Confused about classes in Learn Python the Hard Wa

Question

I'm confused on how the Map and Engine classes together work together to run this Adventureland type game (full code here: http://learnpythonthehardway.org/book/ex43.html). I think I understand what is happening in the Map class, but I'm really confused about what is happening in Engine() and why the scene_map variable is needed.

class Map(object):

    scenes = {
        'central_corridor': CentralCorridor(),
        'laser_weapon_armory': LaserWeaponArmory(),
        'the_bridge': TheBridge(),
        'escape_pod': EscapePod(),
        'death': Death()
    }

    def __init__(self, start_scene):
        self.start_scene = start_scene

    def next_scene(self, scene_name):
        return Map.scenes.get(scene_name)

    def opening_scene(self):
        return self.next_scene(self.start_scene)

class Engine(object):

    def __init__(self, scene_map):
        self.scene_map = scene_map

    def play(self):
        current_scene = self.scene_map.opening_scene()

        while True:
            print "\n--------"
            next_scene_name = current_scene.enter()
            current_scene = self.scene_map.next_scene(next_scene_name)

a_map = Map('central_corridor')
a_game = Engine(a_map)
a_game.play()

Thank you for any help.

Answer 1

The Engine instance's scene_map is an instance of the Map class, just as the global a_map is. In fact, a_game.scene_map is the same instance as a_map.

So, whatever you could do with a_map at the top level, the Engine.play code can do with self.scene_map. It may be worth typing everything into the interactive interpreter up to the a_map definition and playing around with a_map to make sure you know what exactly it can do for you.

So, why does Engine need self.scene_map? Why can't it just use the global a_map?

Well, it could. The problem is that if you did that, you wouldn't be able to create two Engine instances without them fighting over the same a_map. (This is the same reason you don't want to use global variables in functions. Objects don't add a new problem—in fact, a big part of what objects are about is solving the global-variables problem.)