code: def write_shiftfile(image_list, filename, outwcs='tweak_wcs.fits'):
      
      rows = ''
      nrows = 0
      for img in image_list:
          row = img.get_shiftfile_row()
          if row is not None:
              rows += row
              nrows += 1
      if nrows == 0:  # If there are no fits to report, do not write out a file
          return
  
      # write out reference WCS now
      if os.path.exists(outwcs):
          os.remove(outwcs)
      p = fits.HDUList()
      p.append(fits.PrimaryHDU())
      p.append(createWcsHDU(image_list[0].refWCS))
      p.writeto(outwcs)
  
      # Write out shiftfile to go with reference WCS
      with open(filename, 'w') as f:
          f.write('# frame: output\n')
          f.write('# refimage: %s[wcs]\n' % outwcs)
          f.write('# form: delta\n')
          f.write('# units: pixels\n')
          f.write(rows)
      print('Writing out shiftfile :', filename)
docs: Write out a shiftfile for a given list of input Image class objects