__del__上的Python attributeError

我有一个 python类对象,我想分配一个类变量的值

class Groupclass(Workerclass):
    """worker class"""
    count = 0

    def __init__(self):
        """initialize time"""
        Groupclass.count += 1
        self.membercount = 0;
        self.members = []

    def __del__(self):
        """delte a worker data"""
        Groupclass.count -= 1


if __name__ == "__main__":
    group1 = Groupclass()

这个执行结果是正确的,但是有一个错误消息说：

Exception AttributeError: "'NoneType' object has no attribute 'count'" in <bound method Groupclass.__del__ of <__main__.Groupclass instance at 0x00BA6710>> ignored

有人可以告诉我我做错了什么吗？

解决方法

您的__del__方法假定该类在调用时仍然存在.

这个假设是不正确的.当您的Python程序退出时,Groupclass已被清除,现在设置为None.

测试全局对类的引用是否仍然存在：

def __del__(self):
    if Groupclass:
        Groupclass.count -= 1

或者使用type()获取本地引用：

def __del__(self):
    type(self).count -= 1

但请注意,这意味着如果Groupclass被子类化(每个子类获取.count属性,而仅具有.count属性的Groupclass),则计数更改的语义将更改.

引用__del__钩子文档：

Warning: Due to the precarious circumstances under which __del__() methods are invoked,exceptions that occur during their execution are ignored,and a warning is printed to sys.stderr instead. Also,when __del__() is invoked in response to a module being deleted (e.g.,when execution of the program is done),other globals referenced by the __del__() method may already have been deleted or in the process of being torn down (e.g. the import machinery shutting down). For this reason,__del__() methods should do the absolute minimum needed to maintain external invariants. Starting with version 1.5,Python guarantees that globals whose name begins with a single underscore are deleted from their module before other globals are deleted; if no other references to such globals exist,this may help in assuring that imported modules are still available at the time when the __del__() method is called.

如果您使用Python 3,则需要另外两个注意事项：

> CPython 3.3自动将randomized hash salt应用于全局词典中使用的str键;这也会影响全局变量的清除顺序,也可能只在一些运行中看到问题.
>根据Safe Object Finalization,CPython 3.4不再将全局变量设置为None(大多数情况下)见PEP 442.