如何用unittest修补不同返回值的链式函数？

class ChainFunction:
    def __init__(self, value):
        self.value = value

    def add(self, amount):
        self.value += amount
        return self

    def subtract(self, amount):
        self.value -= amount
        return self

    def multiply(self, amount):
        self.value *= amount
        return self

    def result(self):
        return self.value

import unittest

class TestChainFunction(unittest.TestCase):
    def test_addition(self):
        chain = ChainFunction(0)
        result = chain.add(5).result()
        self.assertEqual(result, 5)

    def test_subtraction(self):
        chain = ChainFunction(10)
        result = chain.subtract(3).result()
        self.assertEqual(result, 7)

    def test_multiplication(self):
        chain = ChainFunction(2)
        result = chain.multiply(4).result()
        self.assertEqual(result, 8)

    def test_combined_operations(self):
        chain = ChainFunction(1)
        result = chain.add(2).subtract(1).multiply(3).result()
        self.assertEqual(result, 6)

if __name__ == '__main__':
    unittest.main()