old_contents: import tkinter
  import tk_tools
  
  root = tkinter.Tk()
  root.title('TK Tools Calendar')
  cal = tk_tools.Calendar(root)
  cal.pack()
  
  
  def custom_callback():
      print(cal.selection)
  
  
  cal.add_callback(custom_callback)
  
  root.mainloop()
  
new_contents: import tkinter
  import tk_tools
  
  
  def callback():
      print(calendar.selection)
  
  
  if __name__ == '__main__':
  
      root = tkinter.Tk()
      root.title('TK Tools Calendar')
      calendar = tk_tools.Calendar(root)
      calendar.pack()
  
      calendar.add_callback(callback)
  
      root.mainloop()
  
subject: add if __name__ == '__main__':, renamed one variable, and one function