Lambda Functions

Introduction

A lambda function is a small, anonymous function written in a single expression. They are useful when you need a short function for a brief purpose — most often as an argument to another function.

Syntax

lambda parameters: expression

The expression is evaluated and returned automatically — no return keyword needed.

Basic Example

# Standard function
def square(n):
    return n ** 2

# Equivalent lambda
square = lambda n: n ** 2

print(square(5))   # 25

Where Lambdas Are Most Useful

Lambdas shine when passed directly to functions like sorted(), map(), and filter().

sorted() with a key

employees = [
    {"name": "Charlie", "salary": 55000},
    {"name": "Alice",   "salary": 80000},
    {"name": "Bob",     "salary": 65000},
]

# Sort by salary
by_salary = sorted(employees, key=lambda e: e["salary"])
for emp in by_salary:
    print(emp["name"], emp["salary"])

Output

Charlie 55000
Bob 65000
Alice 80000

map()

Apply a function to every item in an iterable.

salaries = [50000, 65000, 80000]
after_tax = list(map(lambda s: s * 0.8, salaries))
print(after_tax)   # [40000.0, 52000.0, 64000.0]

filter()

Keep only the items where the function returns True.

salaries = [30000, 55000, 80000, 45000, 90000]
high = list(filter(lambda s: s > 60000, salaries))
print(high)   # [80000, 90000]

Lambda vs def

	def	lambda
Lines	Multiple	One
Name	Required	Optional
Docstring	✅	❌
Best for	Complex reusable logic	Short one-off functions

If a lambda grows beyond a simple expression, replace it with a proper def. Readability matters more than brevity.

Practice Exercises

• Use sorted() with a lambda to sort a list of product dictionaries by price, descending.
• Use map() with a lambda to convert a list of prices from GBP to USD (multiply by 1.27).
• Use filter() with a lambda to keep only employees whose department is "Engineering".
• Rewrite the filter() example above using a list comprehension. Which do you prefer?