Flatten arbitrarily nested arrays of integers.

flatten.py

import numbers


def flatten(xs):
    """ 
    # Flatten arbitrarily nested arrays of integers
    # e.g. [[1, 2, [3]], 4] -> [1, 2, 3, 4]
    """
    # if not list, return single item list but only if its a number
    if not isinstance(xs, list):
        return [xs] if isinstance(xs, numbers.Number) else []

    # otherwise, continue splitting by taking off the head,
    # flattening it and prepend it back to the tail
    return flatten(xs[0]) + (flatten(xs[1:]) if len(xs) > 1 else [])

if __name__ == '__main__':
    assert flatten([[1, 2, [3]], 4]) == [1, 2, 3, 4]
    assert flatten([[1, 2, [3]], 4, [5, [6, [7, 8, [9, [10]]]]]]) == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
    assert flatten([1, ['a', 2, 'b'], [3, 'c', ['d', 'e', 4]]]) == [1, 2, 3, 4]