Return the base CLB for a given DU

def base(self, du):
        """Return the base CLB for a given DU"""
        parameter = 'base'
        if parameter not in self._by:
            self._by[parameter] = {}
            for clb in self.upi.values():
                if clb.floor == 0:
                    self._by[parameter][clb.du] = clb
        return self._by[parameter][du]