Face normals correction for toon shader

face_normals_correction.py

# Face normals correction
# Usage:
# 1. Select a mesh
# 2. Enter edit mode
# 3. Select face vertices
# 4. Return to object mode
# 5. Run this script
# 6. Return to object mode
# 7. Enter edit mode
# 8. Normals are displayed correctly.

import bpy
import bmesh
from mathutils import Vector

mesh: bpy.types.Mesh = bpy.context.active_object.data

normals = []
vert: bpy.types.MeshVertex
for vert in mesh.vertices:
    if not vert.select:
        normals.append(Vector())
    else:
        normal = vert.normal.copy()
        normal.z = 0.0
        normal.normalize()
        normals.append(normal)

mesh.normals_split_custom_set_from_vertices(normals)