写一个学生管理系统代码

下面是一个简单的学生管理系统代码示例：

class Student:
    def __init__(self, name, age, gender, grade):
        self.name = name
        self.age = age
        self.gender = gender
        self.grade = grade
        self.courses = []

    def enroll(self, course):
        self.courses.append(course)
    
    def withdraw(self, course):
        self.courses.remove(course)

class Course:
    def __init__(self, name, credits):
        self.name = name
        self.credits = credits

class StudentManager:
    def __init__(self):
        self.students = []

    def add_student(self, student):
        self.students.append(student)
    
    def remove_student(self, student):
        self.students.remove(student)
    
    def get_students(self):
        return self.students

manager = StudentManager()

student1 = Student("张三", 20, "男", "大一")
student2 = Student("李四", 21, "女", "大二")
student3 = Student("王五", 22, "男", "大三")

course1 = Course("数学", 4)
course2 = Course("英语", 4)
course3 = Course("计算机科学", 4)

manager.add_student(student1)
manager.add_student(student2)
manager.add_student(student3)

student1.enroll(course1)
student1.enroll(course2)
student2.enroll(course2)
student2.enroll(course3)
student3.enroll(course1)
student3.enroll(course3)

students = manager.get_students()
for student in students:
    print(f"{student.name}的选修课程: {[course.name for course in student.courses]}")

manager.remove_student(student2)
students = manager.get_students()
print(f"删除后的学生列表: {[student.name for student in students]}")

这个示例代码定义了三个类：`Student`、`Course` 和 `StudentManager`。

- `Student` 类表示一名学生，它有四个属性：名字、年龄、性别和年级。还有两个方法：`enroll` 用于学生选课，`withdraw` 用于学生退课。

- `Course` 类表示一门课