code: def setslice_with_length(self, startindex, endindex, fs, length):
          
          if len(self) < startindex:
              fs = ' '*(startindex - len(self)) + fs
          if len(self) > endindex:
              fs = fs + ' '*(endindex - startindex - len(fs))
              assert len(fs) == endindex - startindex, (len(fs), startindex, endindex)
          result = self.splice(fs, startindex, endindex)
          assert len(result) <= length
          return result
docs: Shim for easily converting old __setitem__ calls