Convert 'cpp.fem.Form' to 'ufl.Form'

Question

I'd like to extract the residual_form and Jacobian_form from a NonlinearVariationalProblem, for use in another solver:

p = NonlinearVariationalProblem(F,u,bc, J)
print p.residual_form()
p2 = NonlinearVariationalProblem(p.residual_form(),p.solution(),p.bcs(), p.jacobian_form)

When I try this, I get an error:

*** Error:   Unable to creating dolfin.Form.
*** Reason:  Expected a ufl.Form..
*** Where:   This error was encountered inside form.py.

The print statement returns that the residual_form() is of type: dolfin.cpp.fem.Form . How can I convert the cpp Form to a UFL form so that the NonlinearVariationalProblem will work?

Thanks!

Answer 1

You can't. A UFL form has all the symbolic data attached that allows the form to be manipulated. a cpp Form just know how the tabulate matrix/vector on cell.

It looks like the p returned by NonlinearVariationalProblem(F,u,bc, J) is coming from the underlying c++ object, which doesn't know about UFL forms.

We probably need to extend the Python NonlinearVariationalProblem class to return the UFL forms. Could you register an issue on Bitbucket?