Thanks for the reply xtreak. I agree that changing the example to include main() isn't necessary - I unintentionally included that from my example of the case where the current version isn't functional. 

In the PR I submitted on Github (https://github.com/python/cpython/pull/10160), the proposed change is simply adding the master Tk instance to self.master then using that to close the window in the Quit button callback.

Sorry for the confusion.