复杂对象上的python垃圾收集器行为

问题描述

是否python垃圾回收器清理了一个复合对象，如果它的某些部分仍然被引用

eg

 
 $ b 
由没有连接回它的原始容器。
 
 
 
请注意，这不是垃圾回收器， GC只处理打破循环引用。这个简单的例子完全由引用计数处理。
 
 
 
当 foo（）返回时，本地名称空间被清除向上。这意味着 A 被删除，这意味着列表对象的引用计数将降为0.这将清除该列表对象，这意味着包含列表也是看他们的参考计数下降一个。对于 A [0] ，这意味着计数也会下降到0并被清除。
 
 
 
对于列表对象由 A [1] 引用，你现在有一个引用 B ，所以它的count仍然是1，仍然'活着'。
 
 
 
要确认相同的代码，只需使用 list 的子类和让我们知道何时该对象正在被删除：
 >>> （自我）
 ... 
> >> def foo（）：
 ... A = DelList（[DelList（[1,3,5,7]），DelList（[2,4,6,8]））
 .. 。return A [1] 
 ... 
>>> B = foo（）
已删除[[1,3,5,7]，[2,4,6,8]] 
已删除[1,3,5,7] 
> ;>>删除B 
删除[2,4,6,8] 
  
所有这些都是特定于CPython（参考Python实现）;其他实现可能会以不同的方式处理对象生命周期（例如，使用垃圾收集器销毁扫描中的对象），但是 A 和 A [0] 在这些情况下不会改变; GC仍会收集其他实现中的那些，尽管也许在不同的时间点。 
Does python garbage collector cleans up a compound object if some of its parts are still referenced
e.g.
def foo():
    A = [ [1, 3, 5, 7], [2, 4, 6, 8]]
    return A[1]
B = foo()
Will A[0] be garbage collected?
Is there a way to confirm the same through code?
 解决方案 
Nothing references the list A and the nested list A[0], so yes, they will be deleted from memory.
The nested list object referenced by A[1] has no connection back to its original container.
Note that it's not the garbage collector that does this; the GC only deals in breaking circular references. This simple case is handled entirely by reference counting.
The moment foo() returns, the local namespace is cleared up. That means A is removed, which means the list object reference count drops to 0. This clears that list object, which means that the contained lists also see their reference count drop by one. For A[0] that means the count drops to 0 too and it is cleared.
For the list object referenced by A[1], you now have a reference B to it, so it's count is  still 1 and it remains 'alive'.
To confirm the same through code, simply use a subclass of list with a __del__ method to let us know when the object is being deleted:
>>> class DelList(list):
...     def __del__(self):
...         print 'Deleted {}'.format(self)
... 
>>> def foo():
...     A = DelList([DelList([1, 3, 5, 7]), DelList([2, 4, 6, 8])])
...     return A[1]
... 
>>> B = foo()
Deleted [[1, 3, 5, 7], [2, 4, 6, 8]]
Deleted [1, 3, 5, 7]
>>> del B
Deleted [2, 4, 6, 8]
All this is specific to CPython (the reference Python implementation); other implementations may handle object lifetimes differently (e.g. do use a garbage collector to destroy objects in sweeps), but the lifetime of A and A[0] doesn't change in those cases; the GC will still collect those in other implementations, albeit at a different point in time perhaps.
                        
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