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

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

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

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

Hint

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

Practice

1. What is the main purpose of LangChain Expression Language (LCEL)?

easy

A. To create full applications without coding

B. To write small expressions that control LangChain chains

C. To replace Python in LangChain completely

D. To design user interfaces for LangChain

Solution

1. Step 1: Understand LCEL's role

   LCEL is designed to write expressions that help control how LangChain chains behave.

2. Step 2: Compare options

   Only To write small expressions that control LangChain chains correctly describes LCEL's purpose; others describe unrelated tasks.

3. Final Answer:

   To write small expressions that control LangChain chains -> Option B

4. Quick Check:

   LCEL purpose = control chains with expressions [OK]

Hint: LCEL is for small expressions controlling chains [OK]

Common Mistakes:

• Thinking LCEL replaces full programming
• Confusing LCEL with UI design tools
• Assuming LCEL builds entire apps

2. Which of the following is the correct syntax to use a variable named input in an LCEL expression?

easy

A. {{input}}

B. {input}

C. $input

D. input

Solution

1. Step 1: Recall LCEL variable usage

   In LCEL, variables are used directly by their name without extra symbols.

2. Step 2: Evaluate options

   Only input uses the variable name plainly, which is correct syntax in LCEL.

3. Final Answer:

   input -> Option D

4. Quick Check:

   Variable usage = plain name [OK]

Hint: Use variable names directly without braces or symbols [OK]

Common Mistakes:

• Adding braces or dollar signs around variables
• Using double curly braces like in templates
• Confusing LCEL with other templating syntaxes

3. Given the LCEL expression input + ' world' where input is 'Hello', what is the output?

medium

A. 'input world'

B. 'Hello+ world'

C. 'Hello world'

D. Error: invalid operation

Solution

1. Step 1: Understand the expression

   The expression adds the string ' world' to the variable input which holds 'Hello'.

2. Step 2: Perform string concatenation

   Concatenating 'Hello' + ' world' results in 'Hello world'.

3. Final Answer:

   'Hello world' -> Option C

4. Quick Check:

   String concat = 'Hello world' [OK]

Hint: Adding strings concatenates them in LCEL [OK]

Common Mistakes:

• Treating + as a literal character
• Expecting variable name output instead of value
• Assuming syntax error on string addition

4. What is wrong with this LCEL expression: if input == 'yes' then 'ok' else 'no'?

medium

A. LCEL does not support if-then-else syntax like this

B. The variable name input is invalid

C. The strings should use double quotes instead of single quotes

D. The expression is correct and will work

Solution

1. Step 1: Check LCEL conditional syntax

   LCEL uses ternary-like syntax: condition ? true_value : false_value, not if-then-else.

2. Step 2: Analyze the given expression

   The expression uses if-then-else which is not valid in LCEL, causing a syntax error.

3. Final Answer:

   LCEL does not support if-then-else syntax like this -> Option A

4. Quick Check:

   Conditional syntax = ternary only [OK]

Hint: Use ternary ?: for conditions, not if-then-else [OK]

Common Mistakes:

• Using if-then-else like in other languages
• Assuming single quotes cause errors
• Thinking variable names are restricted

5. How would you write an LCEL expression to return the length of a variable text only if it is not empty, otherwise return 0?

hard

A. text != '' ? length(text) : 0

B. length(text) if text else 0

C. text ? length(text) : 0

D. if text != '' then length(text) else 0

Solution

1. Step 1: Understand LCEL conditional syntax

   LCEL uses ternary syntax: condition ? true_value : false_value.

2. Step 2: Check condition for non-empty string

   To check if text is not empty, use text != ''.

3. Step 3: Combine condition and function

   Use text != '' ? length(text) : 0 to return length if not empty, else 0.

4. Final Answer:

   text != '' ? length(text) : 0 -> Option A

5. Quick Check:

   Conditional length check = text != '' ? length(text) : 0 [OK]

Hint: Use ternary with condition text != '' for non-empty check [OK]

Common Mistakes:

• Using if-then-else syntax
• Assuming text alone is false if empty
• Using Python-style if expressions