ChatPromptTemplate for conversations in LangChain

Introduction

A ChatPromptTemplate helps you create clear and organized messages for chatbots or AI conversations. It makes building conversations easier and more consistent.

When you want to build a chatbot that talks with users step-by-step.

When you need to organize different parts of a conversation like system instructions and user questions.

When you want to reuse conversation templates with different inputs.

When you want to keep your chat messages clear and easy to update.

When you want to combine multiple messages into one conversation flow.

Syntax

LangChain

from langchain.prompts import ChatPromptTemplate, SystemMessagePromptTemplate, HumanMessagePromptTemplate

chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template("{system_instructions}"),
    HumanMessagePromptTemplate.from_template("{user_input}")
])

Use from_messages to combine different message templates into one chat prompt.

Templates use curly braces {} to mark where input values go.

Examples

This creates a chat prompt with a system message and a fixed user message.

LangChain

chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template("You are a helpful assistant."),
    HumanMessagePromptTemplate.from_template("Hello! Can you help me?")
])

This template lets you insert the city name dynamically when running the chat.

LangChain

chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template("You are a friendly assistant."),
    HumanMessagePromptTemplate.from_template("What is the weather in {city}?")
])

Sample Program

This example creates a chat prompt template with a system message and a user message that asks about a topic. Then it fills in the topic and prints the conversation messages.

LangChain

from langchain.prompts import ChatPromptTemplate, SystemMessagePromptTemplate, HumanMessagePromptTemplate

# Create a chat prompt template with placeholders
chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template("You are a helpful assistant."),
    HumanMessagePromptTemplate.from_template("Tell me about {topic}.")
])

# Fill in the placeholder with actual input
filled_prompt = chat_prompt.format_prompt(topic="Python programming")

# Print the messages in the conversation
for message in filled_prompt.messages:
    print(f"{message.type}: {message.content}")

Important Notes

Always use format_prompt to fill in your template with real values before sending it to the AI.

You can add more message types like AI messages if needed for complex conversations.

Keep your templates simple and clear to avoid confusion in the conversation flow.

Summary

ChatPromptTemplate organizes chat messages into a clear conversation structure.

Use placeholders to insert dynamic user inputs easily.

This helps build reusable and consistent chatbots or AI conversations.

Practice

1. What is the main purpose of ChatPromptTemplate in Langchain conversations?

easy

A. To organize chat messages with placeholders for dynamic inputs

B. To execute database queries automatically

C. To style chat messages with CSS

D. To store user credentials securely

Solution

1. Step 1: Understand ChatPromptTemplate role

   ChatPromptTemplate helps structure chat messages and insert dynamic user inputs using placeholders.

2. Step 2: Eliminate unrelated options

   Options about database queries, styling, or security are unrelated to ChatPromptTemplate's purpose.

3. Final Answer:

   To organize chat messages with placeholders for dynamic inputs -> Option A

4. Quick Check:

   ChatPromptTemplate = Organize messages + placeholders [OK]

Hint: ChatPromptTemplate structures chat with placeholders [OK]

Common Mistakes:

• Confusing ChatPromptTemplate with styling or security features
• Thinking it runs queries or handles authentication

2. Which of the following is the correct way to create a ChatPromptTemplate with a user input placeholder named 'name'?

easy

A. ChatPromptTemplate.from_template('Hello {name}!')

B. ChatPromptTemplate('Hello $name!')

C. ChatPromptTemplate.create('Hello {{name}}!')

D. ChatPromptTemplate.new('Hello %name%!')

Solution

1. Step 1: Recall correct placeholder syntax

   Langchain uses curly braces {name} inside from_template method to define placeholders.

2. Step 2: Check method and syntax correctness

   Only ChatPromptTemplate.from_template('Hello {name}!') uses from_template and correct {name} placeholder syntax.

3. Final Answer:

   ChatPromptTemplate.from_template('Hello {name}!') -> Option A

4. Quick Check:

   Use from_template with {placeholder} [OK]

Hint: Use from_template and {placeholder} syntax [OK]

Common Mistakes:

• Using wrong placeholder symbols like $ or % or double braces
• Using non-existent methods like create or new

3. Given this code snippet:

from langchain.prompts import ChatPromptTemplate

prompt = ChatPromptTemplate.from_template('Hi {user}! How can I help you today?')
message = prompt.format_prompt(user='Alice').to_messages()
print(message[0].content)

What will be printed?

medium

A. Hi {user}! How can I help you today?

B. Hi Alice! How can I help you today?

C. Hi ! How can I help you today?

D. Error: format_prompt missing argument

Solution

1. Step 1: Understand placeholder replacement

   The placeholder {user} is replaced by the value 'Alice' passed to format_prompt.

2. Step 2: Check printed message content

   message[0].content contains the formatted string with 'Alice' inserted.

3. Final Answer:

   Hi Alice! How can I help you today? -> Option B

4. Quick Check:

   Placeholder replaced with 'Alice' in output [OK]

Hint: format_prompt replaces placeholders with given values [OK]

Common Mistakes:

• Thinking placeholders remain unreplaced in output
• Assuming missing argument error without passing user
• Confusing message object with string directly

4. What is wrong with this code snippet?

from langchain.prompts import ChatPromptTemplate

prompt = ChatPromptTemplate.from_template('Hello {name}!')
message = prompt.format_prompt().to_messages()

medium

A. Placeholder syntax should use $name instead of {name}

B. Incorrect method name 'from_template'

C. Missing argument for placeholder 'name' in format_prompt

D. to_messages() is not a valid method

Solution

1. Step 1: Identify placeholder usage

   The template has a placeholder {name} that requires a value when formatting.

2. Step 2: Check format_prompt call

   format_prompt() is called without providing the required 'name' argument, causing an error.

3. Final Answer:

   Missing argument for placeholder 'name' in format_prompt -> Option C

4. Quick Check:

   Placeholder needs matching argument in format_prompt [OK]

Hint: Always pass all placeholders to format_prompt [OK]

Common Mistakes:

• Forgetting to pass required placeholder arguments
• Thinking method names or syntax are incorrect
• Assuming to_messages() doesn't exist

5. You want to create a conversation prompt that greets the user by name and asks their favorite color, then stores both inputs dynamically. Which approach using ChatPromptTemplate is correct?

hard

A. Use a template 'Hello {name}! What is your favorite color?' and call format_prompt(name='Bob') only

B. Use two separate ChatPromptTemplates, one for name and one for color, then combine messages manually

C. Use a template 'Hello $name, favorite color $color?' and call format_prompt(name='Bob', color='blue')

D. Use a template like 'Hello {name}, what is your favorite color {color}?' and call format_prompt(name='Bob', color='blue')

Solution

1. Step 1: Understand placeholder usage for multiple inputs

   ChatPromptTemplate supports multiple placeholders in one template string, e.g., {name} and {color}.

2. Step 2: Check correct syntax and method usage

   Use a template like 'Hello {name}, what is your favorite color {color}?' and call format_prompt(name='Bob', color='blue') uses correct curly brace placeholders and passes both arguments in format_prompt.

3. Step 3: Evaluate other options

   Use two separate ChatPromptTemplates, one for name and one for color, then combine messages manually is more complex than needed; Use a template 'Hello {name}! What is your favorite color?' and call format_prompt(name='Bob') only misses the color input; Use a template 'Hello $name, favorite color $color?' and call format_prompt(name='Bob', color='blue') uses wrong placeholder syntax.

4. Final Answer:

   Use a template like 'Hello {name}, what is your favorite color {color}?' and call format_prompt(name='Bob', color='blue') -> Option D

5. Quick Check:

   Multiple placeholders with format_prompt arguments [OK]

Hint: Use {placeholder} for each input and pass all in format_prompt [OK]

Common Mistakes:

• Using wrong placeholder syntax like $name
• Splitting conversation unnecessarily into multiple templates
• Not passing all required inputs to format_prompt