def _get_localized_fn(path, root_dir):
    """
    Return absolute `path` relative to `root_dir`.

    When `path` == ``/home/xex/somefile.txt`` and `root_dir` == ``/home``,
    returned path will be ``/xex/somefile.txt``.

    Args:
        path (str): Absolute path beginning in `root_dir`.
        root_dir (str): Absolute path containing `path` argument.

    Returns:
        str: Local `path` when `root_dir` is considered as root of FS.
    """
    local_fn = path
    if path.startswith(root_dir):
        local_fn = path.replace(root_dir, "", 1)

    if not local_fn.startswith("/"):
        return "/" + local_fn

    return local_fn