How to flatten a list of lists in Python

Know how vanilla Python and a few libraries can be used to flatten a list of lists.

A flat list is a type of list which is not nested, for example:

#more

["h", "e", "l", "l", "o"]
[True, 1, 2, False]

And nested lists:

[[7], [0, 9, 3], [4, 6, 8]]
[["lorem", "ipsum", "seth", "sir"], ["domat", "texeto", "do"]]

There are several ways in which a nested list can be unpacked and made into a flat list, some of these approaches don’t need a library whereas others use itertools, functools, and numpy.

1. Looping and List comprehension¶

This is the easiest way to flatten a list. It uses a for loop to iterate over the main list and another nested for loop to iterate over the element of the main list.

nested_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
flat_list = []
for sublist in nested_list:
  for element in sublist:
    flat_list.append(element)
print(flat_list)

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

A shorter version of this uses list comprehension:

flat_list = [element for sublist in nested_list for element in sublist]

Alternatively, extend() can be used to create the resulting list without nested loops

flat_list = []
for sublist in nested_list:
  flat_list.extend(element)

2. itertools.chain(*nested_list)¶

Itertools is part of python’s standard libraries and provides a method to create a flat list. The chain method takes lists as arguments, therefore a * is used to unpack the list, read more about *args here, the return value is an iterator and not a list, using list() it is forced to yield all elements.

import itertools

nested_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
flat_list = itertools.chain(*nested_list)
flat_list  = list(flat_list)

3. itertools.chain.from_iterable(nested_list)¶

Similar to itertools.chain() but takes a nested list as argument.

import itertools

nested_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
flat_list = itertools.chain.from_iterable(nested_list)
flat_list  = list(flat_list)

4. functools.reduce(function, nested_list)¶

reduce() works by applying a function to two elements of an iterable cumulatively.

from functools import reduce

nested_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
flat_list = reduce(lambda x, y: x+y, nested_list)

Alternatively, instead of writing lambda function, in-built operator.concat can be used.

import operator
from functools import reduce

nested_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
flat_list = reduce(operator.concat, nested_list)

5. numpy.concatenate(nested_list)¶

Returns merged list instead of an iterator

import numpy

nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat_list = numpy.concatenate(nested_list)

6. numpy.array(nested_list).flat¶

Numpy arrays have a flat property which can be used to get an iterator for a flattened array but it only works if lists inside a nested list are of same lengths.

import numpy

nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(list(numpy.array(nested_list).flat))

Output:

[1, 2, 3, 4, 5, 6, 7, 8, 9]

If sub lists are not of same length

import numpy

nested_list = [[1, 2, 3], [4, 5], [7, 8, 9]]
print(list(numpy.array(nested_list).flat))

Output:

[[1, 2, 3], [4, 5], [7, 8, 9]]