Question

I have some code where instances of classes have parent<->child references to each other, e.g.:

class Node:

    def __init__(self):
        self.parent = None
        self.children = {}

    def AddChild(self, name, child):
        child.parent = self
        self.children[name] = child


def Run():
    root, c1, c2 = Node(), Node(), Node()
    root.AddChild('first', c1)
    root.AddChild('second', c2)


Run()

I think this creates circular references such that root, c1 and c2 won’t be freed after Run() is completed, right?. So, how do get them to be freed? I think I can do something like root.children.clear(), or self.parent = None – but what if I don’t know when to do that?

Is this an appropriate time to use the weakref module? What, exactly, do I weakref’ify? the parent attribute? The children attribute? The whole object? All of the above? I see talk about the WeakKeyDictionary and weakref.proxy, but its not clear to me how they should be used, if at all, in this case.

This is also on Python 2.4 (can’t upgrade).

Update: Example and Summary

What objects to weakref-ify depends on which object can live without the other, and what objects depend on each other. The object that lives the longest should contain weakrefs to the shorter-lived objects. Similarly, weakrefs should not be made to dependencies – if they are, the dependency could silently disappear even though it is still needed.

If, for example, you have a tree structure, root, that has children, kids, but can exist without children, then the root object should use weakrefs for its kids. This is also the case if the child object depends on the existence of the parent object. Below, the child object requires a parent in order to compute its depth, hence the strong-ref for parent. The members of the kids attribute are optional, though, so weakrefs are used to prevent a circular reference.

class Node:

    def __init__(self):
        self.parent = None
        self.kids = weakref.WeakValueDictionary()

    def GetDepth(self):
        root, depth = self, 0
        while root:
            depth += 1
            root = root.parent
        return depth


root = Node()
root.kids['one'] = Node()
root.kids['two'] = Node()

To flip the relationship around, we have something like the below. Here, the Facade classes require a Subsystem instance to work, so they use a strong-ref to the subsystem they need. Subsystems, however, don’t require a Facade to work. Subsystems just provide a way to notify Facades about each other’s actions.

class Facade:

  def __init__(self, subsystem):
    self.subsystem = subsystem
    subsystem.Register(self)


class Subsystem:

    def __init__(self):
        self.notify = []

    def Register(self, who):
        self.notify.append(weakref.proxy(who))


sub = Subsystem()
cli = Facade(sub)

Answer 1

Yep, weakref’s excellent here. Specifically, instead of:

self.children = {}

use:

self.children = weakref.WeakValueDictionary()

Nothing else needs change in your code. This way, when a child has no other differences, it just goes away — and so does the entry in the parent’s children map that has that child as the value.

Avoiding reference loops is up high on a par with implementing caches as a motivation for using the weakref module. Ref loops won’t kill you, but they may end up clogging your memory, esp. if some of the classes whose instances are involved in them define __del__, since that interferes with the gc‘s module ability to dissolve those loops.