Radio buttons in python 3 and tkinter

You want to use the select method on the a radio button like so:

import tkinter as tk

root = tk.Tk()
root.title("Radio Button Example")    
button = tk.Radiobutton(root)  # Make a radio button
button.pack()  # Pack it to the screen
button.select()  #This is the bit that makes it checked
root.mainloop()

For some more information about RadioButtons in tk check out this page on tutorials point:

http://www.tutorialspoint.com/python/tk_radiobutton.htm