Eval in Expression freezes in parallel

dolfinx
2

It freezes as you are not sending in the appropriate MPI communicator to the expression.
Since you are not sending in a communicator (which in your case should be MPI.COMM_SELF, it is using the mesh communicator, thus waiting for a sync step between all processes that never happens. See: dolfinx.fem — DOLFINx 0.7.3 documentation

i.e.

    E = Expression(expr, x_ref, comm=MPI.COMM_SELF)

does the trick