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How to add values to a chain's state

An alternate way of passing data through steps of a chain is to leave the current values of the chain state unchanged while assigning a new value under a given key. The RunnablePassthrough.assign() static method takes an input value and adds the extra arguments passed to the assign function.

This is useful in the common LangChain Expression Language pattern of additively creating a dictionary to use as input to a later step.

Here's an example:

%pip install --upgrade --quiet langchain langchain-openai

import os
from getpass import getpass

os.environ["OPENAI_API_KEY"] = getpass()
from langchain_core.runnables import RunnableParallel, RunnablePassthrough

runnable = RunnableParallel(
extra=RunnablePassthrough.assign(mult=lambda x: x["num"] * 3),
modified=lambda x: x["num"] + 1,

runnable.invoke({"num": 1})
{'extra': {'num': 1, 'mult': 3}, 'modified': 2}

Let's break down what's happening here.

  • The input to the chain is {"num": 1}. This is passed into a RunnableParallel, which invokes the runnables it is passed in parallel with that input.
  • The value under the extra key is invoked. RunnablePassthrough.assign() keeps the original keys in the input dict ({"num": 1}), and assigns a new key called mult. The value is lambda x: x["num"] * 3), which is 3. Thus, the result is {"num": 1, "mult": 3}.
  • {"num": 1, "mult": 3} is returned to the RunnableParallel call, and is set as the value to the key extra.
  • At the same time, the modified key is called. The result is 2, since the lambda extracts a key called "num" from its input and adds one.

Thus, the result is {'extra': {'num': 1, 'mult': 3}, 'modified': 2}.


One convenient feature of this method is that it allows values to pass through as soon as they are available. To show this off, we'll use RunnablePassthrough.assign() to immediately return source docs in a retrieval chain:

from langchain_community.vectorstores import FAISS
from langchain_core.output_parsers import StrOutputParser
from langchain_core.prompts import ChatPromptTemplate
from langchain_core.runnables import RunnablePassthrough
from langchain_openai import ChatOpenAI, OpenAIEmbeddings

vectorstore = FAISS.from_texts(
["harrison worked at kensho"], embedding=OpenAIEmbeddings()
retriever = vectorstore.as_retriever()
template = """Answer the question based only on the following context:

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

generation_chain = prompt | model | StrOutputParser()

retrieval_chain = {
"context": retriever,
"question": RunnablePassthrough(),
} | RunnablePassthrough.assign(output=generation_chain)

stream ="where did harrison work?")

for chunk in stream:
{'question': 'where did harrison work?'}
{'context': [Document(page_content='harrison worked at kensho')]}
{'output': ''}
{'output': 'H'}
{'output': 'arrison'}
{'output': ' worked'}
{'output': ' at'}
{'output': ' Kens'}
{'output': 'ho'}
{'output': '.'}
{'output': ''}

We can see that the first chunk contains the original "question" since that is immediately available. The second chunk contains "context" since the retriever finishes second. Finally, the output from the generation_chain streams in chunks as soon as it is available.

Next steps​

Now you've learned how to pass data through your chains to help to help format the data flowing through your chains.

To learn more, see the other how-to guides on runnables in this section.

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