Lambda Functions

A lambda is a small, unnamed function limited to a single expression — whatever that expression evaluates to is automatically returned.

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

# equivalent regular function
def square(x):
    return x * x

Lambdas can't contain statements (no if/for blocks, no multiple lines, no return keyword) — only a single expression, though that expression can include a ternary.

classify = lambda n: "even" if n % 2 == 0 else "odd"
print(classify(7))   # 'odd'

Where lambdas actually get used

Almost always as a short throwaway function passed into something else — most commonly sorted()'s key argument, or filter()/map().

videos = [
    {"title": "Intro", "views": 500},
    {"title": "Loops", "views": 2000},
    {"title": "Functions", "views": 1200},
]

by_views = sorted(videos, key=lambda v: v["views"], reverse=True)
for v in by_views:
    print(v["title"], v["views"])

key=lambda v: v["views"] tells sorted() "use this value to compare items" — without it, sorted() would try to compare dictionaries directly, which fails.

map() and filter()

numbers = [1, 2, 3, 4, 5]
doubled = list(map(lambda n: n * 2, numbers))
evens = list(filter(lambda n: n % 2 == 0, numbers))
print(doubled, evens)

In modern Python, a list comprehension ([n * 2 for n in numbers]) is usually considered more readable than map/filter + lambda — but you'll see both styles in real codebases, so it's worth recognizing this pattern even if you reach for comprehensions yourself.