聚合后python pandas重新连接列

Question

My DataFrame looks like this

exams = pd.DataFrame({'id1':['1x', '1x','2x','3x','3x'], 'id2':['a','a','b','a','a'],'data':[1,2,3,4,5]}) 

    id1    id2   data
0    1x      a      1
1    1x      a      2
2    2x      b      3
3    3x      a      4
4    3x      a      5

Then I aggregate it to

exams_agg = exams.groupby('id1').agg('mean')

Then exams_agg looks like

       data
id1
 1x      1.5
 2x        3
 3x      4.5

I want to reattach id2 column to exams_agg . So I was thinking about create a lookup table

lookup = exams[['id1', 'id2']]
exams_agg = pd.merge(exams_agg, lookup, left_index=True, right_on='id1')

But since lookup contains duplicate pairs of ids, exams_agg contains duplicates as well. What is a good way to create

       data    id2
id1
 1x      1.5     a
 2x        3     b
 3x      4.5     a

Answer 1

If a unique id1 always corresponds to the same id2 , you can simply add id2 in your groupby :

In [5]: df.groupby(['id1', 'id2']).agg('mean')
Out[5]: 
         data
id1 id2      
1x  a     1.5
2x  b     3.0
3x  a     4.5