On Tue, Aug 4, 2009 at 12:30 PM, kj<no.em...@please.post> wrote: > In <mailman.4183.1249336823.8015.python-l...@python.org> Chris Rebert > <c...@rebertia.com> writes: > >>On Mon, Aug 3, 2009 at 2:47 PM, r<rt8...@gmail.com> wrote: >>> On Aug 3, 4:07=C2=A0pm, kj <no.em...@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.
The builtin types are allowed to take such shortcuts for performance reasons. Cheers, Chris -- http://blog.rebertia.com -- http://mail.python.org/mailman/listinfo/python-list
