code: def length(self):
          
          span = self._stop - self._start
          length, modulo = divmod(span, self._step)
          if length < 0:
              return 0
          if modulo != 0:
              return length + 1
          return length
docs: The number of elements changed.
  
          :rtype: int
  
          .. code:: python
  
              assert change.length == len(change.indices)
              assert change.length == len(change.elements)