A Guide to Flatten List & List of Lists in Python

Converting a 2D array into a 1D array is called flattening. There are many approaches to solve the problem.

We will explore some of them in this tutorial.

Let’s see an example.

Input

[[1, 2, 3], [4, 5], [6, 7, 8, 9, 10]]

Output

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

#1. Loops

The most common way to solve the problem is by using loops. I think most of you already got it. Let’s see the steps to solve the problem using loops.

Initialize the list of lists with dummy data and name it as data.
Now, initialize an empty list called flat_list.
Iterate over the data.
- Unpack all the elements from the current list.
- Add them to the flat_list using the list append method.
Print the result.

See the code for the problem below.

# initializing the data and an empty list
data = [[1, 2, 3], [4, 5], [6, 7, 8, 9, 10]]
flat_list = []

# iterating over the data
for item in data:
    # appending elements to the flat_list
    flat_list += item

# printing the resultantn flat_list
print(flat_list)

You can use another loop to add sub-list elements to flat_list instead of a concatenation operator. We can also use list comprehensions instead of loops.

Both do the same work. Let’s see the next way to solve the problem.

#2. Itertools – Chain

We will use a method called chain from itertools built-in module.

The method chain iterates over each sub-list and returns the elements until there are no sub-lists in it. It returns an iterable that we have to convert it into a list.

Let’s see the steps involved in solving the problem.

Initialize the list of lists with dummy data and name it as data.
Get the flatten iterable using itertools.chain(*data).
Conver the resultant iterable into a list.
Print the flatten list.

You can go through the code in the below snippet.

# importing the module
import itertools

# initializing the data
data = [[1, 2, 3], [4, 5], [6, 7, 8, 9, 10]]

# flattening the list and storing the result
flat_list = itertools.chain(*data)

# converting iterable to list and printing
print(list(flat_list))

#3. Flatten Multi-Level Lists

We have seen how to flatten a list of lists. The above methods that we have discussed to flatten the list won’t work for multi-level lists. Let’s see an example.

Input

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

Output

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

As we don’t know the depth of the lists before the program, we have to use recursion to solve the problem.

Initialize the data as shown in the example and name it as data.
Initialize an empty list called flat_list.
Write a function called flatten_list.
- Iterate over the elements of the given list.
- If the element is a list then recursively call the same function again.
- If the element is not a list, then append the element to the flat_list.
Invoke the function with data.
The function will fill all the elements in the flat_list list.
Print the flat_list to check the output.

Phew! a lot of steps to code. Don’t worry. Converting the above statements into code won’t take more than mins.

# initializing the data and empty list
data = [1, [2, 3, [4, 5]], 6, [[7], [8, 9]]]
flat_list = []

# function
def flatten_list(data):
    # iterating over the data
    for element in data:
        # checking for list
        if type(element) == list:
            # calling the same function with current element as new argument
            flatten_list(element)
        else:
            flat_list.append(element)

# flattening the given list
flatten_list(data)

# printing the flat_list
print(flat_list)

Remember, we didn’t convert the existing list. Instead, we have created a new list with the given list element.