一、问题描述

        程序能正常运行、没什么问题，但是当程序运行结束的时候，却出现以下令人难受的报错：

Exception ignored in: <bound method Viewer.__del__ of <gym.envs.classic_control.rendering.Viewer object at 0x0000012EFC8D99E8>>
Traceback (most recent call last):
  File "D:\root\python3.6.3\lib\site-packages\gym\envs\classic_control\rendering.py", line 165, in __del__
    self.close()
  File "D:\root\python3.6.3\lib\site-packages\gym\envs\classic_control\rendering.py", line 83, in close
    self.window.close()
  File "D:\root\python3.6.3\lib\site-packages\pyglet\window\win32\__init__.py", line 319, in close
    super(Win32Window, self).close()
  File "D:\root\python3.6.3\lib\site-packages\pyglet\window\__init__.py", line 838, in close
    app.windows.remove(self)
  File "D:\root\python3.6.3\lib\_weakrefset.py", line 109, in remove
    self.data.remove(ref(item))
KeyError: (<weakref at 0x0000012E8CC759A8; to 'Win32Window' at 0x0000012EFC976278>,)

Exception ignored in: <bound method Viewer.__del__ of <gym.envs.classic_control.rendering.Viewer object at 0x0000012EFC8D99E8>>
Traceback (most recent call last):
  File "D:\root\python3.6.3\lib\site-packages\gym\envs\classic_control\rendering.py", line 165, in __del__
    self.close()
  File "D:\root\python3.6.3\lib\site-packages\gym\envs\classic_control\rendering.py", line 83, in close
    self.window.close()
  File "D:\root\python3.6.3\lib\site-packages\pyglet\window\win32\__init__.py", line 319, in close
    super(Win32Window, self).close()
  File "D:\root\python3.6.3\lib\site-packages\pyglet\window\__init__.py", line 838, in close
    app.windows.remove(self)
  File "D:\root\python3.6.3\lib\_weakrefset.py", line 109, in remove
    self.data.remove(ref(item))
KeyError: (<weakref at 0x0000012E8CC759A8; to 'Win32Window' at 0x0000012EFC976278>,)

二、解决方案

        出现这样的问题，其实是因为env.close()没有得到正确的使用。而正确使用的方式如下：

        1.定义close()函数

        2.当程序运行结束时调用env.close()

        比如我定义了一个下面这样的环境类：

class GridWorldEnv(gym.Env):
    def you_function():
        pass
    # ...

        同时我需要在这个环境类中定义close函数：

class GridWorldEnv(gym.Env):
    def you_function():
        pass

    def close(self):
        if self.viewer:
            self.viewer.close()
            self.viewer = None
    # ...

        然后我是如何创建环境实例并进行学习的呢？如下：

if __name__ == "__main__":
    env = GridWorldEnv()
    agent = Agent(env)
    # agent learning
    # after learning, then you can close the env
    agent.env.close() # 调用此行，关闭环境

        最后再运行，程序就没有以上的报错了。

三、参考

ImportError: sys.meta_path is None, Python is likely shutting down · Issue #893 · openai/gym · GitHub中回答"nicehiro commented on 24 Aug 2019"