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
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 tosys.stderrinstead. 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.