Normally I would recommend placing global variables in a separate Python file and importing them. I would also separate the functions and classes into modules and packages. I am not recommending this because:

• Class projects are generally smaller that normal projects
• Multiple files are harder to keep track of (multiple files, multiple students)
• No version or content management system is used for the class

Talk to the instructor if you want to use more thant one file.

# =========================================================
# Program Name/Description
# =========================================================
# Name: ..................
# Date: mm/dd/yyyy
# =========================================================
#
# Description / Usage
#
# ..................................................
# ..................................................
# ..................................................
#
# =========================================================
#
# Change History
#
# May 26, 2017 Developer Name
#              original code completed
#
# =========================================================
# Note: If a DVCS (like GIT) is used, some of the data
#       in this header would stored there. Pydoc could be
#       use to create documentation.
# =========================================================

# ---------------------------------------------------------
# --- import
# ---------------------------------------------------------

import sys

# ---------------------------------------------------------
# --- global variables
# ---------------------------------------------------------

buttonFont = ('arial', '14', 'bold')
displayFont = ('courier', '12', 'normal')

# ---------------------------------------------------------
# --- classes
# ---------------------------------------------------------

# ---------------------------------------------------------
# --- functions
# ---------------------------------------------------------

# ---------------------------------------------------------
# --- main
# ---------------------------------------------------------

if __name__ == '__main__':

    if sys.version_info.major == 3:
        from tkinter import *
        from tkinter.font import *
        ##import tkinter as tk
        ##import tkinter.font as tkf
        py3 = True
    else:
        from Tkinter import *
        from tkFont import *
        ##import Tkinter as tk
        ##import tkFont as tkf
        py3 = False

    root = Tk()
    root.title('My Program')
    ##root.resizable(width=False,height=False)

    Grid.columnconfigure(root, 0, weight=1)
    Grid.rowconfigure(root, 0, weight=1)

    ........................
    ........................

    root.mainloop()