IMHO the bug here is a lack of validation. Its perfectly fine to have mutable elements of a vector subspace, but y.__setitem__ should then check that the vector really is in the subspace or raise a ValueError. Of course it would almost always raise, so its not a particularly useful operation.
O Wednesday, May 27, 2015 at 5:38:18 PM UTC+2, Johan S. R. Nielsen wrote: > > Hi Sage-devel > > We just crashed our heads against a bug in our code due to extremely weird > semantics of Sage. Consider the following code which creates a sub-space C > of the four-dimensional vector space over the binary field, and then > modifies a *copy* of a vector from the subspace: > > F = GF(2) > VS = F^4 > C = VS.subspace([ (F.one(),F.one(),F.one(),F.one()) ]) > w = C([F.one(),F.one(),F.one(),F.one()]) > y = copy(w) > y[0] = F.zero() > print "y's parent is ", y.parent() > print "y is in C: ", y in C > print "y = ", y > > The code above is going to print: > > y's parent is Vector space of degree 4 and dimension 1 over Finite Field > of size 2 > Basis matrix: > [1 1 1 1] > y is in C: True > y = (0, 1, 1, 1) > > So even though y is clearly not in our space C, Sage claims it is. The > reason is that y's parent is still this sub-space. > > Sage disallows that we modify w, the original vector from C. But we are > allowed to modify the copy y (even encouraged by a warning that Sage prints > when you try to modify c). > > I assume everyone agrees the above behaviour is bad? In that case, what is > the bug? > > - Is it that the copy of c is allowed to be mutated (for instance, without > first casting it into the ambient vector space)? > > - Is it that the parent of y is still C immediately after copying? > > - Or is it that the parent of y is still C after modification? > > - Or a fourth option I didn't think of? > > Best, > Johan > -- 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 post to this group, send email to sage-devel@googlegroups.com. Visit this group at http://groups.google.com/group/sage-devel. For more options, visit https://groups.google.com/d/optout.
