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.
