import shutil


def create_backup(orig, backup=None):
    """
    Creates a backup copy of original.  If backup is given it must be the full name, otherwise
    if backup is not given, the original file name will be appended with .bak

    :param orig:
    :param backup:
    :return:
    """
    backup_path = backup if backup else (orig + '.bak')
    return shutil.copy(orig, backup_path)