Lambda Functions

Lambda Functions

A lambda is an anonymous, single-expression function. It’s useful for short, throwaway functions — especially as arguments to map, filter, sorted, etc.

Syntax

lambda arguments: expression

Basic Examples

square = lambda x: x ** 2
print(square(5))   # 25

add = lambda a, b: a + b
print(add(3, 4))   # 7

greet = lambda name: f"Hello, {name}!"
print(greet("Alice"))  # Hello, Alice!

With sorted()

people = [("Alice", 30), ("Bob", 25), ("Charlie", 35)]

# Sort by age
sorted_people = sorted(people, key=lambda p: p[1])
# [('Bob', 25), ('Alice', 30), ('Charlie', 35)]

# Sort dicts by value
scores = {"Alice": 90, "Bob": 75, "Charlie": 85}
sorted_scores = sorted(scores.items(), key=lambda x: x[1], reverse=True)

With map()

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

With filter()

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

With reduce()

from functools import reduce

nums = [1, 2, 3, 4, 5]
product = reduce(lambda a, b: a * b, nums)
# 120

Lambda vs Regular Function

# Lambda
double = lambda x: x * 2

# Equivalent regular function
def double(x):
    return x * 2

Use a regular def when:

• The logic spans multiple lines
• You need a docstring
• You need to reuse it in multiple places
• Readability matters more than brevity

Immediately Invoked Lambda

result = (lambda x, y: x + y)(3, 4)
print(result)  # 7

Limitations

• Only a single expression — no statements, no if/else blocks (ternary is OK)
• No return keyword
• No type annotations
• Harder to debug (shows as <lambda> in tracebacks)

# Ternary in lambda is fine
classify = lambda x: "even" if x % 2 == 0 else "odd"
print(classify(4))  # even