material_eraser

import bpy
from bpy.props import BoolProperty
import sys
import logging
argv = sys.argv[sys.argv.index('--') + 1:]
path_to_save = argv[0]


def set_scene_properties(scene):
    bpy.types.Scene.Respect = BoolProperty(
        name='Respect Fake User', description='Respect Fake User for all removal actions'
    )
    scene['Respect'] = True


def erase_unused_materials():
    material_list = []
    scene = bpy.context.scene
    set_scene_properties(scene)
    scenes = bpy.data.scenes
    for i in range(len(scenes)):
        for obj in scenes[i].objects:
            if obj.type == 'MESH' or obj.type == 'SURFACE' \
               or obj.type == 'META' or obj.type == 'CURVE' or obj.type == 'FONT':
                if obj.data.materials == '' or len(obj.material_slots.items()):
                    for slot in obj.material_slots:
                        material_list.append(slot.material)

    material_list = list(set(material_list))  # duplicates removal

    for material in bpy.data.materials:
        if material not in material_list:
            option = False if scene['Respect'] else True

            if option and material.use_fake_user:
                material.use_fake_user = False

            if not material.use_fake_user:
                logging.warning(material.name)
                material.user_clear()
                bpy.data.materials.remove(material, do_unlink=False)
                # bpy.data.materials.remove(material, do_unlink=True)  # to be tested (do_unlink=True)


def save_as_new(path):
    bpy.ops.wm.save_as_mainfile(filepath=path)

erase_unused_materials()
save_as_new(path_to_save)