Modify based on comments

address_book.py

#Name
#Date
#Add global variable - a list of addresses
#open button should open a set text file
#and load addresses
#close should save using the list
#When open first item displayed
#next moves to next item
import tkinter

class MyGui:
    def __init__(self):
        #main window
        self.main_window = tkinter.Tk()

        #menus
        self.menubar = tkinter.Menu(self.main_window)
        self.filemenu = tkinter.Menu(self.main_window,tearoff=0)
        self.filemenu.add_command(label="Open", command=self.fileopen)
        self.filemenu.add_command(label="Save", command=self.filesave)
        self.filemenu.add_command(label="Exit",command=self.main_window.quit)

        self.menubar.add_cascade(label="File",menu=self.filemenu)
        self.main_window.config(menu = self.menubar)

        #frames
        self.topframe = tkinter.Frame(self.main_window)
        self.bottomframe = tkinter.Frame(self.main_window)
        self.topframe.pack()
        self.bottomframe.pack()

        #labels and textboxes
        self.lbl_firstname = tkinter.Label(self.topframe,text="First Name")
        self.lbl_firstname.pack(side="left")
        self.fname_text = tkinter.StringVar()
        self.fname_text.set("First name")
        self.txt_firstname = tkinter.Entry(self.topframe,text=self.fname_text)
        self.txt_firstname.pack(side="left")

        #buttons
        self.btn_next = tkinter.Button(self.bottomframe,text="Next",command=self.btn_next_click)
        self.btn_next.pack()
        #main loop
        tkinter.mainloop()
    def btn_next_click(self):
        self.fname_text.set("Replace with next name")

    def fileopen(self):
        """To-do: add code to open address
        book file. File should be:
        firstname,lastname,phone"""
        print("Open method")
    def filesave(self):
        """To-do: add code to save address
        book file."""
        print("Save method")


gui = MyGui()