code: def start_response(self, status, response_headers, exc_info=None):
          
          if exc_info:
              try:
                  if self.headers_sent:
                      # Re-raise original exception if headers sent
                      # because this violates WSGI specification.
                      raise
              finally:
                  exc_info = None
          elif self.header_set:
              raise AssertionError("Headers already set!")
  
          if PY3K and not isinstance(status, str):
              self.status = str(status, 'ISO-8859-1')
          else:
              self.status = status
          # Make sure headers are bytes objects
          try:
              self.header_set = Headers(response_headers)
          except UnicodeDecodeError:
              self.error = ('500 Internal Server Error',
                            'HTTP Headers should be bytes')
              self.err_log.error('Received HTTP Headers from client that contain'
                                 ' invalid characters for Latin-1 encoding.')
  
          return self.write_warning
docs: Store the HTTP status and headers to be sent when self.write is
          called.