no-clobber dicts?

[kj]

In <mailman.4183.1249336823.8015.python-list at python.org> Chris Rebert <clp2 at rebertia.com> writes:

>On Mon, Aug 3, 2009 at 2:47 PM, r<rt8396 at gmail.com> wrote:
>> On Aug 3, 4:07=C2=A0pm, kj <no.em... at please.post> wrote:
>>> I use the term "no-clobber dict" to refer to a dictionary D with
>>> the especial property that if K is in D, then
>>>
>>> =C2=A0 D[K] =3D V
>>>
>>> will raise an exception unless V =3D=3D D[K]. =C2=A0In other words, D[K]
>>> can be set if K doesn't exist already among D's keys, or if the
>>> assigned value is equal to the current value of D[K]. =C2=A0All other
>>> assignments to D[K] trigger an exception.
>>>
>>> The idea here is to detect inconsistencies in the data.
>>>
>>> This is a data structure I often need. =C2=A0Before I re-invent the
>>> wheel, I thought I'd ask: is it already available?
>>>
>>> TIA!
>>>
>>> kynn
>>
>> Not sure if something like this already exists, but it would be
>> trivial to implement by overriding dict.__setitem__()

>That is, if you don't care about .update() not preserving the
>invariant.

The implication here is that .update() does not in turn use
.__setitem__(), which I find a bit surprising.

kynn