code: def start_response(self, status, headers, exc_info = None):
      
      if self.started_response:
        if not exc_info:
          raise AssertionError("WSGI start_response called a second "
                     "time with no exc_info.")
        else:
          try:
            raise exc_info[0], exc_info[1], exc_info[2]
          finally:
            exc_info = None
      self.started_response = True
      self.status = status
      self.outheaders.extend(headers)
  
      return self.write_buffer.write
docs: WSGI callable to begin the HTTP response.