In Python, a lambda function is a special type of function without the function name. For example,

lambda : print('Hello World')

Here, we have created a lambda function that prints 'Hello World'.

Before you learn about lambdas, make sure to know about Python Functions.

Python lambda Function Declaration

We use the lambda keyword instead of def to create a lambda function. Here’s the syntax to declare the lambda function:

lambda argument(s) : expression 

Here,

• argument(s) – any value passed to the lambda function
• expression – expression is executed and returned

Let’s see an example,

greet = lambda : print('Hello World')

Here, we have defined a lambda function and assigned it to the variable named greet.

To execute this lambda function, we need to call it. Here’s how we can call the lambda function

# call the lambda
greet()

The lambda function above simply prints the text 'Hello World'.

Note: This lambda function doesn’t have any arguments.

Example: Python lambda Function

# declare a lambda function
greet = lambda : print('Hello World')

# call lambda function
greet()

# Output: Hello World

In the above example, we have defined a lambda function and assigned it to the greet variable.

When we call the lambda function, the print() statement inside the lambda function is executed.

Python lambda Function with an Argument

Similar to normal functions, the lambda function can also accept arguments. For example,

# lambda that accepts one argument
greet_user = lambda name : print('Hey there,', name)

# lambda call
greet_user('Delilah')

# Output: Hey there, Delilah

In the above example, we have assigned a lambda function to the greet_user variable.

Here, name after the lambda keyword specifies that the lambda function accepts the argument named name.

Notice the call of lambda function,

greet_user('Delilah')

Here, we have passed a string value 'Delilah' to our lambda function.

And finally, the statement inside the lambda function is executed.

# Program to filter out only the even items from a list
my_list = [1, 5, 4, 6, 8, 11, 3, 12]

new_list = list(filter(lambda x: (x%2 == 0) , my_list))

print(new_list)

# Output: [4, 6, 8, 12]

# Program to double each item in a list using map()

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

new_list = list(map(lambda x: x * 2 , my_list))

print(new_list)

# Output: [2, 10, 8, 12, 16, 22, 6, 24]