singular.invariant_ring in Sagemath

Question

I am trying to use Singular through Sagemath, to compute an the invariant subring of a finite group. Here is a minimal example where the error I get occurs:

singular.lib('finvar.lib')
R = singular.ring(3, '(x0,x1)', 'dp')
g = singular.matrix(2,2,'1,0,1,1')
singular.invariant_ring(g)

I saved this code as test.sage and ran sage test.sage (Debian 9, sagemath installed using apt, with an existing installation of Singular), and got the error (I quote only the last few lines)

  File "/usr/lib/python2.7/dist-packages/sage/interfaces/singular.py", line 653, in eval
    raise SingularError('Singular error:\n%s'%s)
TypeError: Singular error:
   ? assign not impl.
   ? error occurred in or before STDIN line 16: `        return(P,S);`

However the corresponding (or at least I believe so) Singular code

LIB"finvar.lib";
ring R = 3,(x0,x1),dp;
matrix g1[2][2]= 1,0,1,1;
matrix P, S=invariant_ring(g1);

runs properly. What is the problem? Instead of the last line in my sage code, I tried

P, S =     singular.invariant_ring(g)

and

[P, S] =   singular.invariant_ring(g)

but this did not help. I tried the same code at the Sage Cell and on a different machine (Debian 10, sagemath installed using apt), but I got the same error.

Answer 1

I posted an answer in the Ask SageMath thread Calling singular.invariant_ring (it's a bug, it has been reported, and a workaround is available).