Non-matching UFL cell and mesh cell shapes from XDMFFile mesh

General
1

Hi everyone,
I am quite new to FeniCSx, I have dolfinx 0.7.2 installed.

I am sharing a code below, which is creating a 1D mesh, exporting it as an XDMF file, and importing it back to build a FunctionSpace.

By running this code, I get the following error: ValueError: Non-matching UFL cell and mesh cell shapes.

Any ideas of what is causing this? Thank you in advance.

# FeniCSx
from basix.ufl import element, mixed_element
from dolfinx import mesh, fem 
from dolfinx.fem import Function, FunctionSpace
from dolfinx import io
from mpi4py import MPI


nel = 30                                         # number of elements per domain
msh = mesh.create_interval(comm=MPI.COMM_WORLD, nx=3*nel, points=(0, 3))


with io.XDMFFile(msh.comm, "./samples/mymesh.xdmf", "w") as xdmf:
    xdmf.write_mesh(msh)

with io.XDMFFile(MPI.COMM_WORLD, "./samples/mymesh.xdmf", "r") as xdmf:
    msh2 = xdmf.read_mesh()

finite_element = element("P", msh2.basix_cell(), 1)
V = FunctionSpace(msh2, finite_element)
2

I can’t pinpoint what the issue is, but I can tell you that your code runs correctly with 0.8.0. Simplest course of action would be to upgrade your dolfinx installation.

3

There was an issue with how basix compares elements in 0.7.x. There was some discussions regarding this in the #development channel on Slack with @Massimiliano_Leoni and @mscroggs. I think we concluded that it was the unset Lagrange variant (in C++) that caused an issue. I would advice you to upgrade to 0.8.x as Francesco points out, there shouldn’t be an issue there.