def test_tags_must_be_str(self):
contact = Contact("Marvin Goodwright")
with self.assertRaises(TypeError):
contact.add_tag(100)
def test_can_add_tags(self):
contact = Contact("Marvin Goodwright")
contact.add_tag("111")
self.assertEqual(contact._tags, set(["111"]))
contact.add_tag("222")
self.assertEqual(contact._tags, set(["111", "222"]))