def _subst_vars(path, local_vars):
    """In the string `path`, replace tokens like {some.thing} with the
    corresponding value from the map `local_vars`.

    If there is no corresponding value, leave the token unchanged.
    """
    def _replacer(matchobj):
        name = matchobj.group(1)
        if name in local_vars:
            return local_vars[name]
        elif name in os.environ:
            return os.environ[name]
        return matchobj.group(0)
    return _VAR_REPL.sub(_replacer, path)