Using pytest, all you need to do to start writing tests is to create a new file named test_*.py and write test functions that start with test:

    # contents of test_player_mechanics.py
    def test_player_hit():
        player = create_player()
        assert player.health == 100
        undead = create_undead()
        undead.hit(player)
        assert player.health == 80

To execute this test, simply execute pytest, passing the name of the file:

λ pytest test_player_mechanics.py

If you don't pass anything, pytest will look for all of the test files from the current directory recursively and execute them automatically.