Let's give it a try the regular way. Open up your previous code and change it to look like this:

class Window(tk.Tk):
    def __init__(self):
        super().__init__()
        self.title("Hello Tkinter")
        self.label_text = "Choose One"

        self.label = tk.Label(self, text=self.label_text)
        self.label.pack(fill=tk.BOTH, expand=1, padx=100, pady=30)

        hello_button = tk.Button(self, text="Say Hello", 
                                 command=self.say_hello)
        hello_button.pack(side=tk.LEFT, padx=(20, 0), pady=(0, 20))

        goodbye_button = tk.Button(self, text="Say Goodbye",
                                   command=self.say_goodbye)
        goodbye_button.pack(side=tk.RIGHT, padx=(0, 20), pady=(0, 20))

        def say_hello(self):
          self.label_text = "Hello World"

        def say_goodbye(self):
          self.label_text="Goodbye! 
 (Closing in 2 seconds)"
          self.after(2000, self.destroy)


if __name__ == "__main__":
  window = Window()
  window.mainloop()

Give this code a whirl and click on your Say Hello button. Nothing happens. Now try your Say Goodbye button. The label will not update, but the window will still close after 2 seconds. This goes to show that the code written is not invalid, but will not behave as we may expect it to.