BUG: Cofunction object has no attribute '_ufl_class_'

[Ig-dolci]

Steps to Reproduce

from firedrake import *
from firedrake.adjoint import *
continue_annotation()

def test_solve_cofunction_rhs():
    mesh = UnitSquareMesh(2, 2)
    V = FunctionSpace(mesh, "CG", 1)

    u = TrialFunction(V)
    v = TestFunction(V)
    a = inner(u, v) * dx

    L = Cofunction(V.dual())
    L.vector()[:] = 1.

    w = Function(V)
    solve(a == L, w)
    Aw = assemble(action(a, w))
    assert isinstance(Aw, Cofunction)
    
    assert np.allclose(Aw.dat.data_ro, L.dat.data_ro)

test_solve_cofunction_rhs()
get_working_tape().visualise("tape.pdf")

Error message

  File "/Users/ddolci/work/firedrake_main/src/ufl/ufl/formatting/ufl2unicode.py", line 351, in ufl2unicode
    return expression2unicode(ufl.as_ufl(expression))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ddolci/work/firedrake_main/src/ufl/ufl/formatting/ufl2unicode.py", line 357, in expression2unicode
    return map_expr_dag(rules, expression)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ddolci/work/firedrake_main/src/ufl/ufl/corealg/map_dag.py", line 34, in map_expr_dag
    result, = map_expr_dags(function, [expression], compress=compress,
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ddolci/work/firedrake_main/src/ufl/ufl/corealg/map_dag.py", line 100, in map_expr_dags
    r = handlers[v._ufl_typecode_](v, *[vcache[u] for u in v.ufl_operands])
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/ddolci/work/firedrake_main/src/ufl/ufl/corealg/multifunction.py", line 99, in undefined
    raise ValueError(f"No handler defined for {o._ufl_class_.__name__}.")
                                               ^^^^^^^^^^^^^
AttributeError: 'Cofunction' object has no attribute '_ufl_class_'

Environment:

• OS: MacOs
• Python version: 3.11.6

Additional Info
Fix in the upstream for ufl2unicode works properly for Cofunction.

[nbouziani]

I don't think the fact that Cofunctions don't have a _ufl_class_ attribute is a bug as they are not Expr objects. The error you are observing is a consequence of BaseForm objects, such as Cofunction or FormSum, not having ufl2unicode support at the moment, as mentioned in the following issue firedrakeproject/ufl#43, so the error you are observing is expected.