def redirect(self, url, permanent=False, status=None)
      重定向到给定的URL(可以选择相对路径).
      如果指定了 ``status`` 参数, 这个值将作为HTTP状态码; 否则
      将通过 ``permanent`` 参数选择301 (永久) 或者 302 (临时).
      默认是 302 (临时重定向).
    
      if self._headers_written:
          raise Exception("Cannot redirect after headers have been written")
      if status is None:
          status = 301 if permanent else 302
      else:
          assert isinstance(status, int) and 300 <= status <= 399
      self.set_status(status)
      self.set_header("Location", utf8(url))
      self.finish()


  def write(self, chunk):
    """把给定块写到输出buffer.

    为了把输出写到网络, 使用下面的flush()方法.

    如果给定的块是一个字典, 我们会把它作为JSON来写同时会把响应头
    设置为 ``application/json``. (如果你写JSON但是设置不同的
    ``Content-Type``,  可以调用set_header *在调用write()之后* ).

    注意列表不能转换为JSON 因为一个潜在的跨域安全漏洞. 所有的JSON
    输出应该包在一个字典中. 更多细节参考
    http://haacked.com/archive/2009/06/25/json-hijacking.aspx/ 和
    https://github.com/facebook/tornado/issues/1009
    """
    if self._finished:
        raise RuntimeError("Cannot write() after finish()")
    if not isinstance(chunk, (bytes, unicode_type, dict)):
        message = "write() only accepts bytes, unicode, and dict objects"
        if isinstance(chunk, list):
            message += ". Lists not accepted for security reasons; see http://www.tornadoweb.org/en/stable/web.html#tornado.web.RequestHandler.write"
        raise TypeError(message)
    if isinstance(chunk, dict):
        chunk = escape.json_encode(chunk)
        self.set_header("Content-Type", "application/json; charset=UTF-8")
    chunk = utf8(chunk)
    self._write_buffer.append(chunk)