reversed.py

import unittest

TREE = (1, (2, None, None), (3, (4, None, None), (5, None, None)))
EXPECTED_REVERSED = (1, (3, (5, None, None), (4, None, None)), (2, None, None))

def reversed(tree):
    if tree is None:
        return None
    else:
        val, left, right = tree
        return (val, reversed(right), reversed(left))

class TestReversed(unittest.TestCase):
    def test_reversed(self):
        self.assertEqual(reversed(TREE), EXPECTED_REVERSED)

    def test_reversed_not_same(self):
        self.assertNotEqual(reversed(TREE), TREE)

    def test_reversed_reversed(self):
        self.assertEqual(reversed(reversed(TREE)), TREE)

    def test_reversed_asymmetric(self):
        self.assertEqual(reversed((1, None, (2, None, None))),
                        (1, (2, None, None), None))

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