abstract layer question

In the previous layer we had an explicit abstract layer that handled
common functionality. In the current incarnation that layer seems to
have largely vanished away ie the back ends return an instance of the
wrapper class.

How should common functionality be coded

as an example I'm trying to code a progress bar in my 0.1 version I had

a range state item  and want to ensure it's correctly ordered low<high
so in the abstract class I had

        
        def _set_range(self,range):
                assert range[0]<range[1], "reversed or zero range"
                self._range = range
                self.pos = range[0]

in the backend I used _ensure_pos to transmit the value to the
underlying widget.

How should I achieve the same kind of thing in 0.2.
-- 
Robin Becker


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