Lambda Functions

Introduction

A lambda is a small, anonymous function defined with the lambda keyword. It can take any number of arguments but returns only one expression.

Syntax

lambda arguments: expression

Basic Examples

# Regular function vs lambda
def square(x):
    return x ** 2

square_lambda = lambda x: x ** 2

print(square(5))        # 25
print(square_lambda(5)) # 25

Common Use Cases

With map()

numbers = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x ** 2, numbers))
print(squared)  # [1, 4, 9, 16, 25]

With filter()

numbers = [1, 2, 3, 4, 5, 6, 7, 8]
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens)  # [2, 4, 6, 8]

With sorted()

students = [
    {"name": "Alice", "grade": 85},
    {"name": "Bob", "grade": 72},
    {"name": "Charlie", "grade": 95},
]

sorted_students = sorted(students, key=lambda s: s["grade"], reverse=True)
print(sorted_students)
# [{'name': 'Charlie', 'grade': 95}, {'name': 'Alice', 'grade': 85}, {'name': 'Bob', 'grade': 72}]

reduce()

from functools import reduce

numbers = [1, 2, 3, 4, 5]
product = reduce(lambda x, y: x * y, numbers)
print(product)  # 120

When Lambdas Make Sense

# ✅ Good: Simple operation for a sorting key
data.sort(key=lambda x: x["timestamp"])

# ✅ Good: Short callback or event handler
button.on_click(lambda: print("Clicked!"))

# ❌ Bad: Complex logic in a lambda
# Don't do this:
process = lambda x: x ** 2 if x > 0 else (abs(x) if x < 0 else 0)

# Do this instead:
def process(x):
    if x > 0:
        return x ** 2
    elif x < 0:
        return abs(x)
    return 0

Limitations

Can only contain expressions, not statements
No return, if/elif/else (only ternary expressions), loops, or try/except
No annotations/type hints
Less readable for complex operations

Best Practices

Use lambdas for simple operations only — one line, no side effects
Prefer regular functions for anything complex or reusable
Use list comprehensions instead of map()/filter() with lambdas when possible
Don’t assign lambdas to variables — use def instead

Practice Exercises

Exercise 1: Use a lambda with sorted() to sort a list of tuples by the second element.

Exercise 2: Use map() and a lambda to convert a list of Celsius temperatures to Fahrenheit.

Exercise 3: Use filter() and a lambda to extract all words longer than 5 characters from a list.