def join_b2_path(b2_dir, b2_name):
    """
    Like os.path.join, but for B2 file names where the root directory is called ''.

    :param b2_dir: a directory path
    :type b2_dir: str
    :param b2_name: a file name
    :type b2_name: str
    """
    if b2_dir == '':
        return b2_name
    else:
        return b2_dir + '/' + b2_name