code: def _get_dim_size(start, stop, step):
      
      assert step != 0
      if step > 0:
          assert start < stop
          dim_size = (stop - start - 1) // step + 1
      else:
          assert stop < start
          dim_size = (start - stop - 1) // (-step) + 1
      return dim_size
docs: Given start, stop, and stop, calculate the number of elements
      of this slice.