import bpy

def make_mesh(obj_name, set_normals, access_uv_layer_again_after_set_normals):
    mesh = bpy.data.meshes.new("mesh")

    # One triangle
    verts = [
        (0, 0, 0),
        (0, 1, 0),
        (0, 0, 1),
    ]
    faces = [
        (0,1,2),
    ]
    mesh.from_pydata(vertices=verts, edges=[], faces=faces)

    uv_layer = mesh.uv_layers.new()
    uv_layer_data = uv_layer.data
    uv_layer_name = uv_layer.name
    del uv_layer

    if set_normals:
        mesh.use_auto_smooth = True
        mesh.normals_split_custom_set([(0,0,1),]*3)

    if access_uv_layer_again_after_set_normals:
        uv_layer_data = mesh.uv_layers[uv_layer_name].data

    for i in range(3):
        uv_layer_data[i].uv = (0,0)

    obj = bpy.data.objects.new(obj_name, mesh)
    bpy.context.scene.collection.objects.link(obj)
    obj.show_name = True
    return obj

make_mesh("no_set_normals", False, False).location = (0,0,0)
make_mesh("with_set_normals_and_reaccess_uv", True, True).location = (1,0,0)
make_mesh("with_set_normals_but_no_reaccess_uv", True, False).location = (2,0,0)