When I plug the mesh into the x input of the new Python editor, the input mesh becomes an ngon.
Baking the input shows the ngon in the viewport. I ran _DeleteMeshNgons and the mesh has 3 faces again.
Unplugging and recomputing the GH definition also reverts the mesh to a mesh with 3 faces.
ngon_from_mesh_python.gh (14.7 KB)