messages:
+            # This means that the sub-class has implemented an efficient add_messages
+            # method, so we should usage of add_message to that.
+            self.add_messages([message])
+        else:
+            raise NotImplementedError(
+                "add_message is not implemented for this class. "
+                "Please implement add_message or add_messages."
+            )
+
+    def add_messages(self, messages: Sequence[BaseMessage]) -> None:
+        """Add a list of messages.
+
+        Implementations should over-ride this method to handle bulk addition of messages
+        in an efficient manner to avoid unnecessary round-trips to the underlying store.
+
+        Args:
+            messages: A list of BaseMessage objects to store.
+        """
+        for message in messages:
+            self.add_message(message)
 
     @abstractmethod
     def clear(self) -> None:
         """Remove all messages from the store"""
 
     def __str__(self) -> str:
+        """Return a string representation of the chat history."""
         return get_buffer_string(self.messages)

libs/core/tests/unit_tests/chat_history/test_chat_history.py

@@ -0,0 +1,68 @@
+from typing import List, Sequence
+
+from langchain_core.chat_history import BaseChatMessageHistory
+from langchain_core.messages import BaseMessage, HumanMessage
+
+
+def test_add_message_implementation_only() -> None:
+    """Test implementation of add_message only."""
+
+    class SampleChatHistory(BaseChatMessageHistory):
+        def __init__(self, *, store: List[BaseMessage]) -> None:
+            self.store = store
+
+        def add_message(self, message: BaseMessage) -> None:
+            """Add a message to the store."""
+            self.store.append(message)
+
+        def clear(self) -> None:
+            """Clear the store."""
+            raise NotImplementedError()
+
+    store: List[BaseMessage] = []
+    chat_history = SampleChatHistory(store=store)
+    chat_history.add_message(HumanMessage(content="Hello"))
+    assert len(store) == 1
+    assert store[0] == HumanMessage(content="Hello")
+    chat_history.add_message(HumanMessage(content="World"))
+    assert len(store) == 2
+    assert store[1] == HumanMessage(content="World")
+
+    chat_history.add_messages(
+        [HumanMessage(content="Hello"), HumanMessage(content="World")]
+    )
+    assert len(store) == 4
+    assert store[2] == HumanMessage(content="Hello")
+    assert store[3] == HumanMessage(content="World")
+
+
+def test_bulk_message_implementation_only() -> None:
+    """Test that SampleChatHistory works as expected."""
+    store: List[BaseMessage] = []
+
+    class BulkAddHistory(BaseChatMessageHistory):
+        def __init__(self, *, store: List[BaseMessage]) -> None:
+            self.store = store
+
+        def add_messages(self, message: Sequence[BaseMessage]) -> None:
+            """Add a message to the store."""
+            self.store.extend(message)
+
+        def clear(self) -> None:
+            """Clear the store."""
+            raise NotImplementedError()
+
+    chat_history = BulkAddHistory(store=store)
+    chat_history.add_message(HumanMessage(content="Hello"))
+    assert len(store) == 1
+    assert store[0] == HumanMessage(content="Hello")
+    chat_history.add_message(HumanMessage(content="World"))
+    assert len(store) == 2
+    assert store[1] == HumanMessage(content="World")
+
+    chat_history.add_messages(
+        [HumanMessage(content="Hello"), HumanMessage(content="World")]
+    )
+    assert len(store) == 4
+    assert store[2] == HumanMessage(content="Hello")
+    assert store[3] == HumanMessage(content="World")