class - Confused about classes in Learn Python the Hard Way ex43? -

i'm confused on how map , engine classes work run adventureland type game (full code here: http://learnpythonthehardway.org/book/ex43.html). think understand happening in map class, i'm confused happening in engine() , why scene_map variable 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 help.

the engine instance's scene_map instance of map class, global a_map is. in fact, a_game.scene_map same instance a_map.

so, whatever a_map @ top level, engine.play code can self.scene_map. may worth typing interactive interpreter a_map definition , playing around a_map make sure know can you.

so, why engine need self.scene_map? why can't use global a_map?

well, could. problem if did that, wouldn't able create 2 engine instances without them fighting on same a_map. (this same reason don't want use global variables in functions. objects don't add new problem—in fact, big part of objects solving global-variables problem.)