Confused about classes in Learn Python the Hard Wa

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.

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.)