I have the following code :
myList =[\'a\',\'b\']
rb=wx.RadioBox(self.panel, -1, \"Options :\", (0, 0), wx.DefaultSize,myList, 2, wx.RA_SPECIFY_COLS)
The use of a radio box implies, "there is a list of two or more options that are mutually exclusive and the user must select exactly one choice."
The radio box never exists in a state with no choice made. If that's not the case, then don't use a radio box.
If you don't want any of the current radio box options to be selected as default, add another option for "N/A" or "No choice".