老师if not rest 这段代码没看明白，麻烦讲解一下

 # 3. 读取文件内容

            with open(full_dist_path, 'w', encoding='utf-8') as f_dist:

                print('>> 开始备份【{0}】'.format(file_name))

                with open(full_src_path, 'r', encoding='utf-8') as f_src:

                    while True:

                        rest = f_src.read(100)

                        if not rest:

                            break