模拟mvc expect异常

# 模型
class User:
    def __init__(self, username, password):
        self.username = username
        self.password = password

    def save(self):
        # 将用户数据保存到数据库
        pass

# 视图
@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        user = User(username, password)
        user.save()
        return '注册成功！'
    return render_template('register.html')

# 控制器
if __name__ == '__main__':
    app.run()

# 控制器
@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        try:
            # 检查用户名是否已存在
            if is_username_exists(username):
                raise Exception('用户名已存在')
            user = User(username, password)
            user.save()
            return '注册成功！'
        except Exception as e:
            return '注册失败：' + str(e)
    return render_template('register.html')