def test_example(self):
     self.assertEqual(longest_palindrome('4123215'), '12321')
 def test_all_odd(self):
     self.assertEqual(longest_palindrome('aba'), 'aba')
 def test_base1(self):
     self.assertEqual(longest_palindrome('1'), '1')
 def test_all_even(self):
     self.assertEqual(longest_palindrome('aa'), 'aa')
 def test_base0(self):
     self.assertEqual(longest_palindrome(''), '')