Blender Python: seting custom split normals

update_custom_split_normals.py

import bpy

mesh_data = bpy.context.object.data

# Create a list with the same number of elements as the number of loops (so we can do this `new_loop_normals[loop_index]`)
new_loop_normals = [None for _ in mesh_data.loops]

for face in mesh_data.polygons:
    for loop_index in face.loop_indices:
        new_loop_normals[loop_index] = face.normal

mesh_data.use_auto_smooth = True
mesh_data.normals_split_custom_set(new_loop_normals)