PromptTemplate basics in LangChain - Mini Project: Build & Apply

PromptTemplate basics

📖 Scenario: You are building a simple chatbot that uses a prompt template to ask questions. The prompt template will help you create messages with placeholders for user input.

🎯 Goal: Create a PromptTemplate that takes a user's name and question, then formats a message asking the question politely.

📋 What You'll Learn

Create a PromptTemplate with input variables name and question.

Set the template string to: 'Hello {name}, can you please answer: {question}?'

Use the format method to fill in the template with example values.

Print the formatted prompt message.

💡 Why This Matters

🌍 Real World

PromptTemplate helps create dynamic messages for chatbots, AI assistants, or any system that needs to ask questions or generate text with user input.

💼 Career

Understanding PromptTemplate is useful for AI developers, chatbot builders, and anyone working with language models to create flexible and reusable prompts.

1

Import PromptTemplate and create the template

Import PromptTemplate from langchain.prompts and create a variable called template that is a PromptTemplate with input variables name and question, and the template string set to 'Hello {name}, can you please answer: {question}?' .

LangChain

# Import PromptTemplate and create the template
# Your code here

Hint

Use PromptTemplate(input_variables=[...], template=...) to create the template.

2

Create example input values

Create a dictionary called inputs with keys 'name' and 'question'. Set 'name' to 'Alice' and 'question' to 'What is the weather today'.

LangChain

from langchain.prompts import PromptTemplate

template = PromptTemplate(input_variables=["name", "question"], template="Hello {name}, can you please answer: {question}?")

# Create the inputs dictionary
# Your code here

Hint

Use a dictionary with exact keys and values as shown.

3

Format the prompt with inputs

Use the format method of template with the inputs dictionary to create a variable called formatted_prompt.

LangChain

from langchain.prompts import PromptTemplate

template = PromptTemplate(input_variables=["name", "question"], template="Hello {name}, can you please answer: {question}?")

inputs = {"name": "Alice", "question": "What is the weather today"}

# Format the prompt using the inputs
# Your code here

Hint

Use template.format(**inputs) to fill the template.

4

Print the formatted prompt

Write a line to print the variable formatted_prompt.

LangChain

from langchain.prompts import PromptTemplate

template = PromptTemplate(input_variables=["name", "question"], template="Hello {name}, can you please answer: {question}?")

inputs = {"name": "Alice", "question": "What is the weather today"}

formatted_prompt = template.format(**inputs)

# Print the formatted prompt
# Your code here

Hint

Use print(formatted_prompt) to show the final message.

Practice

1. What is the main purpose of PromptTemplate in langchain?

easy

A. To create message templates with placeholders for dynamic content

B. To execute machine learning models directly

C. To store data in a database

D. To visualize data in charts

Solution

1. Step 1: Understand the role of PromptTemplate

   PromptTemplate is designed to create text templates that include placeholders for variables.

2. Step 2: Identify its main use

   It helps organize prompts by allowing you to fill in placeholders later, making prompt reuse easier.

3. Final Answer:

   To create message templates with placeholders for dynamic content -> Option A

4. Quick Check:

   PromptTemplate = Templates with placeholders [OK]

Hint: PromptTemplate = template with blanks to fill [OK]

Common Mistakes:

• Thinking PromptTemplate runs models
• Confusing it with data storage
• Assuming it creates visual charts

2. Which of the following is the correct way to create a PromptTemplate with a placeholder named name?

easy

A. PromptTemplate.from_template("Hello, <name>!")

B. PromptTemplate.from_template("Hello, $name!")

C. PromptTemplate.from_template("Hello, {name}!")

D. PromptTemplate.from_template("Hello, %name%!")

Solution

1. Step 1: Recall placeholder syntax in PromptTemplate

   Langchain uses curly braces {} to mark placeholders in templates.

2. Step 2: Match the syntax to the options

   Only PromptTemplate.from_template("Hello, {name}!") uses curly braces correctly: {name}.

3. Final Answer:

   PromptTemplate.from_template("Hello, {name}!") -> Option C

4. Quick Check:

   Placeholders use curly braces {} [OK]

Hint: Placeholders always use curly braces {} [OK]

Common Mistakes:

• Using $ or % instead of {} for placeholders
• Using angle brackets <> which are invalid
• Confusing placeholder syntax with other languages

3. Given the code:

from langchain.prompts import PromptTemplate
prompt = PromptTemplate.from_template("Hello, {name}!")
result = prompt.format(name="Alice")
print(result)

What will be printed?

medium

A. Hello, {name}!

B. Hello, Alice!

C. Hello, name!

D. Error: missing argument

Solution

1. Step 1: Understand the template and format method

   The template has a placeholder {name}. The format method fills it with the value "Alice".

2. Step 2: Determine the output of print(result)

   After formatting, the placeholder is replaced, so the output is "Hello, Alice!".

3. Final Answer:

   Hello, Alice! -> Option B

4. Quick Check:

   format() replaces {name} with "Alice" [OK]

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

Common Mistakes:

• Printing template without formatting
• Confusing placeholder name with literal text
• Expecting an error without missing arguments

4. What is the error in this code snippet?

from langchain.prompts import PromptTemplate
prompt = PromptTemplate.from_template("Hi, {user}!")
result = prompt.format(name="Bob")
print(result)

medium

A. KeyError because 'user' placeholder is not provided

B. No error; output is 'Hi, Bob!'

C. SyntaxError due to wrong placeholder syntax

D. TypeError because format() is missing arguments

Solution

1. Step 1: Check placeholder and format argument names

   The template uses {user} but format() provides name="Bob" which does not match.

2. Step 2: Understand the error caused

   Since {user} is not given a value, a KeyError occurs during formatting.

3. Final Answer:

   KeyError because 'user' placeholder is not provided -> Option A

4. Quick Check:

   Placeholder name must match format() keys [OK]

Hint: Placeholder and format keys must match exactly [OK]

Common Mistakes:

• Assuming format() keys can differ from placeholders
• Thinking it's a syntax error
• Expecting no error when keys mismatch

5. You want to create a PromptTemplate that asks for a user's name and age, then formats a greeting. Which code correctly defines and uses this template?

hard

A. prompt = PromptTemplate.from_template("Hello {name}, you are {age} years old.")
result = prompt.format(user="Eva", years=30)
print(result)

B. prompt = PromptTemplate.from_template("Hello {name}, you are {age} years old.")
result = prompt.format(name="Eva")
print(result)

C. prompt = PromptTemplate.from_template("Hello {name}, you are {age} years old.")
result = prompt.format("Eva", 30)
print(result)

D. prompt = PromptTemplate.from_template("Hello {name}, you are {age} years old.")
result = prompt.format(name="Eva", age=30)
print(result)

Solution

1. Step 1: Check template placeholders

   The template has placeholders {name} and {age} which must be provided as keyword arguments in format().

2. Step 2: Verify format() usage

   prompt = PromptTemplate.from_template("Hello {name}, you are {age} years old.") result = prompt.format(name="Eva", age=30) print(result) correctly passes name="Eva" and age=30 matching placeholders. Others miss arguments or use wrong keys or positional args.

3. Final Answer:

   Correctly defines and uses placeholders with matching keys -> Option D

4. Quick Check:

   Match placeholders and format() keys exactly [OK]

Hint: Pass all placeholders as keyword args in format() [OK]

Common Mistakes:

• Missing required placeholder arguments
• Using wrong argument names in format()
• Passing positional args instead of keywords