Partial formatting for chat messages (#8450)

libs/langchain/langchain/prompts/chat.py

@@ -57,6 +57,14 @@ class BaseMessagePromptTemplate(Serializable, ABC):
         """
 
     def __add__(self, other: Any) -> ChatPromptTemplate:
+        """Combine two prompt templates.
+
+        Args:
+            other: Another prompt template.
+
+        Returns:
+            Combined prompt template.
+        """
         prompt = ChatPromptTemplate(messages=[self])
         return prompt + other
 
@@ -68,7 +76,7 @@ class MessagesPlaceholder(BaseMessagePromptTemplate):
     """Name of variable to use as messages."""
 
     def format_messages(self, **kwargs: Any) -> List[BaseMessage]:
-        """To a BaseMessage.
+        """Format messages from kwargs.
 
         Args:
             **kwargs: Keyword arguments to use for formatting.
@@ -156,10 +164,17 @@ class BaseStringMessagePromptTemplate(BaseMessagePromptTemplate, ABC):
 
     @abstractmethod
     def format(self, **kwargs: Any) -> BaseMessage:
-        """To a BaseMessage."""
+        """Format the prompt template.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            Formatted message.
+        """
 
     def format_messages(self, **kwargs: Any) -> List[BaseMessage]:
-        """Format messages from kwargs. Should return a list of BaseMessages.
+        """Format messages from kwargs.
 
         Args:
             **kwargs: Keyword arguments to use for formatting.
@@ -187,6 +202,14 @@ class ChatMessagePromptTemplate(BaseStringMessagePromptTemplate):
     """Role of the message."""
 
     def format(self, **kwargs: Any) -> BaseMessage:
+        """Format the prompt template.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            Formatted message.
+        """
         text = self.prompt.format(**kwargs)
         return ChatMessage(
             content=text, role=self.role, additional_kwargs=self.additional_kwargs
@@ -197,6 +220,14 @@ class HumanMessagePromptTemplate(BaseStringMessagePromptTemplate):
     """Human message prompt template. This is a message that is sent to the user."""
 
     def format(self, **kwargs: Any) -> BaseMessage:
+        """Format the prompt template.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            Formatted message.
+        """
         text = self.prompt.format(**kwargs)
         return HumanMessage(content=text, additional_kwargs=self.additional_kwargs)
 
@@ -205,6 +236,14 @@ class AIMessagePromptTemplate(BaseStringMessagePromptTemplate):
     """AI message prompt template. This is a message that is not sent to the user."""
 
     def format(self, **kwargs: Any) -> BaseMessage:
+        """Format the prompt template.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            Formatted message.
+        """
         text = self.prompt.format(**kwargs)
         return AIMessage(content=text, additional_kwargs=self.additional_kwargs)
 
@@ -215,6 +254,14 @@ class SystemMessagePromptTemplate(BaseStringMessagePromptTemplate):
     """
 
     def format(self, **kwargs: Any) -> BaseMessage:
+        """Format the prompt template.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            Formatted message.
+        """
         text = self.prompt.format(**kwargs)
         return SystemMessage(content=text, additional_kwargs=self.additional_kwargs)
 
@@ -241,6 +288,15 @@ class BaseChatPromptTemplate(BasePromptTemplate, ABC):
     """Base class for chat prompt templates."""
 
     def format(self, **kwargs: Any) -> str:
+        """Format the chat template into a string.
+
+        Args:
+            **kwargs: keyword arguments to use for filling in template variables
+                      in all the template messages in this chat template.
+
+        Returns:
+            formatted string
+        """
         return self.format_prompt(**kwargs).to_string()
 
     def format_prompt(self, **kwargs: Any) -> PromptValue:
@@ -261,7 +317,7 @@ class BaseChatPromptTemplate(BasePromptTemplate, ABC):
 
 
 class ChatPromptTemplate(BaseChatPromptTemplate, ABC):
-    """A prompt template for a chat models.
+    """A prompt template for chat models.
 
     Use to create flexible templated prompts for chat models.
 
@@ -292,6 +348,14 @@ class ChatPromptTemplate(BaseChatPromptTemplate, ABC):
     """List of messages consisting of either message prompt templates or messages."""
 
     def __add__(self, other: Any) -> ChatPromptTemplate:
+        """Combine two prompt templates.
+
+        Args:
+            other: Another prompt template.
+
+        Returns:
+            Combined prompt template.
+        """
         # Allow for easy combining
         if isinstance(other, ChatPromptTemplate):
             return ChatPromptTemplate(messages=self.messages + other.messages)
@@ -497,8 +561,42 @@ class ChatPromptTemplate(BaseChatPromptTemplate, ABC):
                 raise ValueError(f"Unexpected input: {message_template}")
         return result
 
-    def partial(self, **kwargs: Union[str, Callable[[], str]]) -> BasePromptTemplate:
-        raise NotImplementedError
+    def partial(self, **kwargs: Union[str, Callable[[], str]]) -> ChatPromptTemplate:
+        """Return a new ChatPromptTemplate with some of the input variables already
+        filled in.
+
+        Args:
+            **kwargs: keyword arguments to use for filling in template variables. Ought
+                        to be a subset of the input variables.
+
+        Returns:
+            A new ChatPromptTemplate.
+
+
+        Example:
+
+            .. code-block:: python
+
+                from langchain.prompts import ChatPromptTemplate
+
+                template = ChatPromptTemplate.from_messages(
+                    [
+                        ("system", "You are an AI assistant named {name}."),
+                        ("human", "Hi I'm {user}"),
+                        ("ai", "Hi there, {user}, I'm {name}."),
+                        ("human", "{input}"),
+                    ]
+                )
+                template2 = template.partial(user="Lucy", name="R2D2")
+
+                template2.format_messages(input="hello")
+        """
+        prompt_dict = self.__dict__.copy()
+        prompt_dict["input_variables"] = list(
+            set(self.input_variables).difference(kwargs)
+        )
+        prompt_dict["partial_variables"] = {**self.partial_variables, **kwargs}
+        return type(self)(**prompt_dict)
 
     @property
     def _prompt_type(self) -> str:

libs/langchain/tests/unit_tests/prompts/test_chat.py

@@ -20,6 +20,7 @@ from langchain.schema.messages import (
     BaseMessage,
     HumanMessage,
     SystemMessage,
+    get_buffer_string,
 )
 
 
@@ -288,3 +289,27 @@ def test_convert_to_message_is_strict() -> None:
         # this test is here to ensure that functionality to interpret `meow`
         # as a role is NOT added.
         _convert_to_message(("meow", "question"))
+
+
+def test_chat_message_partial() -> None:
+    template = ChatPromptTemplate.from_messages(
+        [
+            ("system", "You are an AI assistant named {name}."),
+            ("human", "Hi I'm {user}"),
+            ("ai", "Hi there, {user}, I'm {name}."),
+            ("human", "{input}"),
+        ]
+    )
+    template2 = template.partial(user="Lucy", name="R2D2")
+    with pytest.raises(KeyError):
+        template.format_messages(input="hello")
+
+    res = template2.format_messages(input="hello")
+    expected = [
+        SystemMessage(content="You are an AI assistant named R2D2."),
+        HumanMessage(content="Hi I'm Lucy"),
+        AIMessage(content="Hi there, Lucy, I'm R2D2."),
+        HumanMessage(content="hello"),
+    ]
+    assert res == expected
+    assert template2.format(input="hello") == get_buffer_string(expected)