def test_string1_empty(self):
     self.assertEqual('', longest_common_substring('string1', ''))
 def test_string1_string2(self):
     self.assertEqual('string', longest_common_substring('string1',
                                                         'string2'))
 def test_animal_mallard(self):
     self.assertEqual('mal', longest_common_substring('animal', 'mallard'))
 def test_empty_string2(self):
     self.assertEqual('', longest_common_substring('', 'string2'))