human
Here's a good function header given the following:
I wrote the below code

    cmd = config.DOWNLOAD_COMMAND.get
    (ddir, basename) = (config.DDIR.get, os.path.basename(filename))
    cmd_list = shlex.split(cmd)
    def list_string_sub(orig, repl, lst):
        '    Replace    substrings    for    items    in    a    list.    '
        return [(x if (orig not in x) else x.replace(orig, repl)) for x in lst]
    cmd_list = list_string_sub('%F', filename, cmd_list)
    cmd_list = list_string_sub('%d', ddir, cmd_list)
    cmd_list = list_string_sub('%f', basename, cmd_list)
    cmd_list = list_string_sub('%u', url, cmd_list)
    cmd_list = list_string_sub('%i', song.ytid, cmd_list)
    util.dbg('Downloading    using:    %s', '    '.join(cmd_list))
    subprocess.call(cmd_list)

What's a good function header?
assistant
#Here's a good function header.

def external_download(song, filename, url):