nested structure with "internal references"

Carl Banks pavlovevidence at
Fri Sep 25 20:29:54 CEST 2009

On Sep 25, 10:11 am, Torsten Mohr <tm... at> wrote:
> Hi,
> sorry for posting in german before, that was a mistake.
> I'd like to use a nested structure in memory that consists
> of dict()s and list()s, list entries can be dict()s, other list()s,
> dict entries can be list()s or other dict()s.
> The lists and dicts can also contain int, float, string, ...
> But i'd also like to have something like a "reference" to another
> entry.
> I'd like to refer to another entry and not copy that entry, i need to
> know later that this is a reference to another entry, i need to find
> also access that entry then.
> Is something like this possible in Python?
> The references only need to refer to entries in this structure.
> The lists may change at runtime (entries removed / added), so
> storing the index may not help.

Hmm, I think I understand what you're asking but I don't think there's
an easy way.

Probably the most straightforward way is to use "cells" of some sort
to contain the entries in a list or dict; then take references to the
cells.  That way you can change the cell's value while still
maintaining the reference.  For instance, suppose you have a list like

lst = [ 1, 2, 3, 4 ]

Convert it to this:

lst = [ [1], [2], [3], [4] ]

where you are using a nested singleton list as the cell type.  Then,
you can take a reference like this:

ref = lst[2]

If you insert an item into the list, the reference remains valid:


And, if you change the value of the cell, the reference will see the
updated value.

lst[3][0] = 5

The downside is that you have to add [0] everywhere.  The reference's
value is ref[0], not just ref.  The third item in the list is lst[2]
[0], not just lst[2].  You could define custom list- and dict-like
classes that automatically do this, mitigating the problem somewhat.

I hope you could understand that suggestion, it's tough to describe.

Other than that, you are stuck with massive use of Observer pattern
(you'd have to create list- and dict-like types that know when
something is referencing them, and that notify the references whenever
they change).

Carl Banks

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