code: def secure_filename(filename):
      r
      if isinstance(filename, text_type):
          from unicodedata import normalize
  
          filename = normalize("NFKD", filename).encode("ascii", "ignore")
          if not PY2:
              filename = filename.decode("ascii")
      for sep in os.path.sep, os.path.altsep:
          if sep:
              filename = filename.replace(sep, " ")
      filename = str(_filename_ascii_strip_re.sub("", "_".join(filename.split()))).strip(
          "._"
      )
  
      # on nt a couple of special files are present in each folder.  We
      # have to ensure that the target file is not such a filename.  In
      # this case we prepend an underline
      if (
          os.name == "nt"
          and filename
          and filename.split(".")[0].upper() in _windows_device_files
      ):
          filename = "_" + filename
  
      return filename
docs: r"""Pass it a filename and it will return a secure version of it.  This
      filename can then safely be stored on a regular file system and passed
      to :func:`os.path.join`.  The filename returned is an ASCII only string
      for maximum portability.
  
      On windows systems the function also makes sure that the file is not
      named after one of the special device files.
  
      >>> secure_filename("My cool movie.mov")
      'My_cool_movie.mov'
      >>> secure_filename("../../../etc/passwd")
      'etc_passwd'
      >>> secure_filename(u'i contain cool \xfcml\xe4uts.txt')
      'i_contain_cool_umlauts.txt'
  
      The function might return an empty filename.  It's your responsibility
      to ensure that the filename is unique and that you abort or
      generate a random filename if the function returned an empty one.
  
      .. versionadded:: 0.5
  
      :param filename: the filename to secure