title: Select a tk radiobutton at startup
body: <p>Python 3.4</p>
  
  <p>I'm having what seems like a simple snag. I have a series of radiobuttons, choose one of many, and I just want to pick the first one as the default at startup, but after reading through the docs and a few how-to's I must be overlooking something.</p>
  
  <p>It seems that invoke() or select() might do it, but I get "'NoneType' object has no attribute 'invoke'", same for select. I also tried setting the state to 'selected' but that didn't seem to be a valid state for the radiobutton widget.</p>
  
  <p>EDIT: My main concern was just how do I set a default button. The NoneType error isn't really my question. It's just the most recent error I ran into when experimenting. If I'd put less detail someone would complain "what have you tried?", etc.</p>
  
  <p>I'm new to Python and Tk, so I guess I'm just overlooking something.</p>
  
  <p>Example:</p>
  
  <pre><code>button1 = ttk.Radiobutton(mainframe, text="Start", variable=m_type, value='start', command=start_options).grid(column=2, row=1, sticky=W)
  button1.invoke()
  </code></pre>
  
  <p>Thanks for any help.</p>