flatten_lists.py

def flatten_lists(integer_list):
    list_to_return = []
    for element in integer_list:
        """
            if element is a list, recursivelly call flatten_lists and extend the list
            else, append the element to the list to return
        """
        if type(element) == list:
            list_to_return.extend(flatten_lists(element))
        else:
            list_to_return.append(element)

    return list_to_return


print('[[1, 2, [3]], 4] -> {}'.format(flatten_lists([[1, 2, [3]], 4])))
print('[1, 2, [3, 4], [5, 6], [7], [8, 9, [10]]] -> {}'.format(
    flatten_lists([1, 2, [3, 4], [5, 6], [7], [8, 9, [10]]])))