no-clobber dicts?

[r]

On Aug 3, 4:07 pm, 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
>
>   D[K] = V
>
> will raise an exception unless V == D[K].  In 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].  All 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.  Before 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__()

badda-bing baby!