__subclasses__在删除类时的行为 [英] Behaviour of __subclasses__ when classes are deleted

问题描述

在Python 2.7.3交互式会话中：

 >>> class Foo（object）：
 ... pass 
 ... 
>>> type（Bar，（Foo，），{}）
< class'__main __。Bar'> 
>>> Foo .__子类__（）
 [< class'__main __。Bar'>] 
>>> 
  

另外：

 >>> class Foo（object）：
 ... pass 
 ... 
>>>班级Bar（Foo）：
 ...通过
 ... 
>>> Foo .__子类__（）
 [< class'__main __。Bar'>] 
>>> del Bar 
>>> Foo .__子类__（）
 [< class'__main __。Bar'>] 
  

Bar 如何通过 __子类__ 函数仍然可用？如果我希望它被垃圾收集，我该怎么做？

解决方案

请参阅此线程。看起来会发生什么是该类的 __ mro __ 属性存储对自身的引用，从而创建一个引用循环。您可以强制执行完整的gc运行，它将检测周期并删除对象：

 >>> class Foo（object）：pass 
>>> class Bar（Foo）：pass 
>>> import gc 
>>> del Bar 
>>> gc.collect（）
 3 
>>> Foo .__子类__（）
 [] 
  

或者，如果您输入其他命令一段时间，gc将自行运行并收集周期。

请注意，交互式测试时必须小心，因为交互式解释器存储对最后一个值变量 _ 中最近返回的值。如果您明确地查看子类列表，然后立即尝试收集，它将不起作用，因为 _ 变量将包含对该类的强引用的列表。

Edit: Generalised the question due to NPE's comment.

In a Python 2.7.3 interactive session:

>>> class Foo(object):
...     pass
... 
>>> type("Bar", (Foo,), {})
<class '__main__.Bar'>
>>> Foo.__subclasses__()
[<class '__main__.Bar'>]
>>> 

Also:

>>> class Foo(object):
...     pass
... 
>>> class Bar(Foo):
...     pass
... 
>>> Foo.__subclasses__()
[<class '__main__.Bar'>]
>>> del Bar
>>> Foo.__subclasses__()
[<class '__main__.Bar'>]

How come Bar is still available via the __subclasses__ function? I would have expected it to be garbage collected.

Conversely, if I want it to be garbage collected, how do I do it?

解决方案

See this thread. It would seem that what happens is the class's __mro__ attribute stores a reference to itself, creating a reference cycle. You can force a full gc run which will detect the cycle and delete the object:

>>> class Foo(object): pass
>>> class Bar(Foo): pass
>>> import gc
>>> del Bar
>>> gc.collect()
3
>>> Foo.__subclasses__()
[]

Alternatively, if you enter other commands for a while, the gc will run on its own and collect the cycle.

Note that you have to be a bit careful when testing this interactively, because the interactive interpreter stores a reference to the most recently returned value in the "last value" variable _. If you explicitly look at the subclass list and then immediately try to collect, it won't work, because the _ variable will hold a list with a strong reference to the class.

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