Lambda Functions and Anonymous Functions in Python

The def keyword is used to define a function in Python, as we have seen in the previous chapter. The lambda keyword is used to define anonymous functions in Python. Usually, such a function is meant for one-time use.

lambda [arguments] : expression

The lambda function can have zero or more arguments after the : symbol. When this function is called, the expression after : is executed.

Example: Lambda Function
square = lambda x : x * x

Above, the lambda function starts with the lambda keyword followed by parameter x. An expression x * x after : returns the value of x * x to the caller. The whole lambda function lambda x : x * x is assigned to a variable square in order to call it like a named function. The variable name becomes the function name so that We can call it as a regular function, as shown below.

Example: Calling Lambda Function
>>> square(5)

The above lambda function definition is the same as the following function:

def square(x):
    return x * x

The expression does not need to always return a value. The following lambda function does not return anything.

Example: Lambda Function
>>> greet = lambda name: print('Hello ', name) 
>>> greet('Steve')
Hello Steve
The lambda function can have only one expression. Obviously, it cannot substitute a function whose body may have conditionals, loops, etc.

The following lambda function contains multiple parameters:

Example: Lambda Function
>>> sum = lambda x, y, z : x + y + z 
>>> sum(5, 10, 15)

The following lambda function can take any number of parameters:

Example: Lambda Function
>>> sum = lambda *x: x[0]+x[1]+x[2]+x[3]  
>>> sum(5, 10, 15, 20)

Parameterless Lambda Function

The following is an example of the parameterless lambda function.

Example: Parameterless Lambda Function
>>> greet = lambda : print('Hello World!')
>>> greet()
Hello World!

Anonymous Function

We can declare a lambda function and call it as an anonymous function, without assigning it to a variable.

Example: Parameterless Lambda Function
>>> (lambda x: x*x)(5)

Above, lambda x: x*x defines an anonymous function and call it once by passing arguments in the parenthesis (lambda x: x*x)(5).

In Python, functions are the first-class citizens, which means that just as literals, functions can also be passed as arguments.

The lambda functions are useful when we want to give the function as one of the arguments to another function. We can pass the lambda function without assigning it to a variable, as an anonymous function as an argument to another function.

Example: Parameterless Lambda Function
>>> def dosomething(fn):
	    print('Calling function argument:')
>>> dosomething(lambda : print('Hello World')) # passing anonymous function
Calling function argument:
Hello World
>>> myfn = lambda : print('Hello World') 
>>> dosomething(myfn) # passing lambda function

Above, the dosomething() function is defined with the fn parameter which is called as a function inside dosomething(). The dosomething(lambda : print('Hello World')) calls the dosomething() function with an anonymous lambda function as an argument.

Python has built-in functions that take other functions as arguments. The map(), filter() and reduce() functions are important functional programming tools. All of them take a function as their argument. The argument function can be a normal function or a lambda function.

Example: Pass Lambda Function to map()
>>> sqrList = map(lambda x: x*x, [1, 2, 3, 4]) # passing anonymous function
>>> next(sqrList)
>>> next(sqrList)
>>> next(sqrList)
>>> next(sqrList)
>>> next(sqrList)