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Model I/O

The core element of any language model application is...the model. LangChain gives you the building blocks to interface with any language model.

Flowchart illustrating the Model I/O process with steps Format, Predict, and Parse, showing the transformation from input variables to structured output.


The below quickstart will cover the basics of using LangChain's Model I/O components. It will introduce the two different types of models - LLMs and Chat Models. It will then cover how to use Prompt Templates to format the inputs to these models, and how to use Output Parsers to work with the outputs.

Language models in LangChain come in two flavors:


Chat models are often backed by LLMs but tuned specifically for having conversations. Crucially, their provider APIs use a different interface than pure text completion models. Instead of a single string, they take a list of chat messages as input and they return an AI message as output. See the section below for more details on what exactly a message consists of. GPT-4 and Anthropic's Claude-2 are both implemented as chat models.


LLMs in LangChain refer to pure text completion models. The APIs they wrap take a string prompt as input and output a string completion. OpenAI's GPT-3 is implemented as an LLM.

These two API types have different input and output schemas.

Additionally, not all models are the same. Different models have different prompting strategies that work best for them. For example, Anthropic's models work best with XML while OpenAI's work best with JSON. You should keep this in mind when designing your apps.

For this getting started guide, we will use chat models and will provide a few options: using an API like Anthropic or OpenAI, or using a local open source model via Ollama.

First we'll need to install their partner package:

pip install langchain-openai

Accessing the API requires an API key, which you can get by creating an account and heading here. Once we have a key we'll want to set it as an environment variable by running:

export OPENAI_API_KEY="..."

We can then initialize the model:

from langchain_openai import ChatOpenAI
from langchain_openai import OpenAI

llm = OpenAI()
chat_model = ChatOpenAI(model="gpt-3.5-turbo-0125")

API Reference:

If you'd prefer not to set an environment variable you can pass the key in directly via the api_key named parameter when initiating the OpenAI LLM class:

from langchain_openai import ChatOpenAI
llm = ChatOpenAI(api_key="...")

API Reference:

Both llm and chat_model are objects that represent configuration for a particular model. You can initialize them with parameters like temperature and others, and pass them around. The main difference between them is their input and output schemas. The LLM objects take string as input and output string. The ChatModel objects take a list of messages as input and output a message.

We can see the difference between an LLM and a ChatModel when we invoke it.

from langchain_core.messages import HumanMessage

text = "What would be a good company name for a company that makes colorful socks?"
messages = [HumanMessage(content=text)]

# >> Feetful of Fun

# >> AIMessage(content="Socks O'Color")

API Reference:

The LLM returns a string, while the ChatModel returns a message.

Prompt Templates

Most LLM applications do not pass user input directly into an LLM. Usually they will add the user input to a larger piece of text, called a prompt template, that provides additional context on the specific task at hand.

In the previous example, the text we passed to the model contained instructions to generate a company name. For our application, it would be great if the user only had to provide the description of a company/product without worrying about giving the model instructions.

PromptTemplates help with exactly this! They bundle up all the logic for going from user input into a fully formatted prompt. This can start off very simple - for example, a prompt to produce the above string would just be:

from langchain_core.prompts import PromptTemplate

prompt = PromptTemplate.from_template("What is a good name for a company that makes {product}?")
prompt.format(product="colorful socks")

API Reference:

What is a good name for a company that makes colorful socks?

There are several advantages of using these over raw string formatting. You can "partial" out variables - e.g. you can format only some of the variables at a time. You can compose them together, easily combining different templates into a single prompt. For explanations of these functionalities, see the section on prompts for more detail.

PromptTemplates can also be used to produce a list of messages. In this case, the prompt not only contains information about the content, but also each message (its role, its position in the list, etc.). Here, what happens most often is a ChatPromptTemplate is a list of ChatMessageTemplates. Each ChatMessageTemplate contains instructions for how to format that ChatMessage - its role, and then also its content. Let's take a look at this below:

from import ChatPromptTemplate

template = "You are a helpful assistant that translates {input_language} to {output_language}."
human_template = "{text}"

chat_prompt = ChatPromptTemplate.from_messages([
("system", template),
("human", human_template),

chat_prompt.format_messages(input_language="English", output_language="French", text="I love programming.")

API Reference:

SystemMessage(content="You are a helpful assistant that translates English to French.", additional_kwargs={}),
HumanMessage(content="I love programming.")

ChatPromptTemplates can also be constructed in other ways - see the section on prompts for more detail.

Output parsers

OutputParsers convert the raw output of a language model into a format that can be used downstream. There are a few main types of OutputParsers, including:

  • Convert text from LLM into structured information (e.g. JSON)
  • Convert a ChatMessage into just a string
  • Convert the extra information returned from a call besides the message (like OpenAI function invocation) into a string.

For full information on this, see the section on output parsers.

In this getting started guide, we use a simple one that parses a list of comma separated values.

from langchain.output_parsers import CommaSeparatedListOutputParser

output_parser = CommaSeparatedListOutputParser()
output_parser.parse("hi, bye")
# >> ['hi', 'bye']

Composing with LCEL

We can now combine all these into one chain. This chain will take input variables, pass those to a prompt template to create a prompt, pass the prompt to a language model, and then pass the output through an (optional) output parser. This is a convenient way to bundle up a modular piece of logic. Let's see it in action!

template = "Generate a list of 5 {text}.\n\n{format_instructions}"

chat_prompt = ChatPromptTemplate.from_template(template)
chat_prompt = chat_prompt.partial(format_instructions=output_parser.get_format_instructions())
chain = chat_prompt | chat_model | output_parser
chain.invoke({"text": "colors"})
# >> ['red', 'blue', 'green', 'yellow', 'orange']

Note that we are using the | syntax to join these components together. This | syntax is powered by the LangChain Expression Language (LCEL) and relies on the universal Runnable interface that all of these objects implement. To learn more about LCEL, read the documentation here.


That's it for getting started with prompts, models, and output parsers! This just covered the surface of what there is to learn. For more information, check out:

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