Tkinter Radiobutton Menu: how to select programmatically (newbie)

[Charles Medcoff]

How does one programmatically select the active entry a radio button menu?
For example in a drawing application with a tool menu with entries for
"Selector", "Line", "Rectangle", etc I want to automatically change the
active drawing task to "Selector" and I need to have the radiobutton menu
reflect this.

Regards,
Chuck