Yield None until cmd finished

def exec_nb_cmd(self, cmd):
        '''
        Yield None until cmd finished
        '''
        r_out = []
        r_err = []
        rcode = None
        cmd = self._cmd_str(cmd)

        logmsg = 'Executing non-blocking command: {0}'.format(cmd)
        if self.passwd:
            logmsg = logmsg.replace(self.passwd, ('*' * 6))
        log.debug(logmsg)

        for out, err, rcode in self._run_nb_cmd(cmd):
            if out is not None:
                r_out.append(out)
            if err is not None:
                r_err.append(err)
            yield None, None, None
        yield ''.join(r_out), ''.join(r_err), rcode