In the documentation for tkinter, "A Simple Hello World Program" Application class does not hold onto the master Tk() instance as a class attribute. This is a good practice, and newcomers to tkinter would likely have trouble closing the window without this since root will almost never be in the global namespace.

The original example:

"""""

import tkinter as tk

class Application(tk.Frame):
    def __init__(self, master=None):
        super().__init__(master)
        self.pack()
        self.create_widgets()

    def create_widgets(self):
        self.hi_there = tk.Button(self)
        self.hi_there["text"] = "Hello World\n(click me)"
        self.hi_there["command"] = self.say_hi
        self.hi_there.pack(side="top")

        self.quit = tk.Button(self, text="QUIT", fg="red",
                              command=root.destroy)
        self.quit.pack(side="bottom")

    def say_hi(self):
        print("hi there, everyone!")

root = tk.Tk()
app = Application(master=root)
app.mainloop()

""""

The proposed fix:

""""
import tkinter as tk

class Application(tk.Frame):
    def __init__(self, master=None):
        super().__init__(master)
        self.master = master
        self.pack()
        self.create_widgets()

    def create_widgets(self):
        self.hi_there = tk.Button(self)
        self.hi_there["text"] = "Hello World\n(click me)"
        self.hi_there["command"] = self.say_hi
        self.hi_there.pack(side="top")

        self.quit = tk.Button(self, text="QUIT", fg="red",
                              command=self.master.destroy)
        self.quit.pack(side="bottom")

    def say_hi(self):
        print("hi there, everyone!")
        
def main():
    root = tk.Tk()
    app = Application(master=root)
    app.mainloop()

if __name__ == "__main__":
    main()

""""

Thanks for the report. So the current example in the docs works fine since root is in global namespace. But this seems to be a sensible change to use self.master which references root instead of relying on root to be global though I don't know we should restructure the example to use main(). I am adding Serhiy for review.

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.

New changeset a80af770870937271865b5e2b05a2cfe40b024b6 by Serhiy Storchaka (Daniel Lovell) in branch 'master':
bpo-35086: Fix tkinter example "A Simple Hello World Program". (GH-10160)
https://github.com/python/cpython/commit/a80af770870937271865b5e2b05a2cfe40b024b6

New changeset f51ef51db686938486bff453e791a3093a1df108 by Miss Islington (bot) in branch '3.7':
bpo-35086: Fix tkinter example "A Simple Hello World Program". (GH-10160)
https://github.com/python/cpython/commit/f51ef51db686938486bff453e791a3093a1df108

New changeset c843a47007293d8361d0bfd45bfd7169afaa601c by Miss Islington (bot) in branch '3.6':
bpo-35086: Fix tkinter example "A Simple Hello World Program". (GH-10160)
https://github.com/python/cpython/commit/c843a47007293d8361d0bfd45bfd7169afaa601c