设计一个学生类，一个班级类，实现向班级中添加，删除，查看学生，按照指定顺序排序。（初学到面向对象请多指教）

# 建立学生类
class Student:
    def __init__(self,name,num,age,score):
        self.name = name
        self.num = num
        self.age = age
        self.score = score
    def __str__(self):
        return '姓名：{}，学号：{}，年龄：{}，成绩：{}'.format(self.name,self.num,self.age,self.score)
# 班级类
class Class:
    def __init__(self,name):
        self.name = name
        self.stu_list = []
        self.stu_dict = {}
    # 添加学生
    def add_stu(self,stu):
        self.stu_list.append(stu)
        self.stu_dict[stu.num] = stu
    # 删除学生
    def del_stu(self,num):
        # 从字典中删除
        s = self.stu_dict.pop(num)
        # 从列表中删除
        self.stu_list.reverse(s)
    #学生排序
    def sort_stu(self,key = None, reverse = False):
        self.stu_list.sort(key=key,reverse=reverse)
    #查找学生
    def get_stu(self,num):
        return self.stu_dict.get(num)
    #展示学生信息
    def show_stu(self):
        for s in self.stu_list:
            print(s)
from  random import randint
# 创建班级对象
c = Class('ZZ-JY-PY1805')
# 添加学生
for i in range(10):
    name = 'stu'+str(i+1)
    num = 100 + i
    age = randint(20,30)
    score = randint(0,100)
    #创建学生对象
    stu = Student(name,num,age,score)
    #添加到班级对象中
    c.add_stu(stu)
#按照成绩
c.sort_stu(key=lambda s:s.score,reverse=True)
#展示学生信息
c.show_stu()
# 查找num = 104的学生
s = c.get_stu(104)
print(s)