title: Tkinter: How to set ttk.Radiobutton activated and get its value?
body: <p>1) I need to set one of my three ttk.Radiobuttons activated by default
      when I start my gui app.<br>
      How do I do it?</p>
  
  <p>2) I also need to check if one of my ttk.Radiobuttons was
      activated/clicked by the user.<br>
      How do I do it?</p>
  
  <pre><code>rb1 = ttk.Radiobutton(self.frame, text='5', variable=self.my_var, value=5)
  rb2 = ttk.Radiobutton(self.frame, text='10', variable=self.my_var, value=10)
  rb3 = ttk.Radiobutton(self.frame, text='15', variable=self.my_var, value=15)
  self.rb1.grid(row=0)
  self.rb2.grid(row=1)
  self.rb3.grid(row=2)
  </code></pre>