面向对象编程：类与对象¶

# 定义一个简单的类
class Dog:
    # 类的属性（类变量）
    species = "Canis familiaris"

    # 初始化方法（构造函数）
    def __init__(self, name, age):
        # 实例属性
        self.name = name
        self.age = age

    # 实例方法
    def bark(self):
        return f"{self.name} says woof!"

# 创建Dog类的实例
my_dog = Dog("Buddy", 5)

# 访问对象的属性
print(my_dog.name)  # 输出: Buddy
print(my_dog.age)   # 输出: 5

# 调用对象的方法
print(my_dog.bark())  # 输出: Buddy says woof!

# 访问类变量
print(Dog.species)  # 输出: Canis familiaris

# 修改实例属性
my_dog.age = 6
print(my_dog.age)  # 输出: 6

# 调用实例方法
print(my_dog.bark())  # 输出: Buddy says woof!

class Car:
    # 类变量
    wheels = 4

    # 构造函数
    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year

    # 实例方法
    def description(self):
        return f"{self.year} {self.make} {self.model}"

# 创建Car类的实例
my_car = Car("Toyota", "Corolla", 2020)

# 访问属性和方法
print(my_car.wheels)  # 输出: 4
print(my_car.description())  # 输出: 2020 Toyota Corolla

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def celebrate_birthday(self):
        self.age += 1
        return f"Happy Birthday, {self.name}! You are now {self.age} years old."

# 创建Person类的实例
person = Person("Alice", 30)

# 调用方法修改属性
print(person.celebrate_birthday())  # 输出: Happy Birthday, Alice! You are now 31 years old.

class Circle:
    # 类变量
    pi = 3.14159

    def __init__(self, radius):
        self.radius = radius

    def area(self):
        return self.pi * (self.radius ** 2)

# 创建Circle类的实例
circle = Circle(5)

# 计算面积
print(circle.area())  # 输出: 78.53975