Context formatting and injection in LangChain

Introduction

Context formatting and injection helps you prepare and add useful information to your AI prompts. It makes the AI understand your question better by giving it extra details.

When you want the AI to answer questions using specific background information.

When you need to add user data or previous conversation history into the AI prompt.

When you want to customize how the AI sees the input before generating a response.

When you want to reuse a template with different data for multiple AI calls.

When you want to control the AI's behavior by injecting instructions or context.

Syntax

LangChain

from langchain.prompts import PromptTemplate

# Create a template with placeholders
template = "Hello {name}, today is {day}."

# Format the template with actual values
formatted_prompt = template.format(name="Alice", day="Monday")

Use curly braces {} to mark where data will be inserted.

Call format() with keyword arguments matching the placeholders.

Examples

Inject order ID and delivery date into the prompt.

LangChain

template = "Your order {order_id} will arrive on {date}."
prompt = template.format(order_id="1234", date="Friday")

Inject user name and topic to personalize the prompt.

LangChain

template = "Hi {user}, you asked about {topic}."
prompt = template.format(user="Bob", topic="weather")

Use Langchain's PromptTemplate to manage context and question injection.

LangChain

from langchain.prompts import PromptTemplate

prompt_template = PromptTemplate(
    input_variables=["question", "context"],
    template="Answer the question: {question} using this info: {context}"
)

final_prompt = prompt_template.format(question="What is AI?", context="AI means artificial intelligence.")

Sample Program

This program creates a prompt template with two placeholders: name and task. Then it fills in the placeholders with actual values and prints the final prompt.

LangChain

from langchain.prompts import PromptTemplate

# Define a prompt template with placeholders
prompt_template = PromptTemplate(
    input_variables=["name", "task"],
    template="Hello {name}, your task today is: {task}."
)

# Inject actual values into the template
final_prompt = prompt_template.format(name="Emma", task="write a report")

print(final_prompt)

Important Notes

Always match the placeholder names exactly when formatting.

You can reuse the same template with different data to save time.

Context injection helps the AI give more accurate and relevant answers.

Summary

Context formatting lets you prepare prompts with placeholders.

Injection means filling those placeholders with real data.

This makes AI responses clearer and more useful.