import gmsh
import sys

gmsh.initialize(sys.argv)

gmsh.model.add("test")

# add discrete surface with tag 1
gmsh.model.addDiscreteEntity(2, 1)

# add 4 mesh nodes
gmsh.model.mesh.addNodes(
    2,
    1,
    [1, 2, 3, 4],  # node tags: 1, 2, 3, and 4
    [
        0.,
        0.,
        0.,  # coordinates of node 1
        1.,
        0.,
        0.,  # coordinates of node 2
        1.,
        1.,
        0.,  # ...
        0.,
        1.,
        0.
    ])

# add 2 triangles
gmsh.model.mesh.addElements(
    2,
    1,
    [2],  # single type : 3-node triangle
    [[1, 2]],  # triangle tags: 1 and 2
    [[
        1,
        2,
        3,  # triangle 1: nodes 1, 2, 3
        1,
        3,
        4
    ]])  # triangle 2: nodes 1, 3, 4

# export the mesh ; use explore.py to read and examine the mesh
gmsh.write("test.msh")

gmsh.finalize()