Python Lambda - W3Schools

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Lambda Functions

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

Syntax

lambda arguments : expression

The expression is executed and the result is returned:

Example

Add 10 to argument a, and return the result:

x = lambda a : a + 10print(x(5)) Try it Yourself »

Lambda functions can take any number of arguments:

Example

Multiply argument a with argument b and return the result:

x = lambda a, b : a * bprint(x(5, 6)) Try it Yourself »

Example

Summarize argument a, b, and c and return the result:

x = lambda a, b, c : a + b + cprint(x(5, 6, 2)) Try it Yourself »

Why Use Lambda Functions?

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

def myfunc(n): return lambda a : a * n

Use that function definition to make a function that always doubles the number you send in:

Example

def myfunc(n): return lambda a : a * n mydoubler = myfunc(2) print(mydoubler(11)) Try it Yourself »

Or, use the same function definition to make a function that always triples the number you send in:

Example

def myfunc(n): return lambda a : a * n mytripler = myfunc(3) print(mytripler(11)) Try it Yourself »

Or, use the same function definition to make both functions, in the same program:

Example

def myfunc(n): return lambda a : a * n mydoubler = myfunc(2)mytripler = myfunc(3) print(mydoubler(11))print(mytripler(11)) Try it Yourself »

Use lambda functions when an anonymous function is required for a short period of time.

Lambda with Built-in Functions

Lambda functions are commonly used with built-in functions like map(), filter(), and sorted().

Using Lambda with map()

The map() function applies a function to every item in an iterable:

Example

Double all numbers in a list:

numbers = [1, 2, 3, 4, 5] doubled = list(map(lambda x: x * 2, numbers)) print(doubled) Try it Yourself »

Using Lambda with filter()

The filter() function creates a list of items for which a function returns True:

Example

Filter out even numbers from a list:

numbers = [1, 2, 3, 4, 5, 6, 7, 8] odd_numbers = list(filter(lambda x: x % 2 != 0, numbers)) print(odd_numbers) Try it Yourself »

Using Lambda with sorted()

The sorted() function can use a lambda as a key for custom sorting:

Example

Sort a list of tuples by the second element:

students = [("Emil", 25), ("Tobias", 22), ("Linus", 28)] sorted_students = sorted(students, key=lambda x: x[1]) print(sorted_students) Try it Yourself »

Example

Sort strings by length:

words = ["apple", "pie", "banana", "cherry"] sorted_words = sorted(words, key=lambda x: len(x)) print(sorted_words) Try it Yourself » ❮ Previous Next ❯ ★ +1 Sign in to track progress