title: Tkinter: can't set values in combobox
body: <p>I'm just learning Tkinter and have gone through numerous examples of setting up a combo box with predefined values, but I keep getting an error when I try to run it.</p>
  
  <p><em>combo['Values'] = ("Private", "Commercial", "ATP")
  TypeError: 'NoneType' object does not support item assignment</em></p>
  
  <p>Here is my code:</p>
  
  <pre><code>from tkinter import *
  from tkinter import ttk
  
  root = Tk()
  root.title("Title Here")
  root.geometry("250x300")
  frame = ttk.Frame(root)
  
  labeltext = StringVar()
  labeltext.set("Click the OK button!!") # use the .set option to set the text value
  
  checkboxvar = StringVar()
  radiovar = StringVar()
  entryVar = StringVar()
  comboVar = StringVar()
  
  
  
  
  def changetext():
      labeltext.set("thanks")
      print("OK button clicked")
      return
  
  
  frame.grid(column=0, row=0, sticky = (N,S,E,W))
  ttk.Label(frame, textvariable = labeltext, foreground="red").grid(column=1, row=1, sticky = W)
  
  
  button1 = ttk.Button(frame, text="OK", default="active", command=changetext).grid(column=2, row=1, sticky = W)
  
  checkbox = ttk.Checkbutton(frame, text="check me", variable = checkboxvar).grid(column=1, row = 3, sticky=W)
  
  rbOne = ttk.Radiobutton(frame, text="One", variable=radiovar, value = "1").grid(column=1, row = 4, sticky = W)
  rbTwo = ttk.Radiobutton(frame, text="Two", variable=radiovar, value = "2").grid(column=1, row = 5, sticky = W)
  rbThree = ttk.Radiobutton(frame, text="Three", variable=radiovar, value = "3").grid(column=1, row = 6, sticky = W)
  
  entry = ttk.Entry(frame, textvariable=entryVar).grid(column=1, row =7, sticky=W)
  
  
  combo = ttk.Combobox(frame, textvariable=comboVar).grid(column=1, row =2, sticky=W)
  combo['Values'] = ("Private", "Commercial", "ATP")
  
  
  root.mainloop()
  </code></pre>