Skip to main content

Composition

LangChain provides a user friendly interface for composing different parts of prompts together. You can do this with either string prompts or chat prompts. Constructing prompts this way allows for easy reuse of components.

String prompt composition​

When working with string prompts, each template is joined together. You can work with either prompts directly or strings (the first element in the list needs to be a prompt).

from langchain.prompts import PromptTemplate
prompt = (
    PromptTemplate.from_template("Tell me a joke about {topic}")
    + ", make it funny"
    + "\n\nand in {language}"
)
prompt
PromptTemplate(input_variables=['language', 'topic'], output_parser=None, partial_variables={}, template='Tell me a joke about {topic}, make it funny\n\nand in {language}', template_format='f-string', validate_template=True)
prompt.format(topic="sports", language="spanish")
'Tell me a joke about sports, make it funny\n\nand in spanish'

You can also use it in an LLMChain, just like before.

from langchain.chains import LLMChain
from langchain_openai import ChatOpenAI
model = ChatOpenAI()
chain = LLMChain(llm=model, prompt=prompt)
chain.run(topic="sports", language="spanish")
'¿Por qué el futbolista llevaba un paraguas al partido?\n\nPorque pronosticaban lluvia de goles.'

Chat prompt composition​

A chat prompt is made up a of a list of messages. Purely for developer experience, we’ve added a convinient way to create these prompts. In this pipeline, each new element is a new message in the final prompt.

from langchain.schema import AIMessage, HumanMessage, SystemMessage

First, let’s initialize the base ChatPromptTemplate with a system message. It doesn’t have to start with a system, but it’s often good practice

prompt = SystemMessage(content="You are a nice pirate")

You can then easily create a pipeline combining it with other messages or message templates. Use a Message when there is no variables to be formatted, use a MessageTemplate when there are variables to be formatted. You can also use just a string (note: this will automatically get inferred as a HumanMessagePromptTemplate.)

new_prompt = (
    prompt + HumanMessage(content="hi") + AIMessage(content="what?") + "{input}"
)

Under the hood, this creates an instance of the ChatPromptTemplate class, so you can use it just as you did before!

new_prompt.format_messages(input="i said hi")
[SystemMessage(content='You are a nice pirate', additional_kwargs={}),
 HumanMessage(content='hi', additional_kwargs={}, example=False),
 AIMessage(content='what?', additional_kwargs={}, example=False),
 HumanMessage(content='i said hi', additional_kwargs={}, example=False)]

You can also use it in an LLMChain, just like before.

from langchain.chains import LLMChain
from langchain_openai import ChatOpenAI
model = ChatOpenAI()
chain = LLMChain(llm=model, prompt=new_prompt)
chain.run("i said hi")
'Oh, hello! How can I assist you today?'