How to Flatten a List of Lists in Python

Here are two ways to flatten a list of lists in Python:

(1) Using a list comprehension:

flatten_list = [i for s in list_of_lists for i in s]

(2) Using for loop:

flatten_list = []
for s in list_of_lists:
    for i in s:
        flatten_list.append(i)

Examples

Example 1: Flatten a list of lists in Python using a list comprehension

To start, let’s suppose that you have the following list of lists in Python that contains numeric data:

list_of_lists = [[11, 22, 33], [44, 55, 66], [77, 88, 99]]

print(list_of_lists)

Here is how the list of lists would look like:

[[11, 22, 33], [44, 55, 66], [77, 88, 99]]

You can then flatten the above list of lists using a list comprehension as follows:

list_of_lists = [[11, 22, 33], [44, 55, 66], [77, 88, 99]]

flatten_list = [i for s in list_of_lists for i in s]

print(flatten_list)

You’ll now get the flatten list:

[11, 22, 33, 44, 55, 66, 77, 88, 99]

Example 2: Flatten a list of lists in Python using a for loop

Alternatively, you may use a for loop to flatten the list of lists in Python:

list_of_lists = [[11, 22, 33], [44, 55, 66], [77, 88, 99]]

flatten_list = []
for s in list_of_lists:
    for i in s:
        flatten_list.append(i)

print(flatten_list)

You’ll get the same flatten list:

[11, 22, 33, 44, 55, 66, 77, 88, 99]

Note that the same principles apply if your list of lists contains strings:

list_of_lists = [['aa', 'bb', 'cc'], ['dd', 'ee', 'ff'], ['gg', 'hh', 'ii']]

flatten_list = []
for s in list_of_lists:
    for i in s:
        flatten_list.append(i)

print(flatten_list)

As you can see, the list is now flatten:

['aa', 'bb', 'cc', 'dd', 'ee', 'ff', 'gg', 'hh', 'ii']