How to group dataframe rows into list in pandas groupby

Question

I have a pandas data frame df like:

a b
A 1
A 2
B 5
B 5
B 4
C 6

I want to group by the first column and get second col

Answer 1

Let us using df.groupby with list and Series constructor

pd.Series({x : y.b.tolist() for x , y in df.groupby('a')})
Out[664]: 
A       [1, 2]
B    [5, 5, 4]
C          [6]
dtype: object