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.

Practice

1. What is the main purpose of context formatting in Langchain?

easy

A. To create database connections

B. To execute code asynchronously

C. To prepare prompts with placeholders for dynamic data

D. To store user credentials securely

Solution

1. Step 1: Understand context formatting

   Context formatting is about creating prompt templates with placeholders that can be filled later.

2. Step 2: Identify the purpose

   This allows dynamic injection of data into prompts, making AI responses relevant.

3. Final Answer:

   To prepare prompts with placeholders for dynamic data -> Option C

4. Quick Check:

   Context formatting = placeholders in prompts [OK]

Hint: Think of placeholders as empty boxes to fill later [OK]

Common Mistakes:

• Confusing context formatting with code execution
• Thinking it stores credentials
• Mixing it with database operations

2. Which of the following is the correct way to inject a variable user_name into a Langchain prompt template?

easy

A. prompt_template.inject(user_name)

B. prompt_template.append(user_name)

C. prompt_template.fill(user_name)

D. prompt_template.format(user_name=user_name)

Solution

1. Step 1: Recall Langchain prompt syntax

   Langchain uses the format() method to fill placeholders in prompt templates.

2. Step 2: Match method to injection

   Only format(user_name=user_name) correctly injects the variable into the placeholder.

3. Final Answer:

   prompt_template.format(user_name=user_name) -> Option D

4. Quick Check:

   Use format() to inject variables [OK]

Hint: Use .format() to fill placeholders in templates [OK]

Common Mistakes:

• Using non-existent methods like inject() or fill()
• Trying to append variables instead of formatting
• Forgetting to name the variable in format()

3. Given this code snippet:

template = "Hello, {name}! Today is {day}."
prompt = template.format(name="Alice", day="Monday")
print(prompt)

What will be the output?

medium

A. Hello, Alice! Today is Monday.

B. Hello, name! Today is day.

C. Hello, {name}! Today is {day}.

D. Error: Missing placeholders

Solution

1. Step 1: Understand the template string

   The template has placeholders {name} and {day} to be replaced by actual values.

2. Step 2: Apply the format method

   Calling format(name="Alice", day="Monday") replaces placeholders with given values.

3. Final Answer:

   Hello, Alice! Today is Monday. -> Option A

4. Quick Check:

   Placeholders replaced by values = output string [OK]

Hint: format() replaces placeholders with given values [OK]

Common Mistakes:

• Printing the template without formatting
• Confusing placeholder names with literal text
• Expecting an error when placeholders are correctly filled

4. Identify the error in this Langchain prompt injection code:

template = "Welcome, {user}!"
prompt = template.format(user)
print(prompt)

medium

A. Incorrect placeholder syntax in template

B. Missing variable name in format() call

C. Using print() instead of return

D. Template string should use double braces {{}}

Solution

1. Step 1: Check format() usage

   The format method requires named arguments matching placeholders or positional arguments in order.

2. Step 2: Identify missing variable name

   Here, format(user) passes a positional argument but the placeholder expects a named argument {user}.

3. Final Answer:

   Missing variable name in format() call -> Option B

4. Quick Check:

   Named placeholders need named arguments in format() [OK]

Hint: Match placeholder names with named format() arguments [OK]

Common Mistakes:

• Passing positional argument without naming it
• Thinking placeholder syntax is wrong
• Confusing print() with return statements

5. You want to create a prompt that greets a user and mentions their favorite color using Langchain. Which code correctly formats and injects the context for user_name = "Bob" and color = "blue"?

hard

A. template = "Hi {user_name}, your favorite color is {color}."
prompt = template.format(user_name=user_name, color=color)

B. template = "Hi {user}, your favorite color is {color}."
prompt = template.format(user_name=user_name, color=color)

C. template = "Hi {user_name}, your favorite color is {color}."
prompt = template.format(user=user_name, color=color)

D. template = "Hi {user_name}, your favorite color is {color}."
prompt = template.fill(user_name, color)

Solution

1. Step 1: Match placeholders with variable names

   The template uses placeholders {user_name} and {color}, so format() must use these exact names.

2. Step 2: Check format() arguments

   template = "Hi {user_name}, your favorite color is {color}." prompt = template.format(user_name=user_name, color=color) correctly passes named arguments matching placeholders: user_name=user_name, color=color.

3. Step 3: Eliminate incorrect options

   template = "Hi {user}, your favorite color is {color}." prompt = template.format(user_name=user_name, color=color) uses {user} placeholder mismatching the user_name= argument. template = "Hi {user_name}, your favorite color is {color}." prompt = template.format(user=user_name, color=color) mismatches placeholder and argument names. template = "Hi {user_name}, your favorite color is {color}." prompt = template.fill(user_name, color) uses non-existent fill() method.

4. Final Answer:

   template = "Hi {user_name}, your favorite color is {color}." prompt = template.format(user_name=user_name, color=color) -> Option A

5. Quick Check:

   Placeholder names must match format() argument names [OK]

Hint: Keep placeholder and format() argument names identical [OK]

Common Mistakes:

• Mismatching placeholder and argument names
• Using wrong method like fill()
• Ignoring placeholder names in template