langchain.chains.conversational_retrieval.base.ConversationalRetrievalChain¶
class langchain.chains.conversational_retrieval.base.ConversationalRetrievalChain[source]¶
Bases: BaseConversationalRetrievalChain
Chain for having a conversation based on retrieved documents.
This chain takes in chat history (a list of messages) and new questions,
and then returns an answer to that question.
The algorithm for this chain consists of three parts:
1. Use the chat history and the new question to create a “standalone question”.
This is done so that this question can be passed into the retrieval step to fetch
relevant documents. If only the new question was passed in, then relevant context
may be lacking. If the whole conversation was passed into retrieval, there may
be unnecessary information there that would distract from retrieval.
2. This new question is passed to the retriever and relevant documents are
returned.
3. The retrieved documents are passed to an LLM along with either the new question
(default behavior) or the original question and chat history to generate a final
response.
Example
from langchain.chains import (
    StuffDocumentsChain, LLMChain, ConversationalRetrievalChain
)
from langchain.prompts import PromptTemplate
from langchain.llms import OpenAI
combine_docs_chain = StuffDocumentsChain(...)
vectorstore = ...
retriever = vectorstore.as_retriever()
# This controls how the standalone question is generated.
# Should take `chat_history` and `question` as input variables.
template = (
    "Combine the chat history and follow up question into "
    "a standalone question. Chat History: {chat_history}"
    "Follow up question: {question}"
)
prompt = PromptTemplate.from_template(template)
llm = OpenAI()