def write(self, chunk):
        """WSGI callable to write unbuffered data to the client.
        
        This method is also used internally by start_response (to write
        data from the iterable returned by the WSGI application).
        """
        if not self.started_response:
            raise AssertionError("WSGI write called before start_response.")
        
        if not self.req.sent_headers:
            self.req.sent_headers = True
            self.req.send_headers()
        
        self.req.write(chunk)