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Writing and running tests

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 tools that followed the convention of starting with py.<TAB> for tab completion, but since then, it has been moved into its own project. The old py.test command is still available and is an alias to pytest, but the latter is the recommended modern usage.

Note that there's no need to create classes; just simple functions and plain assert statements are enough, but if you want to use classes to group tests you can do so:

    class TestMechanics:

        def test_player_hit(self):
            ...

        def test_player_health_flask(self):
            ...

Grouping tests can be useful when you want to put a number of tests under the same scope: you can execute tests based on the class they are in, apply markers to all of the tests in a class (Chapter 3, Markers and Parametrization), and create fixtures bound to a class (Chapter 4, Fixtures).

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