code: def _foldername(self, additionalpath=""):
          
          if not self._foldername_cache.get(additionalpath):
              fn = joinpath(self.base, self.folder, additionalpath) \
                  if not self.is_subfolder \
                  else joinpath(self.base, ".%s" % self.folder, additionalpath)
              self._foldername_cache[additionalpath] = fn
          return self._foldername_cache[additionalpath]
docs: Dot decorate a folder name.