Concatenate rows of pandas DataFrame with same id

Question

Say I have a pandas DataFrame such as:

   A  B  id
0  1  1   0
1  2  1   0
2  3  2   1
3  0  2   1

Say I want to combine rows with the same id

Answer 1

You could use groupby for that with groupby agg method and tolist method of Pandas Series:

In [762]: df.groupby('id').agg(lambda x: x.tolist())
Out[762]: 
         A       B
id                
0   [1, 2]  [1, 1]
1   [3, 0]  [2, 2]

groupby return an Dataframe as you want:

In [763]: df1 = df.groupby('id').agg(lambda x: x.tolist())

In [764]: type(df1)
Out[764]: pandas.core.frame.DataFrame

To exactly match your expected result you could additionally do reset_index or use as_index=False in groupby:

In [768]: df.groupby('id', as_index=False).agg(lambda x: x.tolist())
Out[768]: 
   id       A       B
0   0  [1, 2]  [1, 1]
1   1  [3, 0]  [2, 2]

In [771]: df1.reset_index()
Out[771]: 
   id       A       B
0   0  [1, 2]  [1, 1]
1   1  [3, 0]  [2, 2]