title: Radiobutton instance has no attibute 'variable
body: <p>I'm trying to give the user a set of choices using radio buttons.  When I click on the "Get Data Type" button, Python returns "AttributeError: Radiobutton instance has no attribute 'variable'.  Clearly it does, but refuses to see it (them):</p>
  
  <pre><code>class DataType:
  
      def __init__(self, master):
          frame = LabelFrame(master, text = 'Data Type')
          frame.pack()
  
          data_contents = StringVar()
  
          self.radiobutton = Radiobutton(frame,
                       text="Fixed Data",
                       variable = data_contents,
                       value = 'fixed')
          self.radiobutton.pack()
  
          self.radiobutton = Radiobutton(frame,
                       text="Random Data",
                       variable = data_contents,
                       value = 'random')
          self.radiobutton.pack()
  
          self.radiobutton = Radiobutton(frame,
                       text="All 1's",
                       variable = data_contents,
                       value = '1s')
          self.radiobutton.pack()
  
          self.radiobutton = Radiobutton(frame,
                       text="All 0's",
                       variable = data_contents,
                       value = '0s')
          self.radiobutton.pack()
  
  
          data_contents.set(self.radiobutton)
          self.printdata = Button(frame,
                           text="What data?",
                           command=self.write_data)
          self.printdata.pack()
  
      def write_data(self):
          print (self.radiobutton.variable)
  </code></pre>