RAG

Let's look at adding in a retrieval step to a prompt and LLM, which adds up to a "retrieval-augmented generation" chain

pip install langchain openai faiss-cpu tiktoken

from operator import itemgetter

from langchain.prompts import ChatPromptTemplate
from langchain.chat_models import ChatOpenAI
from langchain.embeddings import OpenAIEmbeddings
from langchain.schema.output_parser import StrOutputParser
from langchain.schema.runnable import RunnablePassthrough, RunnableLambda
from langchain.vectorstores import FAISS

vectorstore = FAISS.from_texts(
    ["harrison worked at kensho"], embedding=OpenAIEmbeddings()
)
retriever = vectorstore.as_retriever()

template = """Answer the question based only on the following context:
{context}

Question: {question}
"""
prompt = ChatPromptTemplate.from_template(template)

model = ChatOpenAI()

chain = (
    {"context": retriever, "question": RunnablePassthrough()}
    | prompt
    | model
    | StrOutputParser()
)

chain.invoke("where did harrison work?")

    'Harrison worked at Kensho.'

template = """Answer the question based only on the following context:
{context}

Question: {question}

Answer in the following language: {language}
"""
prompt = ChatPromptTemplate.from_template(template)

chain = (
    {
        "context": itemgetter("question") | retriever,
        "question": itemgetter("question"),
        "language": itemgetter("language"),
    }
    | prompt
    | model
    | StrOutputParser()
)

chain.invoke({"question": "where did harrison work", "language": "italian"})

    'Harrison ha lavorato a Kensho.'

Conversational Retrieval Chain

We can easily add in conversation history. This primarily means adding in chat_message_history

from langchain.schema.runnable import RunnableMap
from langchain.schema import format_document

from langchain.prompts.prompt import PromptTemplate

_template = """Given the following conversation and a follow up question, rephrase the follow up question to be a standalone question, in its original language.

Chat History:
{chat_history}
Follow Up Input: {question}
Standalone question:"""
CONDENSE_QUESTION_PROMPT = PromptTemplate.from_template(_template)

template = """Answer the question based only on the following context:
{context}

Question: {question}
"""
ANSWER_PROMPT = ChatPromptTemplate.from_template(template)

DEFAULT_DOCUMENT_PROMPT = PromptTemplate.from_template(template="{page_content}")


def _combine_documents(
    docs, document_prompt=DEFAULT_DOCUMENT_PROMPT, document_separator="\n\n"
):
    doc_strings = [format_document(doc, document_prompt) for doc in docs]
    return document_separator.join(doc_strings)

from typing import Tuple, List


def _format_chat_history(chat_history: List[Tuple]) -> str:
    buffer = ""
    for dialogue_turn in chat_history:
        human = "Human: " + dialogue_turn[0]
        ai = "Assistant: " + dialogue_turn[1]
        buffer += "\n" + "\n".join([human, ai])
    return buffer

_inputs = RunnableMap(
    standalone_question=RunnablePassthrough.assign(
        chat_history=lambda x: _format_chat_history(x["chat_history"])
    )
    | CONDENSE_QUESTION_PROMPT
    | ChatOpenAI(temperature=0)
    | StrOutputParser(),
)
_context = {
    "context": itemgetter("standalone_question") | retriever | _combine_documents,
    "question": lambda x: x["standalone_question"],
}
conversational_qa_chain = _inputs | _context | ANSWER_PROMPT | ChatOpenAI()

conversational_qa_chain.invoke(
    {
        "question": "where did harrison work?",
        "chat_history": [],
    }
)

    AIMessage(content='Harrison was employed at Kensho.', additional_kwargs={}, example=False)

conversational_qa_chain.invoke(
    {
        "question": "where did he work?",
        "chat_history": [("Who wrote this notebook?", "Harrison")],
    }
)

    AIMessage(content='Harrison worked at Kensho.', additional_kwargs={}, example=False)

With Memory and returning source documents

This shows how to use memory with the above. For memory, we need to manage that outside at the memory. For returning the retrieved documents, we just need to pass them through all the way.

from operator import itemgetter
from langchain.memory import ConversationBufferMemory

memory = ConversationBufferMemory(
    return_messages=True, output_key="answer", input_key="question"
)

# First we add a step to load memory
# This adds a "memory" key to the input object
loaded_memory = RunnablePassthrough.assign(
    chat_history=RunnableLambda(memory.load_memory_variables) | itemgetter("history"),
)
# Now we calculate the standalone question
standalone_question = {
    "standalone_question": {
        "question": lambda x: x["question"],
        "chat_history": lambda x: _format_chat_history(x["chat_history"]),
    }
    | CONDENSE_QUESTION_PROMPT
    | ChatOpenAI(temperature=0)
    | StrOutputParser(),
}
# Now we retrieve the documents
retrieved_documents = {
    "docs": itemgetter("standalone_question") | retriever,
    "question": lambda x: x["standalone_question"],
}
# Now we construct the inputs for the final prompt
final_inputs = {
    "context": lambda x: _combine_documents(x["docs"]),
    "question": itemgetter("question"),
}
# And finally, we do the part that returns the answers
answer = {
    "answer": final_inputs | ANSWER_PROMPT | ChatOpenAI(),
    "docs": itemgetter("docs"),
}
# And now we put it all together!
final_chain = loaded_memory | standalone_question | retrieved_documents | answer

inputs = {"question": "where did harrison work?"}
result = final_chain.invoke(inputs)
result

    {'answer': AIMessage(content='Harrison was employed at Kensho.', additional_kwargs={}, example=False),
     'docs': [Document(page_content='harrison worked at kensho', metadata={})]}

# Note that the memory does not save automatically
# This will be improved in the future
# For now you need to save it yourself
memory.save_context(inputs, {"answer": result["answer"].content})

memory.load_memory_variables({})

    {'history': [HumanMessage(content='where did harrison work?', additional_kwargs={}, example=False),
      AIMessage(content='Harrison was employed at Kensho.', additional_kwargs={}, example=False)]}