L-shape mesh generation with right element

mesh
12

This is way easier in DOLFINx, as you can work directly with numpy arrays

from mpi4py import MPI
import dolfinx
import basix.ufl
import ufl
import numpy

geometrical_dim = 2
c_el = basix.ufl.element("Lagrange", "triangle", 1, shape=(geometrical_dim,))
ufl_domain = ufl.Mesh(c_el)

if MPI.COMM_WORLD.rank == 0:
    nodes = numpy.array(
        [
            [0.0, 0.0],
            [1.0, 0.0],
            [0.0, 1.0],
            [1.0, 1.0],
            [0.0, 2.0],
            [1.0, 2.0],
            [3.0, 0.0],
            [3.0, 1.0],
        ],
        dtype=numpy.float64,
    )
    cells = numpy.array(
        [[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5], [1, 6, 3], [6, 7, 3]],
        dtype=numpy.int64,
    )
else:
    nodes = numpy.empty((0, geometrical_dim), dtype=numpy.float64)
    cells = numpy.empty((0, 3), dtype=numpy.int64)

mesh = dolfinx.mesh.create_mesh(MPI.COMM_WORLD, cells, nodes, ufl_domain)
with dolfinx.io.XDMFFile(mesh.comm, "L_grid.xdmf", "w") as xdmf:
    xdmf.write_mesh(mesh)

This is also documented in Mesh generation — FEniCS Workshop

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