Testing behavior, not implementation details – In-Memory Store

store.py

# behavior/store.py

class TaskStoreInMemory(TaskStore):
    def __init__(self):
        self._tasks = []

    def add_task(self, task: Task) -> None:
        self._tasks.append(task)

    def list_tasks(self) -> list[Task]:
        return self._tasks

tests.py

# behavior/tests.py
@pytest.fixture
def store_() -> TaskStoreInMemory:
    return TaskStoreInMemory()


def test_added_task_listed_(store_: TaskStoreInMemory):
    task = Task(title="Do the dishes", status=TaskStatus.OPEN)

    store_.add_task(task)

    assert store_.list_tasks() == [task]