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Lambda functions

This guide introduces you to the concept of lambda functions.

Passing a function as an argument

Consider the following example:

def get_full_name(first_name, last_name, formatter):
    return formatter(first_name, last_name)

The function get_full_name() takes three parameters — a first name, a last name, and a function that formats the first and last name. Python allows you to pass one function as a parameter to a second function.

In the above example, the only requirement for a formatter() function is that it needs to take two string arguments, representing a first and last name. Here are some examples of potential formatter() functions:

def first_last(first_name, last_name):
    return f"{first_name} {last_name}"


def last_first(first_name, last_name):
    return f"{last_name}, {first_name}"

Imagine that you have first_name = 'Kermit' and last_name = 'The Frog'. Notice that the first function prints Kermit The Frog and the second function prints The Frog, Kermit.

Here is a full program to use this idea:

def get_full_name(first_name, last_name, formatter):
    return formatter(first_name, last_name)

def first_last(first_name, last_name):
    return f"{first_name} {last_name}"


def last_first(first_name, last_name):
    return f"{last_name}, {first_name}"


if __name__ == '__main__':
    full_name = get_full_name('Kermit', 'The Frog', first_last)
    print(full_name) # Kermit The Frog

    full_name = get_full_name('Kermit', 'The Frog', last_first)
    print(full_name) #  The Frog, Kermit

lambda function

Lambda functions enable us to do the same thing but in a simpler way:

def get_full_name(first_name, last_name, formatter):
    return formatter(first_name, last_name)


if __name__ == '__main__':
    full_name = get_full_name(
        'Kermit',
        'The Frog',
        lambda first_name, last_name: f"{first_name} {last_name}"
    )
    print(full_name)

    full_name = get_full_name(
        'Kermit',
        'The Frog',
        lambda first_name, last_name: f"{last_name}, {first_name}"
    )
    print(full_name)

A lambda function is an anonymous function. It has no name. In the above example, it is created and passed as a parameter at the same time, using the lambda keyword

A lambda function can have only one expression in the body of the function. For this reason, it is often called a lambda expression.

Storing a function in a variable

Let’s say we have defined a complete function as a formatter():

def first_last(first_name, last_name):
    return f"{first_name} {last_name}"

We can set this equal to a variable:

formatter = first_last

Then we can use the formatter as before:

if __name__ == '__main__':

    formatter = first_last
    full_name = get_full_name('Kermit', 'The Frog', formatter)
    print(full_name) # Kermit The Frog

    formatter = first_last
    full_name = get_full_name('Kermit', 'The Frog', formatter)
    print(full_name) #  The Frog, Kermit

This example is not particularly useful, because it is an extra step to store the function in a variable. But there may be other cases where this is useful.

You can likewise store a lambda function in a variable:

if __name__ == '__main__':

    formatter = lambda first_name, last_name: f"{first_name} {last_name}
    full_name = get_full_name('Kermit', 'The Frog', formatter)
    print(full_name) # Kermit The Frog

    formatter = lambda first_name, last_name: f"{last_name}, {first_name}
    full_name = get_full_name('Kermit', 'The Frog', formatter)
    print(full_name) #  The Frog, Kermit

Again, this is not particularly useful, because it takes an extra step. However, this illustrates the concept that a function (a full one or a lambda expression) is treated like any other Python data and can be stored in a variable and passed as a parameter. This means Python supports first-class functions, which is an important programming language concept.