LangChain Expression Language (LCEL) basics - Mini Project: Build & Apply

LangChain Expression Language (LCEL) Basics

📖 Scenario: You are building a simple LangChain application that uses LangChain Expression Language (LCEL) to process and transform data dynamically. LCEL lets you write expressions to manipulate data inside LangChain chains.

🎯 Goal: Build a basic LCEL expression that accesses data fields, uses a condition, and formats output in a LangChain chain.

📋 What You'll Learn

Create a dictionary called user_data with keys name, age, and city with exact values

Create a variable called age_limit set to 18

Write an LCEL expression string called expression that checks if age is greater than age_limit and returns a greeting message

Create a LangChain ExpressionChain using the expression and run it with user_data

💡 Why This Matters

🌍 Real World

LCEL expressions let you write flexible, dynamic logic inside LangChain chains without writing full Python code. This is useful for chatbots, data processing, and automation where you want to customize behavior easily.

💼 Career

Understanding LCEL basics helps you build smarter LangChain applications that can adapt to different inputs and conditions, a valuable skill for AI developers and automation engineers.

1

Create the user data dictionary

Create a dictionary called user_data with these exact entries: 'name': 'Alice', 'age': 20, and 'city': 'New York'.

LangChain

# Create the user_data dictionary with name, age, and city
# Your code here

Need a hint?

Use curly braces to create a dictionary with the exact keys and values.

2

Set the age limit variable

Create a variable called age_limit and set it to the integer 18.

LangChain

user_data = {'name': 'Alice', 'age': 20, 'city': 'New York'}
# Create the age_limit variable and set it to 18
# Your code here

Need a hint?

Just assign the number 18 to the variable age_limit.

3

Write the LCEL expression string

Create a string variable called expression that uses LCEL syntax to check if age is greater than age_limit. If true, it should return the string 'Hello, {name} from {city}!' with name and city replaced by the values from user_data. Otherwise, return 'Access denied'. Use LCEL conditional syntax.

LangChain

user_data = {'name': 'Alice', 'age': 20, 'city': 'New York'}
age_limit = 18
# Write the LCEL expression string below
# Your code here

Need a hint?

Use the LCEL ternary conditional syntax: ${condition ? true_value : false_value}. Use backticks for string interpolation inside the true part.

4

Create and run the ExpressionChain

Import ExpressionChain from langchain.chains. Create an ExpressionChain instance called chain using the expression string. Then run chain.run() with user_data as input.

LangChain

user_data = {'name': 'Alice', 'age': 20, 'city': 'New York'}
age_limit = 18
expression = "${age > age_limit ? `Hello, ${name} from ${city}!` : 'Access denied'}"
# Import ExpressionChain and create chain with expression
# Run chain with user_data
# Your code here

Need a hint?

Remember to import ExpressionChain first. Then create the chain with the expression string. Finally, run the chain passing user_data.