python:类成员变量为null或未定义

class MyClass:
    # 类变量，默认为None
    class_var = None

    def __init__(self, instance_var=None):
        # 实例变量，默认为None
        self.instance_var = instance_var

# 创建实例
obj = MyClass()

# 检查变量是否为None
if obj.class_var is None:
    print("class_var is not initialized")

if obj.instance_var is None:
    print("instance_var is not initialized")

class MyClass:
    def __init__(self):
        self.instance_var = "default value"  # 初始化实例变量

# 使用前检查
obj = MyClass()
if obj.instance_var is not None:
    print(obj.instance_var)
else:
    print("instance_var is None")