[Python-ideas] Automatic context managers

[88888 Dihedral]

Chris Angelico於 2013年4月27日星期六UTC+8上午12時52分38秒寫道：
> On Sat, Apr 27, 2013 at 1:54 AM, MRAB <python at mrabarnett.plus.com> wrote:
> 
> > On 26/04/2013 14:02, anatoly techtonik wrote:
> 
> >> This circular reference problem is interesting. In object space it
> 
> >> probably looks like a stellar detached from the visible (attached)
> 
> >> universe. Is the main problem in detecting it?
> 
> >>
> 
> > The problem is in knowing in which order the objects should be
> 
> > collected.
> 
> >
> 
> > For example, if A refers to B and B refers to A, should you collect A
> 
> > then B, or B then A? If you collect A first, then, for a time, B will
> 
> > be referring to a non-existent object. That's not good if the objects
> 
> > have destructors which need to be run.
> 
> 
> 
> Spin-off thread from python-ideas to discuss a more general question
> 
> of garbage collection of cyclic structures.
> 
> 
> 
> Once it's been proven that there's an unreferenced cycle, why not
> 
> simply dispose of one of the objects, and replace all references to it
> 
> (probably only one - preferably pick an object with the fewest
> 
> references) with a special temporary object? In fact, that could
> 
> probably be done in CPython by wiping out the object in memory and
> 
> replacing it with a special marker of some sort, which would then
> 
> automatically "take over" all references to the old object. Any
> 
> attempt to manipulate this object could simply pop back with a
> 
> DestructedObject exception or something.
> 
> 
> 
> Is this a plausible (never mind viable yet, just conceptually
> 
> plausible) alternative to sticking them into gc.garbage and ignoring
> 
> them? It'd allow a double-linked list/tree to function cleanly -
> 
> imagine, for instance, something like the DOM facilities available to
> 
> web browser scripts:
> 
> 
> 
> class DOMObject:
> 
> 	def __init__(self,parent):
> 
> 		self.parent=parent
> 
> 		self.firstchild=self.sibling=None
> 
> 		if not parent: return
> 
> 		if not parent.firstchild:
> 
> 			parent.firstchild=self
> 
> 		else:
> 
> 			child=parent.firstchild
> 
> 			while child.sibling:
> 
> 				child=child.sibling
> 
> 			child.sibling=self
> 
> 	def __del__(self):
> 
> 		print("Disposing of id #%d"%id(self))
> 
> 
> 
> document=DOMObject(None)
> 
> body=DOMObject(document)
> 
> p=DOMObject(body)
> 
> p=DOMObject(body)
> 
> p=DOMObject(body)
> 
> del document,body,p
> 
> gc.collect()
> 
> 
> 
> The __del__ method would need to clean up the external resources used
> 
> by this object, but wouldn't have to walk the tree. Yet, just because
> 
> there is a reference loop and there are __del__ methods, the garbage
> 
> collector gives up and leaves it to the program author to deal with.
> 
> 
> 
> I can understand if this is considered too complicated and too unusual
> 
> a circumstance to be worth bothering to support, but I'm curious as to
> 
> whether it's at least conceptually reasonable to do something like
> 
> this.
> 
> 
> 
> ChrisA

Please use the deep-copy methods  from time to time to disentangle
referenced objects in python.

The cyclic reference cycle has to be broken by some mean first
in python to proceed for further actions in the gc.