Python内置函数之classmethod函数使用详解

class MyClass:
    @classmethod
    def class_method(cls, arg1, arg2, ...):
        # cls 是类本身（相当于 MyClass）
        # 方法体
        return ...

class Counter:
    count = 0  # 类属性

    @classmethod
    def increment(cls):
        cls.count += 1
        return cls.count

print(Counter.increment())  # 输出: 1
print(Counter.increment())  # 输出: 2

from datetime import date

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

    @classmethod
    def from_birth_year(cls, name, birth_year):
        age = date.today().year - birth_year
        return cls(name, age)  # 等同于 Person(name, age)

# 使用工厂方法创建实例
alice = Person.from_birth_year("Alice", 1990)
print(alice.age)  # 输出: 35 (根据当前年份计算)

class Animal:
    @classmethod
    def speak(cls):
        return f"{cls.__name__} says..."

class Dog(Animal):
    @classmethod
    def speak(cls):
        return super().speak() + " Woof!"

class Cat(Animal):
    @classmethod
    def speak(cls):
        return super().speak() + " Meow!"

print(Dog.speak())  # 输出: "Dog says... Woof!"
print(Cat.speak())  # 输出: "Cat says... Meow!"

import json

class AppConfig:
    def __init__(self, host, port, debug):
        self.host = host
        self.port = port
        self.debug = debug

    @classmethod
    def from_json(cls, file_path):
        with open(file_path, 'r') as f:
            config = json.load(f)
        return cls(**config)  # 解包字典参数

# 使用配置文件创建实例
config = AppConfig.from_json("config.json")
print(config.host)  # 输出: "localhost"（假设配置文件中定义）

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    @classmethod
    def from_square(cls, side_length):
        if side_length <= 0:
            raise ValueError("边长必须为正数")
        return cls(side_length, side_length)

# 创建正方形矩形
square = Rectangle.from_square(5)
print(square.width, square.height)  # 输出: 5 5

class Singleton:
    _instance = None

    @classmethod
    def get_instance(cls):
        if not cls._instance:
            cls._instance = cls()
        return cls._instance

# 获取单例实例
obj1 = Singleton.get_instance()
obj2 = Singleton.get_instance()
print(obj1 is obj2)  # 输出: True（同一实例）

class MyClass:
    def __init__(self, value):
        self.value = value

    @classmethod
    def print_value(cls):
        print(cls.value)  # 错误：类方法无法直接访问实例属性

obj = MyClass(42)
# obj.print_value()  # 报错：AttributeError

class Parent:
    @classmethod
    def method(cls):
        print(f"Parent: {cls.__name__}")

class Child(Parent):
    @classmethod
    def method(cls):
        super().method()  # 调用父类的类方法
        print(f"Child: {cls.__name__}")

Child.method()
# 输出:
# Parent: Child
# Child: Child