docs: def import_source(self, CachableSource):
          """Updates cache area and returns number of items updated with all
             available entries in ICachableSource
          """
          _count = 0
          self._import_source_items_id_list = set() # used to help speed up trim()
          for item in CachableSource.items():
              self._import_source_items_id_list.add(item.getId())
              if self.cache(item):
                  _count += 1
          return _count