Lambda Functions

A lambda is a small, anonymous, single-line function.

# Regular function
def double(x):
    return x * 2

# Lambda
double = lambda x: x * 2

double(5)  # 10

Syntax

lambda arguments: expression

• lambda — keyword
• arguments — parameters (same as in def)
• expression — the result (one line, returned automatically)

Examples

# Arithmetic
add = lambda a, b: a + b
print(add(3, 5))  # 8

square = lambda x: x ** 2
print(square(4))  # 16

is_even = lambda n: n % 2 == 0
print(is_even(10))  # True

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

# Conditional
max_num = lambda a, b: a if a > b else b
print(max_num(10, 20))  # 20

When to Use

Lambda is convenient inside other functions — as a key= argument or instead of short helper functions.

numbers = [5, 2, 8, 1, 9]
sorted_desc = sorted(numbers, key=lambda x: -x)
print(sorted_desc)  # [9, 8, 5, 2, 1]

doubled = list(map(lambda x: x * 2, numbers))
print(doubled)  # [10, 4, 16, 2, 18]

even = list(filter(lambda x: x % 2 == 0, numbers))
print(even)  # [2, 8]

When NOT to Use

Complex logic — readability suffers:

# Bad
check = lambda x: x > 0 and x < 100 and x % 2 == 0 and str(x)[0] != "5"

# Good — use a regular function
def check_number(x):
    if x <= 0 or x >= 100:
        return False
    if x % 2 != 0:
        return False
    return str(x)[0] != "5"

Documentation needed — lambda doesn’t support docstrings.

Lambda vs def

Practical Examples

products = [
    {"name": "Laptop", "price": 50000},
    {"name": "Mouse", "price": 500},
    {"name": "Keyboard", "price": 3000}
]

# Sort by price
cheap_first = sorted(products, key=lambda p: p["price"])
print(cheap_first[0]["name"])  # Mouse

# Find the most expensive
most_expensive = max(products, key=lambda p: p["price"])
print(most_expensive["name"])  # Laptop

# Words by length
words = ["Python", "Go", "JavaScript", "Rust"]
by_length = sorted(words, key=lambda w: len(w))
print(by_length)  # ['Go', 'Rust', 'Python', 'JavaScript']

Common Mistakes

Mistake 1: Multiple lines — not allowed

# Error — lambda is only one line
calc = lambda x:
    result = x * 2  # SyntaxError
    return result

# Correct
calc = lambda x: x * 2

Mistake 2: Assignment inside a lambda

update = lambda x: x = x + 1  # SyntaxError
# Use def for this

Mistake 3: Saving a lambda to a variable

# Bad — if you're saving it, use def instead
double = lambda x: x * 2
square = lambda x: x ** 2

# Good
def double(x):
    return x * 2

def square(x):
    return x ** 2

Lambda is for one-time use inside another function. If you’re saving it to a variable — use def.