Discord.py音乐机器人无法播放队列中的下一首歌曲

Question

这是我要创建的机器人的代码，可以很好地播放音乐。

ytdl_options = {
    'format': 'bestaudio/best',
    'restrictfilenames': True,
    'noplaylist': True,
    'nocheckcertificate': True,
    'quiet':True,
    'ignoreerrors': False,
    'logtostderr': False,
    'no_warnings': True,
    'default_search': 'auto',
    'source_address': '0.0.0.0' # using ipv4 since ipv6 addresses causes issues sometimes
    }


        # ffmpeg options
ffmpeg_options= {
    'options': '-vn'
    }



@bot.command()
async def play(ctx, url:str = None): 
    queue = {} 

    channel = ctx.author.voice.channel    
    if ctx.voice_client is not None:  
        await ctx.voice_client.move_to(channel)
    elif ctx.author.voice and ctx.author.voice.channel:       
        await channel.connect()

    if not url:
        await ctx.send("Try adding a URL. e.g. !play https://youtube.com/watch?v=XXXXXXXXX")

    if ctx.voice_client is not None:
        vc = ctx.voice_client #vc = voice client, retrieving it
        ytdl = youtube_dl.YoutubeDL(ytdl_options)


        loop = asyncio.get_event_loop()
        data = await loop.run_in_executor(None, lambda: ytdl.extract_info(url))

        if 'entries' in data:
            data = data['entries'][0] 
        svr_id = ctx.guild.id
        if svr_id in queue:
            queue[svr_id].append(data)

        else:
            queue[svr_id] = [data]
        await ctx.send(data.get('title') + " added to queue")

        source = ytdl.prepare_filename(queue[svr_id][0])
        def pop_queue():
            if queue[svr_id] != []:
                queue[svr_id].pop(0)
                data = queue[svr_id][0]
            else:
                vc.stop()

        if not vc.is_playing():
            vc.play(discord.FFmpegPCMAudio(source, **ffmpeg_options), after=lambda: pop_queue())

下载下一首歌曲并将其排入队列，但是一旦第一首歌曲结束，就不会播放下一首歌曲。第一首歌开始后，我不知道如何播放。我设置了after=来删除队列的顶部项目，但是如何使它再次播放？谢谢