Как работает тестирование функций в Python?

import unittest

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

class TestAddNumbers(unittest.TestCase):
    def test_add_positive_numbers(self):
        self.assertEqual(add_numbers(1, 2), 3)
        
    def test_add_negative_numbers(self):
        self.assertEqual(add_numbers(-1, -1), -2)
        
    def test_add_zero(self):
        self.assertEqual(add_numbers(5, 0), 5)

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

# test_functions.py
def add_numbers(a, b):
    return a + b

def test_add_numbers():
    assert add_numbers(1, 2) == 3
    assert add_numbers(-1, -1) == -2
    assert add_numbers(5, 0) == 5