code: def base(self, 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]
docs: Return the base CLB for a given DU