关键点： 每次调用实例方法时，Python会自动将对象作为self参数传递。 这就是为什么my_dog.bark()和your_dog.bark()会产生不同结果的原因。

修改对象状态的实例方法

class BankAccount:
    def __init__(self, owner, balance=0):
        self.owner = owner
        self.balance = balance
    
    def deposit(self, amount):
        """存款方法"""
        self.balance += amount
        return f"存入{amount}元。当前余额: {self.balance}元"
    
    def withdraw(self, amount):
        """取款方法"""
        if amount > self.balance:
            return "余额不足"
        self.balance -= amount
        return f"取出{amount}元。当前余额: {self.balance}元"

# 创建银行账户
account = BankAccount("张三", 1000)

# 调用修改状态的实例方法
print(account.deposit(500))  
# 输出: 存入500元。当前余额: 1500元

print(account.withdraw(200)) 
# 输出: 取出200元。当前余额: 1300元

print(account.withdraw(2000))
# 输出: 余额不足

带参数的实例方法

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height
    
    def area(self):
        """计算面积"""
        return self.width * self.height
    
    def resize(self, new_width, new_height):
        """调整矩形大小"""
        self.width = new_width
        self.height = new_height
        return f"尺寸已更新为: {self.width}x{self.height}"

# 创建矩形对象
rect = Rectangle(5, 10)

# 调用带参数的方法
print("初始面积:", rect.area()) 
# 输出: 初始面积: 50

# 调用resize方法改变对象状态
print(rect.resize(7, 8))      
# 输出: 尺寸已更新为: 7x8

print("新面积:", rect.area())   
# 输出: 新面积: 56

class Student:
    def __init__(self, name, student_id):
        self.name = name
        self.student_id = student_id
        self.courses = {}
    
    def enroll(self, course_name):
        """注册课程"""
        if course_name not in self.courses:
            self.courses[course_name] = None
            return f"{self.name}已注册{course_name}课程"
        return f"{self.name}已经注册过{course_name}课程"
    
    def update_grade(self, course_name, grade):
        """更新课程成绩"""
        if course_name in self.courses:
            self.courses[course_name] = grade
            return f"{course_name}成绩更新为: {grade}"
        return f"{self.name}未注册{course_name}课程"
    
    def get_gpa(self):
        """计算平均成绩"""
        if not self.courses:
            return 0.0
        
        total = 0
        count = 0
        for grade in self.courses.values():
            if grade is not None:
                total += grade
                count += 1
        return total / count if count > 0 else 0.0
    
    def display_info(self):
        """显示学生信息"""
        info = f"学生姓名: {self.name}\n学号: {self.student_id}\n课程成绩:"
        for course, grade in self.courses.items():
            info += f"\n  - {course}: {grade if grade is not None else '未评分'}"
        info += f"\n当前GPA: {self.get_gpa():.2f}"
        return info

# 使用示例
student1 = Student("张三", "S2023001")
print(student1.enroll("数学"))    # 张三已注册数学课程
print(student1.enroll("英语"))    # 张三已注册英语课程

print(student1.update_grade("数学", 90))   # 数学成绩更新为: 90
print(student1.update_grade("英语", 85))   # 英语成绩更新为: 85

print("\n学生信息:")
print(student1.display_info())