Python 小萌新遭遇 AttributeError: module '__main__' has no attribute 'xxx' 错误，如何解决？

import sys
from core import enter
from core import student
from core import manager

flag0 = 1

def main():
    print('\033[1;34m欢迎进入学生选课系统\033[0m'.center(50, '*'))
    ret = enter.login()
    if ret:
        cls = getattr(sys.modules['__main__'], ret['status'])  # 得到类名
        if cls == 'student':
            obj = student.cls(ret['username'])  # 根据类名实例化对象
        else:
            obj = manager.cls(ret['username'])
        while flag0:
            if ret['status'] == 'student':
                print('\033[1;34m学生选课系统:学生端\033[0m'.center(50, '*') + '\n')
                print(('\033[1;34m当前登录用户:%s\033[0m' % obj.name).center(50))
            else:
                print('\033[1;34m学生选课系统:管理员端\033[0m'.center(50, '*') + '\n')
            for key, item in enumerate(cls.li, 1):
                print(str(key).center(20), item[0])
            num = int(input('\n输入您要做的操作序号：'))
            if num <= len(cls.li):
                getattr(obj, cls.li[num - 1][1])()
            else:
                print('\033[1;32m\n你的输入有误,请重新输入!\033[0m')

>>> type(cls)
<class 'function'>

class Student:
    # ...

class Manager:
    # ...