Dear all,
I am trying to use DolfinX to solve a problem with two different meshes.
I create the submesh from a cellmarker from gmsh.
The issue is when I try to create a function from the Mixed Function Space.
It says:

AttributeError: 'MixedFunctionSpace' object has no attribute '_cpp_object'

I cannot create a function from the space. Is this functionality not implemented in DolfinX or am I making some mistake.

As Francesco mentioned, this is possible with the branches of FEniCSx Iâ€™ve been working on. If you are interested, please checkout GitHub - jpdean/mixed_domain_demos. However, this functionality is still a work in progress and has not yet been merged into main, so Multiphenicsx might be the better option for you.

Dear Francesco,
Thank you so much for your fast response!
I tried to study a bit multiphenicsx and also jpdean implementation of mixed element problems.
Unfortunately my background on applied math is not that vast.
Thus, Iâ€™d like to ask you some help.
On the MWE that I described on the post that I made, I was trying to create a Function Space that comprised both a variable defined on the whole domain, and a second one which was only defined on a subdomain. I was looking to implement on FEniCSx something that was closer to what was proposed on the latest development version of FEniCS and bumped on the fact that this is not available yet.
From what I understood, on your very nice tutorials of multiphenicsx, the proposed solution would be to create the subspaces and â€śbindâ€ť them through a mapping of the DOFâ€™s. Then, create a newton solver object that could solve the problem.
Is that right?
Thank you so much!
All the best,
Murilo

I took a look at this page in your tutorial (tutorial_block_poisson) and this one (tutorial_navier_stokes). It looks you developed a package based on fenicsx to solve a coupled system of equations. Is this one application of your package? I am looking for a way to impose two coupled equation and solve the nonlinear system simultaneously in fenicsx. It looks your package is exactly what I am looking for and your tutorial contains a lot of resouces including the implementation by just using fenicsx and also by using your package. Am I correct? Thanks!

Yes, that is correct. I imagine that your equations involve restricting unknowns to some subdomain: in that case, please have a look at the third tutorial too.