Now, we will write the first round of unit tests. Specifically, we will write unit tests related to the LED resources. Test fixtures provide a fixed baseline to enable us to reliably and repeatedly execute tests. Pytest makes it easy to declare a test fixture function by marking a function with the @pytest.fixture decorator. Then, whenever we use the fixture function name as an argument in a test function declaration, pytest will make the fixture function provide the fixture object.
The pytest-tornasync plugin provides us with many fixtures that we will use to easily write tests for our Tornado API. In order to work with this plugin, we must declare a fixture function, named app, that returns a tornado.web.Application instance. In our case, this fixture function will return an instance of the Application class, which maps the URL patterns to asynchronous and non-blocking request handlers. We don't need to specify an app as an argument for the test functions...