def _write_url_file(images, separator=" "):
  """ ImageNet generally uses a specific format to store the URLs of images.
  This converts a set of images into this format.
  Args:
    images: The set of images to write. It should contain tuples with image
    WNIDs and URLs.
    separator: Default string used to separate WNID from URL in the file.
  Returns:
    A raw string which can then be written to a file. """
  file_data = ""
  for wnid, url in images:
    line = "%s%s%s\n" % (wnid, separator, url)
    file_data += line
  return file_data