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.


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


[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.

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 ā€“ 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.

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.


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


[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.

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.


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 šŸ™‚