Python: Lambda, Map, Filter and Reduce functions

Mayank Porwal
3 min readNov 12, 2020


python lambda function

In Python, normal functions are defined using def keyword followed by function name. For example:

def myFunc(x):
return x

you can see that myFunc takes x as argument and return x.

But when we talk about: Lambda function, is an anonymous functions are defined using lambda keyword. it uses lambda keyword without any function name. It can have any number of arguments but only one expression. For example:

Above lambda function is equivalent to:

In python lambda function you can take number of arguments with comma separated but without parenthesis.

Lambda function is assigned to add_together takes two arguments and return their sum. As per the definition of lambda, list of the arguments with no parenthesis, whereas calling the function is exactly same as normal function with parenthesis surrounding the arguments.


In python, map() is a inbuilt function that allows you to process the each item in an iterable and transform them into new iterable. According to the documentation map takes function and iterable as an argument and return a new iterable.

Python map() function syntax is:

map(function, iterables)

function: the function to execute for each item.

iterable: an iterable like: list,tuple etc. You can pass one or more iterable to map.

For example Let’s create a simple function:

In the above example we pass map object result to list(to create a list). We can also use lambda expression with above example.


The next function is filter() function. Filter function takes a filtering function and an iterable.

Let’s see a example first. Suppose we have list of integers and want to create a list of even and odd numbers by filtering the integers.

In the above example we use a lambda function as a filtering function. Inside filter function we pass lambda as a filtering function and integer_list as an argument. The filter() function return a filter object then to create a list we pass that filter object in list.


Like Filter function, reduce is also a inbuilt function defined in “functools” module in python. The reduce() function takes a function and an interable and return a single value.

In the above example value is calculated as follow:

  1. Initially function is called with first two items in the list and return a value.
  2. The function is again called with previously value returned in step first and the next value in the list. This process is keep repeating till last value in the list.

Optional arguments: You can also pass an optional argument initialzer . Let’s add optional argument initializer to above example.

That’s all for today. We learned a bit about python3 lambda expression, how to use it with map(), filter() and reduce() functions with examples.

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Mayank Porwal

Software Developer | Machine Learning Enthusiast