how to flatten a list in python

Converting a list of lists (2D), into a list (1D) is called flattening. There are several approaches to this problem. We will look at a few important ones.

Approaches to flattening lists

Nested loops

This is perhaps the most intuitive approach to flattening. We pick every element from the list of lists and put it into our flat list.

List_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattened

List_flat = []

for i in range(len(List_2D)): #Traversing through the main list

for j in range (len(List_2D[i])): #Traversing through each sublist

List_flat.append(List_2D[i][j]) #Appending elements into our flat_list

print(“Original List:”,List_2D)

print(“Flattened List:”,List_flat)

Functools (reduce-iconcat)

Perhaps the fastest execution can be achieved through this approach. However, it isn’t very intuitive and requires importing a few libraries.

import functools

import operator

List_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattened

List_flat = functools.reduce(operator.iconcat, List_2D, [])

print(“Original List:”,List_2D)

print(“Flattened List:”,List_flat)

Itertools(chain)

This is another notable approach. Although not as fast as the previous one, it’s still faster than using Nested-loops.

import itertools

List_2D = [[1,2,3],[4,5,6],[7,8,9]] #List to be flattened

List_flat = list(itertools.chain(*List_2D))

print(“Original List:”,List_2D)

print(“Flattened List:”,List_flat)

Other approaches

The approaches we discussed so far are a few of the fastest known ones. However, there are other approaches to the problem, including:

Functools – reduce
NumPy – concatenate
NumPy – flat
Summing inner lists

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