Python: lambda, map(), reduce(), filter()


One of the guiding principles of Python is simplicity. In fact, you will often see well coded, simplistic Python programs called Pythonic as tribute to the programmer’s talent at crafting clean code.

When I think of Pythonic code, the first thing that comes to my mind is lambda

The best way I can describe lambda is as a short cut function. Look at the function in ln[12]. This function accepts 1 argument “y”.  It then multiplies it by 2 and returns the result.

Now look at ln[13]:


x               = lambda            y        :    y*2





Let’s build on lambda and introduce a new function: map()

Start with ln[15] :

  • lst = [1,2,3,4] – we assign lst a list of numbers
  • n = 0 – setting a counter variable
  • for i in lst: – start a for loop, iterating through the numbers in lst
  • lst[n] = i * 2 – replace each value in lst with the original value * 2
  • n += 1 – iterates n by 1
  • lst – outputs new values of lst


Now let’s examine ln[16]:

  • lst = [1,2,3,4] – we assign lst a list of numbers
  • x = map(lambda y:y*2, lst) – one at a time, pass each value in lst to the lambda function – multiply these values by 2 and assign the results to x
  • x – outputs results


Reduce takes all the elements in a list an combines them.

In the example below, x is fed into the lambda function a * b as such:




Filter allows you to run a set of values through a function and filter out the false results.

Below we filter out all values from x that are less than 0



If you enjoyed this lesson, click LIKE below, or even better, leave me a COMMENT

Follow this link for more Python content: Python

Please Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s