How have you applied test-driven development (TDD) in your projects?

In projects using TDD, I start by writing an automated test that describes the required behavior of a function or component. This test initially fails because the functionality has not yet been implemented. Then I write the minimal code to pass the test, and afterwards I refactor the code while keeping the tests passing. This approach helps create more reliable and well-tested code, as well as improves understanding of the requirements.

Example in Python using unittest:

import unittest

def add(a, b):
    return a + b

class TestAddFunction(unittest.TestCase):
    def test_add_positive_numbers(self):
        self.assertEqual(add(2, 3), 5)

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

First, the test test_add_positive_numbers is written, which initially fails, then the add function is implemented.