def _build_from_requirements(cls, req_spec):
        """
        Build a working set from a requirement spec. Rewrites sys.path.
        """
        # try it without defaults already on sys.path
        # by starting with an empty path
        ws = cls([])
        reqs = parse_requirements(req_spec)
        dists = ws.resolve(reqs, Environment())
        for dist in dists:
            ws.add(dist)

        # add any missing entries from sys.path
        for entry in sys.path:
            if entry not in ws.entries:
                ws.add_entry(entry)

        # then copy back to sys.path
        sys.path[:] = ws.entries
        return ws

        Build a working set from a requirement spec. Rewrites sys.path.
        
product/runtime/src/main/python/pkg_resources/__init__.py