Lambda functions

A lambda is a small anonymous function written in a single expression. It is handy when you need a function for a moment, usually to pass into another function like sorted or map.

Syntax

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

lambda is followed by parameters, a colon and one expression. The expression's value is returned automatically. There is no return keyword and no multi line body.

The same thing with def:

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

If you are going to assign a lambda to a name, use def instead. Lambdas are for the cases below, where the function is used in place.

Multiple parameters

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

As a sort key

This is the most common use.

people = [("Zara", 31), ("Amit", 25), ("Neel", 28)]
print(sorted(people, key=lambda p: p[1]))
print(max(people, key=lambda p: p[1]))

With map and filter

map applies a function to each item; filter keeps the items where the function returns true.

nums = [1, 2, 3, 4, 5, 6]
print(list(map(lambda n: n * 10, nums)))
print(list(filter(lambda n: n % 2 == 0, nums)))

Comprehensions usually read better for these two:

nums = [1, 2, 3, 4, 5, 6]
print([n * 10 for n in nums])
print([n for n in nums if n % 2 == 0])

Returning a lambda from a function

A function can build and return a customised function.

def multiplier(factor):
    return lambda x: x * factor

double = multiplier(2)
triple = multiplier(3)
print(double(7), triple(7))

Conditional expression inside a lambda

label = lambda n: "even" if n % 2 == 0 else "odd"
print(label(4), label(9))

When not to use lambda

• If it needs more than one expression, it is a def.
• If it needs a docstring or a meaningful name, it is a def.
• If you find yourself writing lambda x: str(x), just pass str.

Practice

1. Sort ["banana", "fig", "apple", "kiwi"] by word length using a lambda.
2. Use filter with a lambda to keep only the names that start with "A" in a list.
3. Write make_adder(n) that returns a lambda which adds n to its argument.