Making a list out of a list of lists in Python?

James W.

The Problem

You want to flatten both regular and irregular lists of lists. Nested for loops could be used to solve this problem.

But are there any simpler solutions in Python?

The Solution

Using Python Libraries

The Python itertools module is a simple and readable method for flattening regular lists of lists:

import itertools

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

Output:

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

Using List Comprehension

Use list comprehension to flatten a regular list:

regular_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat_list = [item for inner_list in regular_list for item in inner_list]
print(flat_list)

Output:

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

Using Nested Loops

Using nested loops might not be a shortcut, but this method is useful for flattening both regular and irregular lists of lists:

def flatten_list(list_of_lists):
    flattened_list = []

    for element in list_of_lists:
        if type(element) is list:
            for item in element:
                flattened_list.append(item)
        else:
            flattened_list.append(element)
    return flattened_list

# Regular list of lists
regular_list_of_lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# Irregular list of lists
irregular_list_of_lists = [1, [2, 3, 4], 5, [6, 7], 8, 9]

print(flatten_list(regular_list_of_lists))
print(flatten_list(irregular_list_of_lists))

Output:

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