How to flatten list in Python?

In Python, a list of lists (or cascaded lists) resembles a two-dimensional array - although Python doesn't have a concept of the array as in C or Java. Hence, flattening such a list of lists means getting elements of sublists into a one-dimensional array-like list. For example, a list [[1,2,3],[4,5,6]] is flattened into [1,2,3,4,5,6].

This can be achieved by different techniques, each one of them is explained below:

Flatten List using Nested for Loop

Using the nested for loop to append each element of sublist in a flat list

nestedlist = [ [1, 2, 3, 4], ["Ten", "Twenty", "Thirty"], [1.1,  1.0E1, 1+2j, 20.55, 3.142]]
flatlist=[]
for sublist in nestedlist:
    for element in sublist:
        flatlist.append(element)
print(flatlist)

[1, 2, 3, 4, 'Ten', 'Twenty', 'Thirty', 1.1, 10.0, (1+2j), 20.55, 3.142]

Flatten List using List Comprehension

List comprehension technique gives the same result in a single compact statement.

nestedlist = [ [1, 2, 3, 4], ["Ten", "Twenty", "Thirty"], [1.1,  1.0E1, 1+2j, 20.55, 3.142]]
flatlist=[element for sublist in nestedlist for element in sublist]
print(flatlist)

[1, 2, 3, 4, 'Ten', 'Twenty', 'Thirty', 1.1, 10.0, (1+2j), 20.55, 3.142]

Flatten List using the reduce() Function of functools Module

The first argument to the reduce() function is a function itself with two arguments, and the second argument is a list. Argument function is applied cumulatively to elements in the list. For example, reduce(lambda a,b:a+b, [1,2,3,4,5]) returns cumulative sum of numbers in the list.

from functools import reduce
flatlist = reduce(lambda a,b:a+b, nestedlist)
print(flatlist)

[1, 2, 3, 4, 'Ten', 'Twenty', 'Thirty', 1.1, 10.0, (1+2j), 20.55, 3.142]

Recall that the + symbol is defined as a concatenation operator for sequence data types (list, tuple, and string). Instead of a lambda function, we can also use the concat() function defined in the operator module as an argument to cumulatively append the sublists to flatten.

from functools import reduce
from operator import concat

nestedlist = [ [1, 2, 3, 4], ["Ten", "Twenty", "Thirty"], [1.1,  1.0E1, 1+2j, 20.55, 3.142]]
flatlist = reduce(concat, nestedlist)
print(flatlist)

[1, 2, 3, 4, 'Ten', 'Twenty', 'Thirty', 1.1, 10.0, (1+2j), 20.55, 3.142]

Malhar Lathkar

Author

Malhar Lathkar is an independent software professional having 30+ years of experience in various technologies such as Python, Java, Android, PHP, and Databases. He is an author of Python Data Persistence: With SQL and NOSQL Databases