flatten_list.py

import unittest


def flatten(it):
    """Flatten an arbitrarily-nested list of integers into a single list.

    Args:
        it (list)

    Returns: list of integers
    """
    flat = []
    for x in it:
        if type(x) is int:
            flat.append(x)
        else:
            flat.extend(flatten(x))
    return flat


class TestFlatten(unittest.TestCase):
    def _test(self, inputs, expected):
        self.assertEqual(list(flatten(inputs)), expected)

    def test_empty_list(self):
        self._test([], [])

    def test_one_level(self):
        self._test([1, 2, 3], [1, 2, 3])

    def test_nested_lists(self):
        self._test([[1,2,[3]],4], [1,2,3,4])


if __name__ == '__main__':
    unittest.main()