How do I use subprocess to tar & compress a folder?

Question

I have used the below code, it can compress folders. But when I uncompress folders, it will throw an error. I think there is something wrong with my code. Any advice is welcome.

def compress_index():
    "将/var/solr/core0/data索引目录进行打包压缩至同目录下，以该日时间命名的data.tar.gz.20101013形式的压缩包"
    parentdirs,folder=os.path.split(g_index_path)
    tar_filename=os.path.join(parentdirs,folder+".tar.gz.%s"%datetime.datetime.now().strftime("%Y%m%d"))
    if os.path.exists(tar_filename):
        print "已经存在同名压缩文件，删除再压缩"
        delete(tar_filename)
    cmd="tar -zcvf %s %s"%(tar_filename,g_index_path)
    print cmd
    subprocess.call(cmd.split())
    print "将索引%s压缩打包到%s完成"%(g_index_path,tar_filename)