python - 艰难地学习 Python 示例 #43

Question

我正在学习 Zed Shaw 的 LearnPythontheHardWay，但我被难住了。

我收到错误:

[Attribute Error:'NoneType' object has no attribute 'enter']

在行:Engine 类下的 next_scene_name = current_scene.enter()。

from sys import exit

class Scene(object):

    def enter(self):
        print "This scene is not yet configured. Subclass it and implement\
enter()."
        exit(1)

class Engine(object):

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

    def play(self):
        current_scene = self.scene_map.opening_scene()
        last_scene = self.scene_map.next_scene('finished')

        while current_scene != last_scene:
            next_scene_name = current_scene.enter()
            current_scene = self.scene_map.next_scene(next_scene_name)

        current_scene.enter()   

class EmptyScene(Scene):

    def enter(self):
        pass

class FinishScene(Scene):

    def enter(self):
        pass

class Map(object): 

    scenes = {
        'empty_scene': EmptyScene(),
        'finished': FinishScene(),
    }

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

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

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


a_map = Map('empty_scene')
a_game = Engine(a_map)
a_game.play()`

Answer 1

问题是 current_scene 实际上是 None。这是由于 self.scene_map.next_scene 调用 dict.get 引起的。使用 dict.get 的问题在于，如果键不在字典中，那么它只会返回 None 而不是像您期望的那样抛出异常。因此，您的程序会继续运行，认为一切正常，直到代码稍后出现问题。