string calculator kata python

kata.py

def sum_numbers_in(expression: str) -> int:
    if expression is None or expression == "":
        return 0
    if "," in expression:
        tokens = expression.split(',')
        return int(tokens[0]) + int(tokens[1])
    return int(expression)


class StringCalculatorTests(unittest.TestCase):
    def test_none_and_empty_compute_as_zero(self):
        self.assertEqual(sum_numbers_in(""), 0)
        self.assertEqual(sum_numbers_in(None), 0)

    def test_numbers_in_expression_are_converted_to_integers(self):
        self.assertEqual(sum_numbers_in("8"), 8)

    def test_numers_in_expression_are_separated_by_commas(self):
        self.assertEqual(sum_numbers_in("1,4"), 5)
        self.assertEqual(sum_numbers_in("1,4,1"), 6)