Day 15/100: Lambda Functions – Python’s Anonymous One-Liners

This content originally appeared on DEV Community and was authored by Rahul Gupta

Welcome to Day 15 of the 100 Days of Python series!
Today, we explore lambda functions — short, simple, and anonymous functions that fit in one line of code. They’re great for quick calculations, especially when paired with functions like map(), filter(), and sorted().

Let’s break it down with examples and real-world use cases.

What You’ll Learn

• What a lambda function is
• Syntax of a lambda
• How it’s different from def
• Real-world use cases
• Common mistakes to avoid

What Is a Lambda Function?

A lambda function is a small anonymous function (no name). It’s often used when you need a quick function for a short task.

Syntax:

lambda arguments: expression

It can have any number of arguments, but only one expression.

Example 1: Basic Lambda Function

square = lambda x: x * x
print(square(5))  # Output: 25

This is equivalent to:

def square(x):
    return x * x

Example 2: Multiple Arguments

add = lambda a, b: a + b
print(add(3, 7))  # Output: 10

Common Use: Inside map(), filter(), sorted()

Using with map()

numbers = [1, 2, 3, 4]
squares = list(map(lambda x: x**2, numbers))
print(squares)  # [1, 4, 9, 16]

Using with filter()

nums = [5, 12, 17, 18, 24, 3]
evens = list(filter(lambda x: x % 2 == 0, nums))
print(evens)  # [12, 18, 24]

Using with sorted() (custom key)

students = [("John", 88), ("Alice", 92), ("Bob", 75)]
sorted_students = sorted(students, key=lambda x: x[1], reverse=True)
print(sorted_students)
# [('Alice', 92), ('John', 88), ('Bob', 75)]

Real-World Example: Tip Calculator

calculate_tip = lambda bill, percent: round(bill * percent / 100, 2)
print(calculate_tip(200, 10))  # Output: 20.0

When to Use Lambda Functions

Use lambdas when:

• You need a short function for a quick task
• You don’t want to define a full function using def
• You’re using functions like map(), filter(), sorted()

Avoid lambdas when:

• The logic is complex
• You need multiple lines
• You need reusability or debugging support

Common Mistakes

• Trying to write multiple lines: Lambdas only support single expressions
• Overusing lambdas: Makes code harder to read and maintain
• Using lambdas when def is more appropriate

lambda vs def

Recap

Today you learned:

• What lambda functions are and how to write them
• The syntax: lambda args: expression
• How to use them with map(), filter(), and sorted()
• When and when not to use lambdas
• Real-world examples like tip calculators and sorting

This content originally appeared on DEV Community and was authored by Rahul Gupta