blender/release/scripts/modules/bpy_sys.py

import bpy
import os

def expandpath(path):
    if path.startswith("//"):
        return os.path.join(os.path.dirname(bpy.data.filename), path[2:])

    return path

import types
bpy.sys = types.ModuleType("bpy.sys")
bpy.sys.expandpath = expandpath