On Tue, Dec 30, 2014 at 9:33 PM, Nils Bruin <bruin.nils@gmail.com> wrote:

We ran into this for sage (www.sagemath.org) and we implemented "MonoDict" with basically the semantics you describe. It is implemented as a cython extension class. See:

http://git.sagemath.org/sage.git/tree/src/sage/structure/coerce_dict.pyx#n24...

I can confirm your observation that it's hard to get right, but I think our implementation by now is both correct and fast. The idiosyncratic name reveals its origin in "TripleDict" defined in the same file, which was developed first.

Incidentally, we also have a WeakValueDict implementation that is faster and a little safer than at least the Python2 version:

http://git.sagemath.org/sage.git/tree/src/sage/misc/weak_dict.pyx

These implementations receive quite a bit of exercise in sage, so I would recommend that you look at them before developing something from scratch yourself.

This is quite interesting. Is there any interest in maintaining that code in a separate library? Sage is a rather large dependency which, for better or for worse, has little to do with my own work. I'm reluctant to add something that large for a single utility class. If there is no such interest, I might consider forking. In my code, I've worked around it by having the owner class and the descriptor collude (basically, the owner's instances have a private attribute which the descriptor uses), but it's far from the cleanest solution. -- Kevin Norris