Blender collada_import with imported_objects return

collada_importer.py

def diff_list(l1, l2):
    return [obj for obj in l1 if obj not in l2]

import bpy

def objects_names():
    return [obj.name for obj in bpy.data.objects]

def collada_importer(filepath):
    l1=objects_names()
    bpy.ops.wm.collada_import(filepath=filepath)
    return diff_list(objects_names(), l1)

f='pioneer3dx.dae'

imported_objects = collada_importer(f)

# in case
import os
p=os.path.abspath(f)