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  • 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.

    • 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

    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

    #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.

    • 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
    # flattening the given list
    # printing the 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 🙂