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Lambda Functions in Python

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

Syntax

The syntax for a lambda function is:

lambda arguments : expression

The expression is executed and the result is returned.

Example

A lambda function that adds 10 to the number passed in as an argument:

x = lambda a : a + 10
print(x(5)) # Output: 15

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 by an unknown number:

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


mydoubler = myfunc(2)
mytripler = myfunc(3)


print(mydoubler(11)) # Output: 22
print(mytripler(11)) # Output: 33

Practical Examples with Built-in Functions

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

Using filter()

The filter() function takes a function and a list as arguments and returns a new list containing elements for which the function returns True.

numbers = [1, 5, 4, 6, 8, 11, 3, 12]


# Filter out only the even numbers
new_list = list(filter(lambda x: (x % 2 == 0), numbers))


print(new_list) # Output: [4, 6, 8, 12]

Using map()

The map() function takes a function and a list and returns a new list which contains items returned by that function for each item.

numbers = [1, 2, 3, 4, 5]


# Double each number in the list
new_list = list(map(lambda x: x * 2, numbers))


print(new_list) # Output: [2, 4, 6, 8, 10]

Using reduce()

The reduce() function (from functools module) performs a repetitive operation over the pairs of an iterable.

from functools import reduce


numbers = [1, 2, 3, 4, 5]


# Sum all numbers in the list
sum_result = reduce(lambda x, y: x + y, numbers)


print(sum_result) # Output: 15

Key Characteristics

  • They are anonymous (no name).
  • They are one-liners.
  • They are used for short-term purposes or functions passed as arguments.
  • They cannot contain statements like return, pass, assert, or raise.