ChatPromptTemplate for conversations in LangChain - Mini Project: Build & Apply

Create a ChatPromptTemplate for Conversations with Langchain

📖 Scenario: You are building a chatbot that uses Langchain to manage conversations. You want to create a prompt template that formats the chat messages properly before sending them to the language model.

🎯 Goal: Build a ChatPromptTemplate that takes a list of messages and formats them for a conversation with a user and an assistant.

📋 What You'll Learn

Create a list of messages with exact roles and content

Define a variable for the template format

Use ChatPromptTemplate.from_messages with the correct message classes

Add the final call to format the messages with input variables

💡 Why This Matters

🌍 Real World

Chatbots and virtual assistants use prompt templates to organize conversations before sending them to AI models.

💼 Career

Understanding how to build and format chat prompts is essential for AI developers and software engineers working with conversational AI.

1

Create the initial list of messages

Create a list called messages with two dictionaries: the first with 'role': 'system' and 'content': 'You are a helpful assistant.', the second with 'role': 'user' and 'content': 'Hello, who are you?'.

LangChain

# Create the messages list with system and user roles
# Your code here

Hint

Use a list of dictionaries with keys 'role' and 'content'.

2

Define the template format string

Create a variable called template and set it to the string '{role}: {content}' to format each message.

LangChain

messages = [
    {'role': 'system', 'content': 'You are a helpful assistant.'},
    {'role': 'user', 'content': 'Hello, who are you?'}
]
# Define the template string
# Your code here

Hint

Use a simple string with placeholders {role} and {content}.

3

Create the ChatPromptTemplate from messages

Import ChatPromptTemplate and SystemMessagePromptTemplate, HumanMessagePromptTemplate from langchain.prompts.chat. Then create a variable called chat_prompt using ChatPromptTemplate.from_messages with a list containing SystemMessagePromptTemplate.from_template(template) and HumanMessagePromptTemplate.from_template(template).

LangChain

messages = [
    {'role': 'system', 'content': 'You are a helpful assistant.'},
    {'role': 'user', 'content': 'Hello, who are you?'}
]
template = '{role}: {content}'

# Import classes and create chat_prompt from messages
# Your code here

Hint

Use the from langchain.prompts.chat import ... statement and call ChatPromptTemplate.from_messages with the correct message templates.

4

Format the chat prompt with input variables

Call chat_prompt.format_prompt with keyword arguments role='user' and content='What can you do?' and assign the result to a variable called formatted_prompt.

LangChain

from langchain.prompts.chat import ChatPromptTemplate, SystemMessagePromptTemplate, HumanMessagePromptTemplate

messages = [
    {'role': 'system', 'content': 'You are a helpful assistant.'},
    {'role': 'user', 'content': 'Hello, who are you?'}
]
template = '{role}: {content}'

chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template(template),
    HumanMessagePromptTemplate.from_template(template)
])

# Format the prompt with role and content
# Your code here

Hint

Call format_prompt on chat_prompt with the exact keyword arguments.

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