docs: def safe_join(directory, *pathnames):
      """Safely join `directory` and one or more untrusted `pathnames`.  If this
      cannot be done, this function returns ``None``.
  
      :param directory: the base directory.
      :param pathnames: the untrusted pathnames relative to that directory.
      """
      parts = [directory]
      for filename in pathnames:
          if filename != "":
              filename = posixpath.normpath(filename)
          for sep in _os_alt_seps:
              if sep in filename:
                  return None
          if os.path.isabs(filename) or filename == ".." or filename.startswith("../"):
              return None
          parts.append(filename)
      return posixpath.join(*parts)