def test_same_letters(self):
self.assertEqual(count_letters('a'), {'a': 1})
self.assertEqual(count_letters('aa'), {'a': 2})
def test_distinct_letters(self):
self.assertEqual(count_letters('ab'), {'a' :1, 'b': 1})
def test_if_exists(self):
self.assertEqual(count_letters(''), {})
def test_different_letters(self):
self.assertEqual(count_letters('b'), {'b': 1})
