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.
You might encounter examples on the internet that use py.test in the command line instead of pytest. The reason for that is historical: pytest used to be part of the py package, which provided several general purpose utilities, including...