import unittest

def primes(max_n):
    found = [2, 3]
    for candidate in xrange(4, max_n + 1):
        for prime in found:
            if not candidate % prime:
                import pdb; pdb.set_trace()
                break
        else:
            found.append(candidate)
    return found

class PrimeTest(unittest.TestCase):

    def test_primes(self):
        actual = primes(20)
        self.assertEqual(actual, [2, 3, 5, 7, 11, 13, 17, 19])

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