On 27/05/15 17:38, 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?
The behavior is bad but I would not call it a bug. You modified wrongly
the vector.
- 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?
The parent is not modified by FreeModuleElement.__setitem__. It is of
course not a problem for element of a free module. But on the other hand
you can not hope to preserve the fact that you belong to a given
subspace by modifying entries.
If it was me I would set the parent to be the ambient space and not the
subspace itself (but it might break a lot of code).
Vincent
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