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.

Conclusion

Hope you have enjoyed the tutorial. There might be many other ways to flatten a list in Python but I felt the above are probably the easiest ones.

Happy Coding 🙂