'Focus a caption line as specified by `line_number`
Arguments:
line_number (int): caption line number'

def focus_caption_line(self, line_number):

    caption_line_selector = ".subtitles    li    span[data-index='{index}']".format(index=(line_number - 1))
    self.q(css=caption_line_selector).results[0].send_keys(Keys.ENTER)
