Related: https://trac.sagemath.org/ticket/29101 (Refined protocol for _element_constructor_ in element classes with mutability)
On Thursday, August 5, 2021 at 4:25:08 PM UTC-7 Nils Bruin wrote: > > Hi all, > > I am fully aware of why vectors *can* be mutable and that having them > start out life mutable makes sense (since you cannot really change an > immutable vector back to mutable), but there is a scenario where it's super > annoying. I ran into this recently in an actual practical piece of code. I > won't bore you with the details. This is the setup: > > V = VectorSpace(GF(2),6) > D = dict() > > I end up using elements of V as keys into the dictionary [in this case, > theta characteristics, which are naturally indexed by such vectors]. I end > up doing arithmetic with these indices, so I end up with expressions such > as: > > W = V.subspace([V.1,V.2,V.3]) > > sum( D[v0+w] for w in W) > > except that that doesn't work, because v0+w ends up being a fresh vector > and hence is mutable; so not hashable. > > One can work around it with a little helper function > > def imm(v): > v.set_immutable() > return v > > and then one can write: > > sum( D[imm(v0+w)] for w in W) > > it's just super annoying to have to do that everywhere, and of course one > ends up getting an error message each time before actually putting this in > place. > > So perhaps for convenience in interactive use, it would be useful to have > > VectorSpace(R,n,immutable_vectors=True) > > or something similar. It's particularly for these kinds of situations > where one wants to use *vectors*, not tuples, for indexing, because one > WANTS the vector arithmetic. It's not for high-performance scenarios (that > vectors in other situations definitely need to be able to accommodate). > > Ideas? > -- You received this message because you are subscribed to the Google Groups "sage-devel" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-devel+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-devel/34a1125e-ff85-4d67-8156-f52d363ddc35n%40googlegroups.com.
