class TargetPriority(enum.Enum):
    LOW_HEALTH = 1
    HIGH_HEALTH = 2
    FIRST = 3
    LAST = 4
    CLOSEST = 5


class Tower(Entity):
    def __init__(self, *args, **kwargs, target_priority=TargetPriority.FIRST):
        super().__init__(*args, **kwargs)
        self.target_priority = target_priority

    def find_target(self, all_monsters):
        sort_functions = {
            TargetPriority.LOW_HEALTH: lambda monster: monster.health,
            TargetPriority.HIGH_HEALTH: lambda monster: -monster.health,
            TargetPriority.FIRST: ???,
            TargetPriority.LAST: ???,
            TargetPriority.CLOSEST: self.distance_to,
        }
        func = sort_functions[self.target_priority]
        return next(sorted(all_monsters, key=func), None)

class Monster(Entity):
    def __init__(self, *args, **kwargs, health, checkpoint):
        super().__init__(*args, **kwargs)
        self.health = health
        self.checkpoint = checkpoint

    def update(self, dt):
        travel = self.velocity / 1000 * dt
        remaining_distance = self.distance_to(self.checkpoint)
        if travel.length < remaining_distance:
            self.position += travel
        else:
            self.checkpoint.check_in(self)

self.position += travel
self.total_distance_travelled += travel.length