def commit_check(self, commands="", req_format="text"):
        if not commands:
            raise InvalidCommandError('No commands specified')
        clean_cmds = []
        for cmd in clean_lines(commands):
            clean_cmds.append(cmd)
        self.lock()
        self._session.load_configuration(action='set', config=clean_cmds)
        results = ET.fromstring(self._session.validate(
            source='candidate').tostring)
        self.unlock()
        if req_format == "xml":
            return ET.tostring(results)
        out = ""
        for i in results.iter():
            if i.tag == 'commit-check-success':
                out += 'configuration check succeeds\n'
            elif i.text is not None:
                if i.text.strip() + '\n' != '\n':
                    out += i.text.strip() + '\n'
            elif i.text is None:
                if i.tag + '\n' != '\n':
                    out += i.tag + '\n'
        return out