Python pandas分组、转换和加入

我有一个数据帧，我想按GROUP BY并将列的字符串连接在一起.所以类似于下面的内容.

df = pd.DataFrame({
    'id': [1, 1, 2, 2, 3, 3], 
    'txt': ['sth', 'sth else', 'sth', 'one more thing', 'sth else', 'sth else'],
    'status': ['open', 'open', 'closed', 'open', 'open', 'open']})

df.assign(output=
    df.where(df.status=='open')
      .groupby(df.id)
      .txt.transform(lambda col: ', '.join(col.fillna(''))))

这给了我这个

   id   txt             status  output
0   1   sth             open    sth, sth else
1   1   sth else        open    sth, sth else
2   2   sth             closed  , one more thing
3   2   one more thing  open    , one more thing
4   3   sth else        open    sth else, sth else
5   3   sth else        open    sth else, sth else

有没有办法

1. 没有重复的值(如第4行和第5行)
2. 如果状态为"已关闭"，则不使用前导逗号(如第2行和第3行) 这样我就能得到

   id   txt             status  output
0   1   sth             open    sth, sth else
1   1   sth else        open    sth, sth else
2   2   sth             closed  one more thing
3   2   one more thing  open    one more thing
4   3   sth else        open    sth else
5   3   sth else        open    sth else