艰苦学习Python-练习43-类：实例化时将一个类实例传递给另一个类

Question

From Learn Python the Hard Way, 3rd Edition - exercise 43:

In the code below, a Map object instance is created named a_map and Engine object instance is created named a_game.

1. In passing a_map as a variable during a_game instantiation is this code setting a_game.scene_map as an object variable for the entire a_map object?

2. Does this provide the essentials of inheritance functionality to the a_game instance of Engine without using subclass inheritance?

Given that a_game.play() calls self.scene_map.opening_scene() successfully from the a_map instance I'm assuming both of the above questions' answers are 'yes' but it seems like there's a bit of simplified hand waving magic here for something pretty complicated.

I'd like to read more about this topic but most of what I've come across on classes has dealt with more straight forward examples.

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()

Answer 1

Well, look at what's happening.

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

So, we're creating a map, and passing that map to the Engine. What does that do? Depends on what Engine.__init__ does with it.

class Engine(object):

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

Stores the map as an attribute of the engine instance, so that other methods of the engine can use the map.

You've got this far already, seems like. So the answers to your questions are:

1. Yes.
2. No. The Engine instance has a Map . It is not, in itself, a map, nor does it behave like one (ie its interface is not like Map ).

To delve further into this question, why aren't you asking the same question about Map ? After all, it takes a data element at instantiation and stores it as an attribute, so that other methods of Map have access to it.

In short: the Map instance is a data element to Engine the same way the str instance is a data element to Map . The engine needs the map to play the game, but the engine is not a map.