new_contents: """Read an image index from the index file and write it back.
  """
  
  import os.path
  import shutil
  import filecmp
  import pytest
  import photo.index
  from conftest import tmpdir, gettestdata
  
  testimgs = [ 
      "dsc_4623.jpg", "dsc_4664.jpg", "dsc_4831.jpg", 
      "dsc_5126.jpg", "dsc_5167.jpg" 
  ]
  testimgfiles = [ gettestdata(i) for i in testimgs ]
  
  refindex = gettestdata("index-tagged.yaml")
  
  @pytest.fixture(scope="module")
  def imgdir(tmpdir):
      for fname in testimgfiles:
          shutil.copy(fname, tmpdir)
      shutil.copy(refindex, os.path.join(tmpdir, ".index.yaml"))
      return tmpdir
  
  def test_read_write(imgdir):
      """Read the index file and write it out again.
      """
      idx = photo.index.Index(idxfile=imgdir)
      idx.write()
      idxfile = os.path.join(imgdir, ".index.yaml")
      assert filecmp.cmp(refindex, idxfile), "index file differs from reference"
  
subject: Add a test to read an image index and write it back.