Lambda functions

2 min read 5 runnable examples Quiz included

A lambda is a small function written in a single expression, without a name.

Python

square = lambda n: n * n
print(square(6))

add = lambda a, b: a + b
print(add(2, 3))

lambda n: n * n means "a function that takes n and returns n * n". It is exactly the same as:

Python

def square(n):
    return n * n
print(square(6))

A lambda can only contain one expression, whose value is returned automatically. No statements, no loops, no return keyword.

Where lambdas shine: as arguments

Assigning a lambda to a name, as above, is legal but pointless; use def for named functions. Lambdas are for when a function needs to be passed somewhere briefly, especially as a key for sorting:

Python

words = ["banana", "Kiwi", "apple", "cherry"]

print(sorted(words))                          # default: capitals first
print(sorted(words, key=lambda w: w.lower())) # ignore case
print(sorted(words, key=lambda w: len(w)))    # by length
print(sorted(words, key=lambda w: w[-1]))     # by last letter

The key function is called on each item, and the items are sorted by what it returns.

Sorting records

Python

students = [
    {"name": "Asha", "grade": 92},
    {"name": "Leo", "grade": 78},
    {"name": "Mei", "grade": 85},
]
for s in sorted(students, key=lambda s: s["grade"], reverse=True):
    print(s["name"], s["grade"])

best = max(students, key=lambda s: s["grade"])
print("Best:", best["name"])

max and min accept a key too.

Sorting by two things

Return a tuple from the key to sort by one value, then break ties with another:

Python

people = [("Mei", 24), ("Leo", 30), ("Asha", 24), ("Raj", 30)]
print(sorted(people, key=lambda p: (p[1], p[0])))    # by age, then name
print(sorted(people, key=lambda p: (-p[1], p[0])))   # oldest first, then name

Finished this lesson?Mark it complete to earn +20 XP and move your progress on.