property collections

[Bernhard Herzog]

Robin Becker <robin at jessikat.fsnet.co.uk> writes:

> In article <9e90k20aiu at enews1.newsguy.com>, Alex Martelli
> <aleaxit at yahoo.com> writes
> >"Robin Becker" <robin at jessikat.fsnet.co.uk> wrote in message
> >news:g44pZKATy8B7Ewa2 at jessikat.fsnet.co.uk...
> >    ...
> >> My solutions seem to involve elements which need to refer to the parent
> >> and thus create a loop.
> >
> >Not sure I understand your general problem, but, regarding
> >this last item, have you tried the (new in 2.1) weak refs...?
> >
> >
> >Alex
> well now I would certainly use weak refs if that were feasible for all,
> but some people are still using 1.5.2 and so I still have to worry about
> memory loops. Weak refs are exactly the python only solution for this
> sort of acquisition style object.

This seems to work without cyclic references:

class ItemProxy:

    def __init__(self, parent, index):
        self.__dict__["_parent"] = parent
        self.__dict__["_index"] = index

    def __getattr__(self, attr):
        return self._parent.get_attribute(self._index, attr)

    def __setattr__(self, attr, value):
        self._parent.set_attribute(self._index, attr, value)

class Parent:

    def __init__(self):
        self.items = {}

    def __getitem__(self, index):
        return ItemProxy(self, index)

    def set_attribute(self, index, attr, value):
        item = self.items.get(index)
        if item is None:
            item = self.items[index] = {}
        item[attr] = value

    def get_attribute(self, index, attr):
        item = self.items.get(index)
        if item is not None and item.has_key(attr):
            return item[attr]
        return getattr(self, attr)


Sample session:

$ python1.5 -i itemproxy.py 
>>> p = Parent()
>>> p.a = 1
>>> p.b = 2
>>> p[1].a = 0.5
>>> p[1].a
0.5
>>> p[1].b
2
>>> p[2].a
1
>>> p[2].b
2
>>> 


The trick here is that the parent does not have any references to the
objects returned by the __getitem__ method, so there are no cyclic
references.


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