[Python知识库]python类变量和实例变量的区别

先说结论：

1、类变量属于类所有，建议引用时使用类名引用，形如 cls.attr。也可以使用实例引用 self.attr。

• 直接修改类变量。各实例未自行修改类变量时，引用到的值都会改变
• 如果在实例中对类变量赋值，会复制一份为实例变量。覆盖了类变量
• 通过实例引用类变量会比较慢

2、实例变量属于实例所有，引用方式形如 self.attr

代码示例如下：

class TestClass:
    project_id: str = "类变量"
    cluster_id: Optional[str] = None

    def __init__(self):
        self.count = 1

# 类变量 当使用实例变量进行类变量赋值之后，就在实例中重新生成了一份变量
if __name__ == "__main__":
    test_obj1 = TestClass()
    test_obj2 = TestClass()
    # 实例变量
    print("实例变量：")
    print(f"修改前，test_obj1实例变量count内存地址: {id(test_obj1.count)}，值：{test_obj1.count}")
    print(f"修改前，test_obj2实例变量count内存地址: {id(test_obj2.count)}，值：{test_obj2.count}")

    # 类名引用实例变量会报错
    # print(f"修改前，类名引用实例变量count内存地址: {id(TestClass.count)}，值：{TestClass.count}")
    test_obj2.count = 2
    test_obj1.count += 3
    print(f"修改后，test_obj1实例变量count内存地址: {id(test_obj1.count)}，值：{test_obj1.count}")
    print(f"修改后，test_obj2实例变量count内存地址: {id(test_obj2.count)}，值：{test_obj2.count}")

    # 类变量
    print("")
    print("类变量：")
    print(f"修改前，test_obj1类变量project_id内存地址: {id(test_obj1.project_id)}，值：{test_obj1.project_id}")
    print(f"修改前，test_obj2类变量project_id内存地址: {id(test_obj2.project_id)}，值：{test_obj2.project_id}")
    print(f"修改前，类变量project_id内存地址: {id(TestClass.project_id)}，值：{TestClass.project_id}")
    # 通过类修改类bianl
    print("通过类修改类变量")
    TestClass.project_id = "第一次通过类修改类变量"
    print(f"修改后：test_obj1类变量project_id内存地址: {id(test_obj1.project_id)}，值：{test_obj1.project_id}")
    print(f"修改后：test_obj2类变量project_id内存地址: {id(test_obj2.project_id)}，值：{test_obj2.project_id}")
    print(f"修改后：类变量project_id内存地址: {id(TestClass.project_id)}，值：{TestClass.project_id}")

    # 修改实例应用的类变量
    print("")
    print("修改实例应用的类变量，全部实例和类引用的类变量均会改变")
    test_obj1.project_id = "实例1类变量"
    test_obj2.project_id = "实例2中类变量"
    print(f"修改后：test_obj1类变量project_id内存地址: {id(test_obj1.project_id)}，值：{test_obj1.project_id}")
    print(f"修改后：test_obj2类变量project_id内存地址: {id(test_obj2.project_id)}，值：{test_obj2.project_id}")
    print(f"修改后：类变量project_id内存地址: {id(TestClass.project_id)}，值：{TestClass.project_id}")

    # 此时再修改类变量，不会再影响实例中变量了
    print("再次修改类变量，此时不会影响实例引用的类变量了")
    TestClass.project_id = "类变量新"
    print(f"修改后：test_obj1类变量project_id内存地址: {id(test_obj1.project_id)}，值：{test_obj1.project_id}")
    print(f"修改后：test_obj2类变量project_id内存地址: {id(test_obj2.project_id)}，值：{test_obj2.project_id}")
    print(f"修改后：类变量project_id内存地址: {id(TestClass.project_id)}，值：{TestClass.project_id}")

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