def test_complete_task_does_nothing_when_task_is_already_completed():
completed_at = date(2020, 12, 31)
task = Task(name="Foo", completed_at=completed_at)
task.complete(at=date(2021, 1, 17))
assert task.completed_at == completed_at
def test_complete_task_sets_completed_at():
task = Task(name="Foo")
now = date(2020, 12, 31)
task.complete(at=now)
assert task.completed_at is not None
assert task.completed_at == now