new_contents: #!/usr/bin/env python3
  # -*- coding: utf-8 -*-
  
  # Copyright (c) 2015 Jérémie DECOCK (http://www.jdhp.org)
  
  # Permission is hereby granted, free of charge, to any person obtaining a copy
  # of this software and associated documentation files (the "Software"), to deal
  # in the Software without restriction, including without limitation the rights
  # to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  # copies of the Software, and to permit persons to whom the Software is
  # furnished to do so, subject to the following conditions:
  
  # The above copyright notice and this permission notice shall be included in
  # all copies or substantial portions of the Software.
   
  # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  # IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  # FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  # AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  # THE SOFTWARE.
  
  import tkinter as tk
  
  def main():
      """Main function"""
  
      root = tk.Tk()
  
      label = tk.Label(root, text="Please resize the window...")
      label.pack(fill=tk.BOTH, expand=1)
  
      def resize_callback(event):
          # http://stackoverflow.com/questions/4065783/how-do-i-get-a-windows-current-size-using-tkinter
          widget_current_width = label.winfo_width()      # Current label width (this work for every widgets!)
          widget_current_height = label.winfo_height()    # Current label height (this work for every widgets!)
  
          label["text"] = "Label widget size: {}x{}".format(widget_current_width, widget_current_height)
  
      label.bind('<Configure>', resize_callback)
  
      root.mainloop()
  
  if __name__ == '__main__':
      main()
  
subject: Add a widget (Tkinter).