什么时候收集python类和类属性垃圾？

2019年8月5日 110次阅读 
class Member(object):
    def __init__(self, identifier):
        self.identifier = identifier
        print "Member __init__", self.identifier

    def __del__(self):
        print "Member __del__", self.identifier
        with open("/home/might/" + self.identifier, "w") as outF:
            outF.write(self.identifier)

class WithMembers(object):
    def __init__(self):
        print "WithMembers __init__"
        print WithMembers.classMem
        self.instanceMem = Member("instance mem")

    def __del__(self):
        print "WithMembers __del__"

    classMem = Member("class mem")

if __name__ == "__main__":
    print "main"
    WithMembers()
    #del WithMembers.classMem       # "Member __del__ class mem" before "end"
    #del WithMembers                # "Member __del__ class mem" after "end"
    print "end"

上面的代码在Hidden.py中,运行python Hidden.py会产生以下输出：

Member __init__ class mem
main
WithMembers __init__
<__main__.Member object at 0x935aeec>
Member __init__ instance mem
WithMembers __del__
Member __del__ instance mem
end

我没有在输出或类mem文件中看到Member __del__类mem,除非我取消注释其中一个del语句.为什么是这样？什么时候收集python类和类属性垃圾？

最佳答案 这被报告为
http://bugs.python.org/issue1545463中的一个错误,固定在3.4但没有向后移植(我运行2.7).这在
http://code.activestate.com/lists/python-list/504216/也有解释.请参阅下面的python 3.5中的输出.

基于以上所述,我的理解是在2.7中,当解释器退出时,新的样式类WithMembers仍然存在(不会被GC清理).因此,classMem不会被垃圾收集,因为WithMembers仍然引用它.

请注意,新样式类具有来自__mro__和一些内置描述符的循环引用(http://bugs.python.org/issue17950).尽管模块清理后模块级新风格类被GC认为是死的,但是在模块清理后GC调用它们会被禁用,因为它会导致太多其他问题.

这不会导致内存泄漏,因为操作系统会在解释器退出后清理资源.

class Member(object):
    def __init__(self, identifier):
        self.identifier = identifier
        print("Member __init__ " + self.identifier)

    def __del__(self):
        print("Member __del__ " + self.identifier)
        with open("/home/might/" + self.identifier, "w") as outF:
            outF.write(self.identifier)

class WithMembers(object):
    def __init__(self):
        print("WithMembers __init__")
        print(WithMembers.classMem)
        self.instanceMem = Member("instance mem")

    def __del__(self):
        print("WithMembers __del__")

    classMem = Member("class mem")

if __name__ == "__main__":
    print("main")
    WithMembers()
    print("end")

使用python3 Hidden.py运行时输出以下内容：

Member __init__ class mem
main
WithMembers __init__
<__main__.Member object at 0xb6fc8e2c>
Member __init__ instance mem
WithMembers __del__
Member __del__ instance mem
end
Member __del__ class mem
Exception ignored in: <bound method Member.__del__ of <__main__.Member object at 0xb6fc8e2c>>
Traceback (most recent call last):
  File "class_member_gc.py", line 8, in __del__
NameError: name 'open' is not defined
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