core[minor]: Add aformat_messages to FewShotChatMessagePromptTemplate and ChatPromptTemplate (#19648)

Needed since the example selector may use a vector store.

libs/core/langchain_core/prompts/chat.py

@@ -66,6 +66,17 @@ class BaseMessagePromptTemplate(Serializable, ABC):
             List of BaseMessages.
         """
 
+    async def aformat_messages(self, **kwargs: Any) -> List[BaseMessage]:
+        """Format messages from kwargs. Should return a list of BaseMessages.
+
+        Args:
+            **kwargs: Keyword arguments to use for formatting.
+
+        Returns:
+            List of BaseMessages.
+        """
+        return self.format_messages(**kwargs)
+
     @property
     @abstractmethod
     def input_variables(self) -> List[str]:
@@ -594,6 +605,10 @@ class BaseChatPromptTemplate(BasePromptTemplate, ABC):
     def format_messages(self, **kwargs: Any) -> List[BaseMessage]:
         """Format kwargs into a list of messages."""
 
+    async def aformat_messages(self, **kwargs: Any) -> List[BaseMessage]:
+        """Format kwargs into a list of messages."""
+        return self.format_messages(**kwargs)
+
     def pretty_repr(self, html: bool = False) -> str:
         """Human-readable representation."""
         raise NotImplementedError
@@ -901,19 +916,31 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
             partial_variables=partial_vars,
         )
 
-    def format(self, **kwargs: Any) -> str:
-        """Format the chat template into a string.
+    def format_messages(self, **kwargs: Any) -> List[BaseMessage]:
+        """Format the chat template into a list of finalized messages.
 
         Args:
             **kwargs: keyword arguments to use for filling in template variables
                       in all the template messages in this chat template.
 
         Returns:
-            formatted string
+            list of formatted messages
         """
-        return self.format_prompt(**kwargs).to_string()
+        kwargs = self._merge_partial_and_user_variables(**kwargs)
+        result = []
+        for message_template in self.messages:
+            if isinstance(message_template, BaseMessage):
+                result.extend([message_template])
+            elif isinstance(
+                message_template, (BaseMessagePromptTemplate, BaseChatPromptTemplate)
+            ):
+                message = message_template.format_messages(**kwargs)
+                result.extend(message)
+            else:
+                raise ValueError(f"Unexpected input: {message_template}")
+        return result
 
-    def format_messages(self, **kwargs: Any) -> List[BaseMessage]:
+    async def aformat_messages(self, **kwargs: Any) -> List[BaseMessage]:
         """Format the chat template into a list of finalized messages.
 
         Args:
@@ -931,7 +958,7 @@ class ChatPromptTemplate(BaseChatPromptTemplate):
             elif isinstance(
                 message_template, (BaseMessagePromptTemplate, BaseChatPromptTemplate)
             ):
-                message = message_template.format_messages(**kwargs)
+                message = await message_template.aformat_messages(**kwargs)
                 result.extend(message)
             else:
                 raise ValueError(f"Unexpected input: {message_template}")

libs/core/langchain_core/prompts/few_shot.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from pathlib import Path
 from typing import Any, Dict, List, Literal, Optional, Union
 
+from langchain_core.example_selectors import BaseExampleSelector
 from langchain_core.messages import BaseMessage, get_buffer_string
 from langchain_core.prompts.chat import (
     BaseChatPromptTemplate,
@@ -27,7 +28,7 @@ class _FewShotPromptTemplateMixin(BaseModel):
     """Examples to format into the prompt.
     Either this or example_selector should be provided."""
 
-    example_selector: Any = None
+    example_selector: Optional[BaseExampleSelector] = None
     """ExampleSelector to choose the examples to format into the prompt.
     Either this or examples should be provided."""
 
@@ -72,6 +73,24 @@ class _FewShotPromptTemplateMixin(BaseModel):
                 "One of 'examples' and 'example_selector' should be provided"
             )
 
+    async def _aget_examples(self, **kwargs: Any) -> List[dict]:
+        """Get the examples to use for formatting the prompt.
+
+        Args:
+            **kwargs: Keyword arguments to be passed to the example selector.
+
+        Returns:
+            List of examples.
+        """
+        if self.examples is not None:
+            return self.examples
+        elif self.example_selector is not None:
+            return await self.example_selector.aselect_examples(kwargs)
+        else:
+            raise ValueError(
+                "One of 'examples' and 'example_selector' should be provided"
+            )
+
 
 class FewShotPromptTemplate(_FewShotPromptTemplateMixin, StringPromptTemplate):
     """Prompt template that contains few shot examples."""
@@ -325,6 +344,28 @@ class FewShotChatMessagePromptTemplate(
         ]
         return messages
 
+    async def aformat_messages(self, **kwargs: Any) -> List[BaseMessage]:
+        """Format kwargs into a list of messages.
+
+        Args:
+            **kwargs: keyword arguments to use for filling in templates in messages.
+
+        Returns:
+            A list of formatted messages with all template variables filled in.
+        """
+        # Get the examples to use.
+        examples = await self._aget_examples(**kwargs)
+        examples = [
+            {k: e[k] for k in self.example_prompt.input_variables} for e in examples
+        ]
+        # Format the examples.
+        messages = [
+            message
+            for example in examples
+            for message in await self.example_prompt.aformat_messages(**example)
+        ]
+        return messages
+
     def format(self, **kwargs: Any) -> str:
         """Format the prompt with inputs generating a string.
 

libs/core/tests/unit_tests/prompts/test_few_shot.py

@@ -308,7 +308,7 @@ def test_prompt_jinja2_extra_input_variables(
     ).input_variables == ["bar", "foo"]
 
 
-def test_few_shot_chat_message_prompt_template() -> None:
+async def test_few_shot_chat_message_prompt_template() -> None:
     """Tests for few shot chat message template."""
     examples = [
         {"input": "2+2", "output": "4"},
@@ -333,8 +333,7 @@ def test_few_shot_chat_message_prompt_template() -> None:
         + HumanMessagePromptTemplate.from_template("{input}")
     )
 
-    messages = final_prompt.format_messages(input="100 + 1")
-    assert messages == [
+    expected = [
         SystemMessage(content="You are a helpful AI Assistant", additional_kwargs={}),
         HumanMessage(content="2+2", additional_kwargs={}, example=False),
         AIMessage(content="4", additional_kwargs={}, example=False),
@@ -343,6 +342,11 @@ def test_few_shot_chat_message_prompt_template() -> None:
         HumanMessage(content="100 + 1", additional_kwargs={}, example=False),
     ]
 
+    messages = final_prompt.format_messages(input="100 + 1")
+    assert messages == expected
+    messages = await final_prompt.aformat_messages(input="100 + 1")
+    assert messages == expected
+
 
 class AsIsSelector(BaseExampleSelector):
     """An example selector for testing purposes.
@@ -355,11 +359,9 @@ class AsIsSelector(BaseExampleSelector):
         self.examples = examples
 
     def add_example(self, example: Dict[str, str]) -> Any:
-        """Adds an example to the selector."""
-        raise NotImplementedError()
+        raise NotImplementedError
 
     def select_examples(self, input_variables: Dict[str, str]) -> List[dict]:
-        """Select which examples to use based on the inputs."""
         return list(self.examples)
 
 
@@ -387,8 +389,63 @@ def test_few_shot_chat_message_prompt_template_with_selector() -> None:
         + few_shot_prompt
         + HumanMessagePromptTemplate.from_template("{input}")
     )
+    expected = [
+        SystemMessage(content="You are a helpful AI Assistant", additional_kwargs={}),
+        HumanMessage(content="2+2", additional_kwargs={}, example=False),
+        AIMessage(content="4", additional_kwargs={}, example=False),
+        HumanMessage(content="2+3", additional_kwargs={}, example=False),
+        AIMessage(content="5", additional_kwargs={}, example=False),
+        HumanMessage(content="100 + 1", additional_kwargs={}, example=False),
+    ]
     messages = final_prompt.format_messages(input="100 + 1")
-    assert messages == [
+    assert messages == expected
+
+
+class AsyncAsIsSelector(BaseExampleSelector):
+    """An example selector for testing purposes.
+
+    This selector returns the examples as-is.
+    """
+
+    def __init__(self, examples: Sequence[Dict[str, str]]) -> None:
+        """Initializes the selector."""
+        self.examples = examples
+
+    def add_example(self, example: Dict[str, str]) -> Any:
+        raise NotImplementedError
+
+    def select_examples(self, input_variables: Dict[str, str]) -> List[dict]:
+        raise NotImplementedError
+
+    async def aselect_examples(self, input_variables: Dict[str, str]) -> List[dict]:
+        return list(self.examples)
+
+
+async def test_few_shot_chat_message_prompt_template_with_selector_async() -> None:
+    """Tests for few shot chat message template with an async example selector."""
+    examples = [
+        {"input": "2+2", "output": "4"},
+        {"input": "2+3", "output": "5"},
+    ]
+    example_selector = AsyncAsIsSelector(examples)
+    example_prompt = ChatPromptTemplate.from_messages(
+        [
+            HumanMessagePromptTemplate.from_template("{input}"),
+            AIMessagePromptTemplate.from_template("{output}"),
+        ]
+    )
+
+    few_shot_prompt = FewShotChatMessagePromptTemplate(
+        input_variables=["input"],
+        example_prompt=example_prompt,
+        example_selector=example_selector,
+    )
+    final_prompt: ChatPromptTemplate = (
+        SystemMessagePromptTemplate.from_template("You are a helpful AI Assistant")
+        + few_shot_prompt
+        + HumanMessagePromptTemplate.from_template("{input}")
+    )
+    expected = [
         SystemMessage(content="You are a helpful AI Assistant", additional_kwargs={}),
         HumanMessage(content="2+2", additional_kwargs={}, example=False),
         AIMessage(content="4", additional_kwargs={}, example=False),
@@ -396,3 +453,5 @@ def test_few_shot_chat_message_prompt_template_with_selector() -> None:
         AIMessage(content="5", additional_kwargs={}, example=False),
         HumanMessage(content="100 + 1", additional_kwargs={}, example=False),
     ]
+    messages = await final_prompt.aformat_messages(input="100 + 1")
+    assert messages == expected