Gui.py

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()



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

        self.entrythingy = tk.Entry()
        self.entrythingy.pack()

        # here is the application variable
        self.contents = tk.StringVar()
        # set it to some value
        self.contents.tk.set("this is a variable")
        # tell the entry widget to watch this variable
        self.entrythingy["textvariable"] = self.tk.contents

        # and here we get a callback when the user hits return.
        # we will have the program print out the value of the
        # application variable when the user hits return
        self.entrythingy.bind('<Key-Return>',
                              self.print_contents)

    def print_contents(self, event):
        print("hi. contents of entry is now ---->",
              self.contents.get())