import unittest


def get_max_profit(xs):
    if len(xs) < 2:
        raise Exception('Can only trade with two or more ticker values.')
    lowest_buy = xs[0]
    max_profit = None
    for x in xs[1:]:
        if not max_profit:
            max_profit = x - lowest_buy
        else:
            max_profit = max(max_profit, x - lowest_buy)
        lowest_buy = min(lowest_buy, x)
    return max_profit


# Tests
class Test(unittest.TestCase):
    def test_price_goes_up_then_down(self):
        actual = get_max_profit([1, 5, 3, 2])
        expected = 4
        self.assertEqual(actual, expected)

    def test_price_goes_down_then_up(self):
        actual = get_max_profit([7, 2, 8, 9])
        expected = 7
        self.assertEqual(actual, expected)

    def test_price_goes_up_all_day(self):
        actual = get_max_profit([1, 6, 7, 9])
        expected = 8
        self.assertEqual(actual, expected)

    def test_price_goes_down_all_day(self):
        actual = get_max_profit([9, 7, 4, 1])
        expected = -2
        self.assertEqual(actual, expected)

    def test_price_stays_the_same_all_day(self):
        actual = get_max_profit([1, 1, 1, 1])
        expected = 0
        self.assertEqual(actual, expected)

    def test_error_with_empty_prices(self):
        with self.assertRaises(Exception):
            get_max_profit([])

    def test_error_with_one_price(self):
        with self.assertRaises(Exception):
            get_max_profit([1])


unittest.main(verbosity=2)