ChatPromptTemplate for conversations in LangChain - Step-by-Step Execution

Concept Flow - ChatPromptTemplate for conversations

Define Template Variables

↓

Create ChatPromptTemplate

↓

Input User Data

↓

Format Prompt with Variables

↓

Send Prompt to Language Model

↓

Receive and Use Response

This flow shows how you define variables, create a ChatPromptTemplate, input data, format the prompt, send it to the model, and get a response.

Execution Sample

LangChain

from langchain.prompts import ChatPromptTemplate, SystemMessagePromptTemplate, HumanMessagePromptTemplate

chat_prompt = ChatPromptTemplate.from_messages([
    SystemMessagePromptTemplate.from_template("You are a helpful assistant."),
    HumanMessagePromptTemplate.from_template("Hello, {name}!")
])

formatted = chat_prompt.format_prompt(name="Alice")
print(formatted.to_messages())

This code creates a chat prompt template with a system and human message, fills in the name variable, and prints the formatted messages.

Execution Table

Step	Action	Input/Variable	Result/Output
1	Define system message template	Template: 'You are a helpful assistant.'	SystemMessagePromptTemplate created
2	Define human message template	Template: 'Hello, {name}!'	HumanMessagePromptTemplate created
3	Create ChatPromptTemplate	Messages: [system, human]	ChatPromptTemplate instance created
4	Format prompt with name='Alice'	name='Alice'	Prompt filled with 'Hello, Alice!'
5	Convert formatted prompt to messages	Formatted prompt	List of messages ready to send to model
6	Print messages	Messages list	[SystemMessage('You are a helpful assistant.'), HumanMessage('Hello, Alice!')]

💡 All steps complete, prompt formatted and ready for model input

Variable Tracker

Variable	Start	After Step 4	Final
chat_prompt	None	ChatPromptTemplate instance	ChatPromptTemplate instance
name	None	"Alice"	"Alice"
formatted	None	FormattedPrompt with messages	FormattedPrompt with messages
messages	None	None	[SystemMessage, HumanMessage with 'Alice']

Key Moments - 3 Insights

Why do we use {name} inside the human message template?

The {name} is a placeholder variable that gets replaced with actual data ('Alice') during formatting, as shown in step 4 of the execution_table.

What does format_prompt() do exactly?

format_prompt() fills all template variables with provided values and prepares the messages for the language model, as seen in step 4 and 5.

Why do we convert the formatted prompt to messages before sending?

The language model expects a list of message objects, so formatted.to_messages() converts the filled template into that format, shown in step 5.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after step 4?

A ChatPromptTemplate instance created

B Prompt filled with 'Hello, Alice!'

C List of messages ready to send

D SystemMessagePromptTemplate created

💡 Hint

Check the 'Result/Output' column for step 4 in the execution_table

At which step do we create the ChatPromptTemplate instance?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look for 'ChatPromptTemplate instance created' in the execution_table

If we change the name variable to 'Bob', what changes in the execution_table?

A Step 5 no longer converts to messages

B Step 3 creates a different template

C Step 4 output changes to 'Hello, Bob!'

D Step 1 system message changes

💡 Hint

Variable 'name' affects the formatted prompt output at step 4

Concept Snapshot

ChatPromptTemplate lets you create chat message templates with placeholders.
Use from_messages() to combine system and human templates.
Fill variables with format_prompt() to get ready-to-send messages.
Convert with to_messages() before sending to the language model.
This helps keep conversations dynamic and reusable.

Full Transcript

This visual execution trace shows how to use ChatPromptTemplate in langchain for conversations. First, you define message templates with placeholders like {name}. Then you create a ChatPromptTemplate combining these messages. When you have actual data, you call format_prompt() with variables to fill placeholders. This produces a formatted prompt object. You convert it to a list of messages with to_messages(), which you can send to a language model. The execution table walks through each step, showing how templates become ready messages. The variable tracker shows how variables like 'name' change from None to 'Alice'. Key moments clarify why placeholders are used and how formatting works. The quiz tests understanding of each step and variable effects. This method keeps chat prompts flexible and easy to reuse.

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