To set a breakpoint under idle:
-have a "Python Shell" (the one which opens up when you fire up idle) open
-Select Debug->Debugger, a window should pop open having the GUI for the debugger
Now you can set breakpoint in your code.
If you dont have this debug window open, you get a bell from the function
PyShellEditorWindow.set_breakpoint_here(self, event=none) as the attribute
self.flist.pyshell.interp.debugger is <None>.
It seems that the function PyShell.toggle_debugger() (called when you select the above
menu option) gives this attribute a valid value.