code: def write_packed(self, outfile, rows):
          
  
          if self.rescale:
              raise Error("write_packed method not suitable for bit depth %d" %
                self.rescale[0])
          return self.write_passes(outfile, rows, packed=True)
docs: Write PNG file to `outfile`.  The pixel data comes from `rows`
          which should be in boxed row packed format.  Each row should be
          a sequence of packed bytes.
  
          Technically, this method does work for interlaced images but it
          is best avoided.  For interlaced images, the rows should be
          presented in the order that they appear in the file.
  
          This method should not be used when the source image bit depth
          is not one naturally supported by PNG; the bit depth should be
          1, 2, 4, 8, or 16.