find_mode.py

import unittest


def find_mode(l):
    if len(l) == 0:
        return "ZeroLength"

    l.sort()

    sums = {}
    for item in l:
        if item in sums.keys():
            sums[item] += 1
        else:
            sums[item] = 1

    max_count = 0
    max_item = l[0]
    for k, v in sums.items():
        if v > max_count:
            max_count = v
            max_item = k

    return max_item


class TestFindMode(unittest.TestCase):

    def test_empty(self):
        self.assertEqual(find_mode([]), "ZeroLength")

    def test_equal(self):
        self.assertEqual(find_mode([1, 2]), 1)

    def test_positives(self):
        self.assertEqual(find_mode([2, 3, 5, 5, 5, 2, 4, 1, 3, 1]), 5)

    def test_negatives(self):
        self.assertEqual(find_mode([-1, -3, -1, -1, -2, -4, -7, -1]), -1)


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

This should run O(N log N).