Lambda Function in Python

In Python, lambda keyword is used to declare an anonymous function, which is called as Lambda functions. An anonymous function refers to a function declared with no name. They behave in the same way as regular functions that are declared using the def keyword.

Syntax

Lambda function has following syntax

lambda arguments: expression

where lambda is keyword. It can have any number of arguments but only one expression. The expression is evaluated and returned.

The expression lambda parameters: yields a function object. The unnamed object behaves like a function object defined with:

def <lambda>(arguments):
    return expression

Example

In below example, x and y are argument of lambda function, and x * y is the expression that is evaluated and the result of the expression is returned.

product = lambda x, y : x * y

print(product(2, 3))

# Output
# 6

Like nested function definitions, lambda functions can reference variables from the containing scope. Below example lambda expression access the variable passed to function make_incrementor().

def make_incrementor(n):
    return lambda x: x + n

incrementor = make_incrementor(100)

print(incrementor(-50))

# Output
# 50

Lambda expression return a function, which can be passed as function argument. In the below example, list is sorted based on second value of each element.

pairs = [(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four')]
pairs.sort(key=lambda pair: pair[1])

print(pairs)

# Output
# [(4, 'four'), (1, 'one'), (3, 'three'), (2, 'two')]