no-clobber dicts?

kj at
Mon Aug 3 23:07:32 CEST 2009

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?



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