code: def Proxy(self, status, headers, exc_info=None):
      
      self.call_context['status'] = status
      self.call_context['headers'] = headers
      self.call_context['exc_info'] = exc_info
  
      return self.body_buffer.write
docs: Save args, defer start_response until response body is parsed.
  
      Create output buffer for body to be written into.
      Note: this is not quite WSGI compliant: The body should come back as an
        iterator returned from calling service_app() but instead, StartResponse
        returns a writer that will be later called to output the body.
      See google/appengine/ext/webapp/__init__.py::Response.wsgi_write()
          write = start_response('%d %s' % self.__status, self.__wsgi_headers)
          write(body)
  
      Args:
        status: Http status to be sent with this response
        headers: Http headers to be sent with this response
        exc_info: Exception info to be displayed for this response
      Returns:
        callable that takes as an argument the body content