title: Trace method in tkinter
body: <p>I want Combobox 2 to display diffrent values depending on the state of a 1 Combobox. Combobox 2 also changes the state of a Entry box. What I don't understand is why the on_field_change function work's without a problem but the check_website one gives me a <code>TypeError: trace_variable() missing 1 required positional argument: 'callback'</code> error. Why? Any suggestions?</p>
  
  <pre><code>import tkinter as tk
  from tkinter import ttk
  
  
  main = tk.Tk()
  app = tk.Frame(main)
  app.grid()
  
  
  def on_field_change(index, value, op):
      if novel.get() in has_book_website1:
          book_number_chosen.configure(state="enabled")
      else:
          book_number_chosen.configure(state="disabled")
  
  
  def check_website(index, value, op):
      if website.get() == "Website1":
          novel_chosen["values"] = namelist_website1
      elif website.get() == "Website2":
          novel_chosen["values"] = namelist_website2
  
  
  website = tk.StringVar
  website_chosen = ttk.Combobox(app, width=42, textvariable=website,                         
                   state="readonly")
  website.trace("w", check_website)
  website_chosen["values"] = ["Website1", "Website12"]
  website_chosen.current(0)
  website_chosen.grid(column=1, row=0)
  
  book_number = tk.IntVar()
  book_number_chosen = ttk.Entry(app, width=5, textvariable=book_number,             
  state="disabled")
  
  
  novel = tk.StringVar()
  novel_chosen = ttk.Combobox(app, width=42, textvariable=novel, 
                 state="readonly")
  novel.trace("w", on_field_change)
  novel_chosen["values"] = ["1", "2"]
  novel_chosen.grid(column=1, row=1)
  novel_chosen.current(0)
  
  has_book_website1 = ["2"]
  
  namelist_website1 = ["Just", "some", "text"]
  namelist_website2 = ["lorem", "ipsum"]
  
  main.mainloop()
  </code></pre>