Lambda Functions

A lambda is a small, unnamed function defined in a single expression. They exist for situations where writing a full def function would be overkill for something you'll only use once, right where it's defined.

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

25

lambda x: x * x means "a function that takes x and returns x * x" — functionally identical to writing def square(x): return x * x, just condensed to one line. Lambdas can only contain a single expression, not multiple statements.

Where lambdas actually get used

You'll see lambdas most often passed directly into functions like map() (transform every item) and filter() (keep only items matching a condition), where defining a separate named function would be unnecessary ceremony for something this small.

numbers = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x * x, numbers))
print(squared)

evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens)

[1, 4, 9, 16, 25]
[2, 4]

Next: list comprehensions — Python's more idiomatic alternative to map() and filter() for exactly this kind of thing.