Flatten multiple columns in a dataframe to a single column

Question

I have a dataframe like this:

id    other_id_1    other_id_2    other_id_3
1     100           101           102
2     200           201           202
3              


        

Answer 1

By using pd.wide_to_long:

pd.wide_to_long(df,'other_id_',i='id',j='drop').reset_index().drop('drop',axis=1).sort_values('id')
    Out[36]: 
       id  other_id_
    0   1        100
    3   1        101
    6   1        102
    1   2        200
    4   2        201
    7   2        202
    2   3        300
    5   3        301
    8   3        302

or unstack

df.set_index('id').unstack().reset_index().drop('level_0',1).rename(columns={0:'other_id'})

Out[43]: 
   id  other_id
0   1       100
1   2       200
2   3       300
3   1       101
4   2       201
5   3       301
6   1       102
7   2       202
8   3       302