(1)  Why did you change the name of the parameter from index to element?  If the underlying engine also accepts indices (e.g., self.activate(4) ) then the name should stay.  If it really is only meaningful to use the name of an element -- maybe it should still stay for backwards compatibility.  Or at least accept the old name too for a release.

Either way, please provide a test case showing that it works under the new name; there may also be doc fixes.  (I'm not sure there is documentation for this widget, though, and providing some in the first place is good, but perhaps a different task.)

FWIW, 
http://docs.python.org/dev/library/tkinter.html#the-index-parameter
suggests that the name should stay index, and can be far more than an element; migrating some of that to the docstring might be helpful.

(2)  It looks like the set command took *args to give some freedom.  There may be extensions that take a third argument.  It may well be valid to call it without any arguments, or with only one.  So the signature may turn into set(first=0, last=1, *args) or some such.  Whatever the answer about what arguments are really needed, there should be test cases to demonstrate this before the API is changed.