Can you give an example of a test that checks individual system components?

An example of a unit test for checking a single component is a test of a function that calculates the sum of two numbers:

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)

    def test_add_negative_numbers(self):
        self.assertEqual(add(-1, -1), -2)

    def test_add_zero(self):
        self.assertEqual(add(0, 5), 5)

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

This test checks the functionality of a specific function without dependence on other parts of the system, allowing for quick identification of errors in the component's logic.