How to flatten a shallow list in Python?

Flattening a shallow list means converting a nested list into a simple single dimensional list. In other words, converting a multidimensional list into a one-dimensional list.

The process of flattening can be performed by using different techniques like nested for loops, list comprehensions, list concatenation, and by using built-in functions.

In this article, we will discuss a few techniques to flatten a shallow python list.

Using nested for loop

By using nested for loop and list.append() method we can flatten the shallow list. Let’s have a look and see how this can be done in a program.

Example

This simple example successfully flattens a shallow list by using nested for loop. The first loop is for iterating sub lists in list and the second loop is for iterating an item in sublist.

li = [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
print("Original shallow list:", li)

flatlist = []   
for sub_li in li:
    for ele in sub_li:
        flatlist.append(ele)
print("After flattening :", flatlist)

Output

Original shallow list: [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
After flattening : [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]

Using a list comprehension

This technique works very similar to the nested for loop, like the above solution is packed in a single line using a list comprehension.

The list comprehension is an easy and elegant way of creating lists based on existing iterable (list, string, tuple).

Syntax

newList = [ expression(element) for element in oldList if condition ]

Example

We flattened a shallow list in a single line of code using a list comprehension method.

li = [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
print("Original shallow list:", li)

flatlist = [ele for ele in sub_li for sub_li in li]   

print("List after flattening :", flatlist)

Output

Original shallow list: [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
List after flattening : [90, 90, 90, 100, 100, 100, 110, 110, 110, 120, 120, 120]

Using itertools (chain())

The chain() is a method that accepts a series of iterables and then returns a single iterable which cannot be used directly and has to be explicitly converted. To access the chain() method initially we need to Import itertools module to our program.

Syntax

chain (*iterables)

Example

To unpack the nested list, we have mentioned the *operator.

import itertools

li = [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
print("Original shallow list:", li)

flatlist = list(itertools.chain(*li))
print("List after flattening :", flatlist)

Output

Original shallow list: [[10, 20, 30, 40], [50, 60, 70, 80], [90, 100, 110, 120]]
List after flattening : [10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]

Using sum() function

Summing over inner lists(sub lists) is another solution. Here we will use the python built-in function sum().

Syntax

sum(iterable, start)

Example

In this example, the parameters iterable takes a list of lists and start is an empty list that serves as the initial flat list to which items of the inner sublists are added.

li = [[1, 2, 3, 4], [5, 6, 7], [8, 9]]
print("Original shallow list:", li)

faltlist = sum(li, [])
print("List after flattening :", faltlist)

Output

Original shallow list: [[1, 2, 3, 4], [5, 6, 7], [8, 9]]
List after flattening : [1, 2, 3, 4, 5, 6, 7, 8, 9]

We have discussed a few techniques to flatten a shallow python list.