code: def density(self):
          
          r = float(self.sp_index.npoints) / float(self.sp_index.length)
          return r
docs: The percent of non- ``fill_value`` points, as decimal.
  
          Examples
          --------
          >>> s = SparseArray([0, 0, 1, 1, 1], fill_value=0)
          >>> s.density
          0.6