Python tkinter tidbit: centred window

This little application simply creates a window and centres it on the screen (gist):

   1 #! /usr/bin/env python3
   2
   3 from tkinter import *
   4 from tkinter.ttk import *
   5
   6 def centred_win(root, width=400, height=300):
   7         screen_width = root.winfo_screenwidth()
   8         screen_height = root.winfo_screenheight()
   9         x = (screen_width/2) - (width/2)
  10         y = (screen_height/2) - (height/2)
  11         root.geometry('%dx%d+%d+%d' % (width, height, x, y))
  12
  13 root = Tk()
  14 centred_win(root, 500, 300)
  15 root.mainloop()