未将嵌套类视为异常

以下代码表示找不到MyError:

   class Test:
       class MyError(ValueError):
           pass

       def bad(self):
           raise MyError

       def myfunc(self):
           try:
               self.bad()
           except MyError as me:
               print(me)


   tt = Test()
   tt.myfunc()

当我运行它时,我得到以下信息:

 Traceback (most recent call last):
   File "testcase.py", line 16, in <module>
     tt.myfunc()
   File "testcase.py", line 11, in myfunc
     except MyError as me:
 NameError: name 'MyError' is not defined

关于引用嵌套类,我缺少什么?