from tkinter import *
from tkinter.ttk import *
import os, arcpy

tk=Tk()
progress=Progressbar(tk,orient=HORIZONTAL,length=100,mode='determinate')

def bar():
    import time
    os.startfile("Assignment.mxd")
    progress['value']=20
    tk.update_idletasks()
    time.sleep(8)
    progress['value']=50
    tk.update_idletasks()
    time.sleep(8)
    progress['value']=80
    tk.update_idletasks()
    time.sleep(8)
    progress['value']=100


progress.pack()
Button(tk,text='Open',command=bar).pack()
mainloop()