Programming Fundamentals › Functional Programming

Lambda / Anonymous Function

A function without a name, defined inline.

Also known as: anonymous function, lambda expression

A lambda is a function without a name, written inline where you need it. It’s handy for short throwaway functions passed to other functions.

double = lambda x: x * 2
double(4)                                      # 8

sorted(users, key=lambda u: u["age"])          # the usual use
list(map(lambda n: n + 1, [1, 2, 3]))

users.sort((a, b) => a.age - b.age);           // JavaScript: an arrow function

list.removeIf(x -> x < 0);                     // Java lambda

Python’s lambda is limited to a single expression, with no statements. For anything longer, write a normal def.

When to use one

• A short function used once, as an argument: key=, filter, callbacks.
• When naming it would add nothing.

When not to

• Long or complicated logic. A named function is easier to read, test and debug.
• Assigning a lambda to a name (f = lambda x: ...). Just use def f(x): ..., which also gives better stack traces.
• Lambdas created in a loop can capture the variable instead of its value, a classic surprise (closures):

fs = [lambda: i for i in range(3)]
[f() for f in fs]      # [2, 2, 2], not [0, 1, 2]

Lambdas are first-class functions written compactly. In JavaScript, the arrow function plays the same role.