How to flatten nested list in python (5 ways)

A list in python is one of the most flexible data types. It can have multiple dimensions i.e it can contain lists inside a list. It is known as the nested list.

When a list contains sublists in python, it is commonly known as a 2D list. And flattening a list of lists into a one-dimensional list is called flattening a list.

For example,[[1,2,3],[4,5,6]] is flatten into [1,2,3,4,5,6]

So let’s see how to merge nested lists into a single list in python.

Flatten a List in Python

There are a few different methods to flatten a nested list in python.

  1. Using for loop
  2. Using list comprehension
  3. Using reduce() function
  4. Using the itertools.chain() method
  5. Using numpy’s flat and concatenate function

Let’s see each method with examples.

Flatten a Nested List using for loop

We can flatten a nested list by using the nested for loop in python.

When a loop contains another loop inside we called it a nested for loop.

Since in a two-dimensional list, we have lists inside a list, so we have to loop through the first list and then loop through each item in the sub-list.


nested_list = [['a','b','c'],['d','e','f'],[7,8,9]]

flatten_list = []

for sub_list in nested_list:
    for item in sub_list:



['a', 'b', 'c', 'd', 'e', 'f', 7, 8, 9]

Here, we have the first loop through the sub list and then iterated through each item of the sub list and append it to the empty list i.e flatten_list.

Flatten List using List Comprehension

List comprehension is a shorthand syntax for creating a list based on existing lists in Python.

So, we can write the above for loop statement to merge and flatten the list using list comprehension.

nested_list = [['a','b','c'],['d','e','f'],[7,8,9]]

flatten_list = [item for sub_list in nested_list for item in sub_list]



['a', 'b', 'c', 'd', 'e', 'f', 7, 8, 9]

Using reduce() method to Flatten List

We can use the reduce() function to merge the nested list in python. The reduce function is a part of the functools module.

The reduce() function does functional computation by taking in two arguments, a function and the list (sequence) as an argument.


reduce(function, iterable)

The function argument will be applied to all the elements of the iterable in a cumulative manner to output the result.

With the reduce function we can process the nested list without explicitly using the for loop statement.

from functools import reduce

nested_list = [['a','b','c'],['d','e','f'],[7,8,9]]

flatten_list = reduce(lambda a,b:a+b, nested_list)



['a', 'b', 'c', 'd', 'e', 'f', 7, 8, 9]

Here, we have passed the lambda function as the first parameter and then the nested list.

The function will all the items in the list and print it out as an output.

Flatten List of lists using itertools module

To flatten a list of lists into a single list we can use the chain iterator of the itertools module.

The itertools module provides us with some utility functions that can be used to iterate over data structures that can be looped over using the for loop statement.

This module provides an iterator called chain(), which can help us to flatten a 2D list into a regular python list.


from itertools import chain

nested_list = [['a','b','c'],['d','e','f'],[7,8,9]]

flatten_list = list(chain(*nested_list))



['a', 'b', 'c', 'd', 'e', 'f', 7, 8, 9]

Here, we pass on the nested list as a parameter to the chain() function.

The chain() function iterates through the first sublist, returns all the elements, and then moves to the second one, and so on until all the sublists are iterated through.

We then wrap the chain function with the list() to get the flat list of the elements.

Flatten List of Lists using numpy

Numpy is a library in python that provides different utility functions that can help us to flatten a nested list easily.

We can use the flat() and the concatenate() functions that can help to achieve our goal.

import numpy as np

nested_list = [['a','b','c'],['d','e','f'],[7,8,9]]

flatten_list = list(np.concatenate(nested_list).flat)



['a', 'b', 'c', 'd', 'e', 'f', '7', '8', '9']

The output is a 1D flat array.

The concatenate() function concatenates a sequence of an array and the flat iterator is used to make a one-dimensional iterator over an array


Here, we have learned four different ways to flatten a nested list in python. And also we used modules like itertools, functools, and numpy which helped to flatten the list of lists in python.

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