Use the given context parameter, not bpy.context

[hapit]

Note Rather than using bpy.context.scene, we use the context.scene argument passed to execute(). In most cases these will be the same however in some cases operators will be passed a custom context so script authors should prefer the context argument passed to operators. (from http://www.blender.org/api/blender_python_api_2_73a_release/info_tutorial_addon.html)