import bpy
import bmesh
 
obj = bpy.context.object
 
# マテリアル名と同名のfacemapを生成してマテリアルスロット番号->FaceMap番号を紐付ける
mat_to_fm = {}
for slot in obj.material_slots:
    mat = slot.material
    fm = obj.face_maps.get(mat.name)
    if fm is None:
        fm = obj.face_maps.new(name=mat.name)
    mat_to_fm[slot.slot_index] = fm.index
 
# 面に割り当てられているマテリアルスロット番号(material_index)をfacemapに反映
bm = bmesh.new()
bm.from_mesh(obj.data)
fm = bm.faces.layers.face_map.verify()
for face in bm.faces:
    face[fm] = mat_to_fm[face.material_index]
bm.to_mesh(obj.data)
 

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