What will you do if there are no tests in the project and the customer does not want to spend time and money on their development?

I would take the following steps:

• Discuss with the client the risks of not having tests (increased error rate, support and development difficulties, potential financial losses due to failures).
• Suggest starting with the most critical parts of the project where errors would be most costly.
• Show examples of how test automation can speed up development in the future, reducing costs for bug fixes.
• Begin implementing unit tests for new code or during refactoring of existing parts. This is less costly and gradually demonstrates the benefits of testing.
• Propose implementing basic integration tests for key interactions within the system.
• Set up CI/CD for automatic test execution upon changes.
• Keep track of found bugs and demonstrate how they could have been prevented with tests.
• Look for opportunities to introduce automated tests based on analysis of existing code and its frequently used parts during other work (e.g., optimization or enhancement of functionality).

Example of implementing a test for a frequently breaking function:

# Original function without tests
def calculate_discount(price, discount_percentage):
    if discount_percentage > 100 or discount_percentage < 0:
        raise ValueError("Discount percentage invalid")
    discount = price * (discount_percentage / 100)
    return price - discount

# Adding a simple unit test
import unittest

class TestDiscountCalculation(unittest.TestCase):

    def test_valid_discount(self):
        # Test for correct case
        self.assertAlmostEqual(calculate_discount(100, 10), 90.0)

    def test_zero_discount(self):
        # Test for zero discount
        self.assertAlmostEqual(calculate_discount(100, 0), 100.0)

    def test_full_discount(self):
        # Test for 100% discount
        self.assertAlmostEqual(calculate_discount(100, 100), 0.0)

    def test_invalid_discount_negative(self):
        # Test for negative discount (expecting exception)
        with self.assertRaises(ValueError):
            calculate_discount(100, -10)

    def test_invalid_discount_too_high(self):
        # Test for discount above 100% (expecting exception)
        with self.assertRaises(ValueError):
            calculate_discount(100, 110)

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

Thus, I would gradually introduce testing, demonstrating its value to the client in practice, even if initially they are reluctant to invest.