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

One more (or rather two):)

pd.melt(df, id_vars='id', value_vars=['other_id_1', 'other_id_2', 'other_id_3'], value_name='other_id')\
.drop('variable', 1).sort_values(by = 'id')

Option 2:

df.set_index('id').stack().reset_index(1,drop = True).reset_index()\ 
.rename(columns = {0:'other_id'})

Both ways you get

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