Bug in free module homomorphism creation

[williamstein]

Consider the following

sage: V = (QQ^3).span_of_basis([[1,1,0],[1,0,2]]); V
Vector space of degree 3 and dimension 2 over Rational Field
User basis matrix:
[1 1 0]
[1 0 2]
sage: V.hom([V.0, V.1])
Traceback (most recent call last):
...
TypeError: entries has the wrong length

The hom command above should create the identity morphism, since according to the docs hom takes as input a list of the images of the generators. Instead, what is happening is that the list is being turned into a matrix directly. Conclusion, the above just goes boom, since the matrix would have to be 2x2, as V has rank 2. E.g., this works:

sage: V.hom([[1,2],[3,4]], V)
Free module morphism defined by the matrix
[1 2]
[3 4]
Domain: Vector space of degree 3 and dimension 2 over Rational Field
User ...
Codomain: Vector space of degree 3 and dimension 2 over Rational Field
User ...

The really scary thing is that this broken code will accidentally and silently give totally wrong answers in some cases, e.g.,

sage: V = (QQ^2).span_of_basis([[1,2],[3,4]])
sage: I = V.hom([V.0,V.1]); I
Free module morphism defined by the matrix
[1 2]
[3 4]
Domain: Vector space of degree 2 and dimension 2 over Rational Field
User ...
Codomain: Vector space of degree 2 and dimension 2 over Rational Field
User ...
sage: I(V.0)
(7, 10)
sage: V.0
(1, 2)

Notice above that I should be the identity map, but it's most certainly not -- it's the map defined by the matrix [[1,2],[3,4]].

Component: linear algebra

Issue created by migration from https://trac.sagemath.org/ticket/5886

[robertwb]

comment:2

Attachment: trac_5886.patch.gz

It looks good. I'm having trouble running any doctests (applied to a pre-3.4.1 branch after upgrading) but the examples I tried manually work.

[williamstein]

comment:3

No doctests break :-)

Seriously, I did apply this patch to a clean 3.4.1 build on sage.math, run --long doctests, and got no failures.

[robertwb]

comment:5

Good.

[sagetrac-mabshoff]

comment:6

I can confirm William's observation: no doctest breakage with -long in my 3.4.2.alpha0 merge tree.

Cheers,

Michael

[sagetrac-mabshoff]

comment:7

Merged in Sage 3.4.2.alpha0.

Cheers,

Michael