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

If id isn't the index, set it first:

df = df.set_index('id')

df

    other_id_1  other_id_2  other_id_3
id                                    
1          100         101         102
2          200         201         202
3          300         301         302

Now, call the pd.DataFrame constructor. You'll have to tile the index using np.repeat.

df_new = pd.DataFrame({'other_id' : df.values.reshape(-1,)}, 
                         index=np.repeat(df.index, len(df.columns)))
df_new

    other_id
id          
1        100
1        101
1        102
2        200
2        201
2        202
3        300
3        301
3        302