How to beginner · 3 min read

How to format prompt with variables in LangChain

Q: How to format prompt with variables in LangChain

Use ChatPromptTemplate from langchain_core.prompts to define prompts with variables by specifying a template string with placeholders and a list of input variables. Then call format with variable values to generate the final prompt content.

Quick answer

Use ChatPromptTemplate from langchain_core.prompts to define prompts with variables by specifying a template string with placeholders and a list of input variables. Then call format with variable values to generate the final prompt content.

PREREQUISITES

Python 3.8+
OpenAI API key (free tier works)
pip install langchain_openai langchain_core

Setup

Install the required LangChain packages and set your OpenAI API key as an environment variable.

Install LangChain OpenAI and core prompt packages:

bash

pip install langchain_openai langchain_core

Step by step

Define a prompt template with variables using ChatPromptTemplate, then format it with actual values to create a prompt for the model.

python

from langchain_core.prompts import ChatPromptTemplate
from langchain_openai import ChatOpenAI
import os

# Define a prompt template with variables
prompt_template = ChatPromptTemplate.from_template(
    "Translate the following text to {language}: {text}"
)

# Format the prompt with variable values
formatted_prompt = prompt_template.format(language="French", text="Hello, how are you?")

# Initialize the OpenAI chat client
client = ChatOpenAI(model_name="gpt-4o", openai_api_key=os.environ["OPENAI_API_KEY"])

# Send the formatted prompt to the model
response = client.chat([{"role": "user", "content": formatted_prompt}])

print("Prompt sent to model:", formatted_prompt)
print("Model response:", response.content)

output

Prompt sent to model: Translate the following text to French: Hello, how are you?
Model response: Bonjour, comment ça va ?

Common variations

You can use multiple variables in the template, async calls, or different models. For example, use ChatOpenAI with model_name="gpt-4o-mini" or format prompts dynamically in loops.

python

from langchain_core.prompts import ChatPromptTemplate
from langchain_openai import ChatOpenAI
import os

# Template with multiple variables
prompt_template = ChatPromptTemplate.from_template(
    "Summarize the article titled '{title}' in {language}."
)

# Format with variables
prompt = prompt_template.format(title="AI advancements", language="Spanish")

# Async example (requires async environment)
# async def main():
#     client = ChatOpenAI(model_name="gpt-4o-mini", openai_api_key=os.environ["OPENAI_API_KEY"])
#     response = await client.chat.acreate([{"role": "user", "content": prompt}])
#     print(response.content)

# Use different model
client = ChatOpenAI(model_name="gpt-4o-mini", openai_api_key=os.environ["OPENAI_API_KEY"])
response = client.chat([{"role": "user", "content": prompt}])
print(response.content)

output

El artículo titulado 'AI advancements' trata sobre los últimos avances en inteligencia artificial.

Troubleshooting

If you see KeyError when formatting, ensure all variables in the template are provided in format().
If the prompt is not sent correctly, verify your API key is set in os.environ["OPENAI_API_KEY"].
For model errors, confirm you are using a current model name like gpt-4o or gpt-4o-mini.

✅

Key Takeaways

Use ChatPromptTemplate.from_template to create prompts with variables for dynamic content.
Always provide all required variables when calling format() to avoid errors.
Integrate formatted prompts with ChatOpenAI for seamless AI interactions.
You can switch models or use async calls with the same prompt formatting approach.
Set your API key securely in environment variables to avoid authentication issues.

Verified 2026-04 · gpt-4o, gpt-4o-mini

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