A function to flatten an arbitrary list of lists, tuples, and other elements using recursion and list comprehension.

Flatten List

#!/usr/bin/python3
""" Demo of recursive list comprehension to flatten arbitrary list of lists and other objects.
"""

from typing import Any

def flatten_list(source: Any) -> list:
    """ Flatten arbitrary list of lists or tuples to a simple list of elements.  This method works
        even if the list contains non-list objects.
    """
    if isinstance(source, (list, tuple)):
        return [item for sub_list in source for item in flatten_list(sub_list)]
    new_list = []
    new_list.append(source)
    return new_list

def main():
    """ Demo of flatten_list
    """
    test_cases = [[1, [2, 3]],
                  [[1, 2], [3, 4]],
                  1,
                  [1, [2, 3, (4, '5', 6, [7, 8, '9'])]],
                  'a']
    for test_case in test_cases:
        print('{} -> {}'.format(test_case, flatten_list(test_case)))


if __name__ == '__main__':
    main()

Output for the test cases:

[1, [2, 3]] -> [1, 2, 3]
[[1, 2], [3, 4]] -> [1, 2, 3, 4]
1 -> [1]
[1, [2, 3, (4, '5', 6, [7, 8, '9'])]] -> [1, 2, 3, 4, '5', 6, 7, 8, '9']
'a' -> ['a']