How to create new list column values from groupby

Question

My goal is to create a new column c_list that contains a list after an groupby (without merge function): df['c_list'] = df.groupby('a').agg({'c':lambda x: list(x)})

df = pd.DataFrame(
                  {'a': ['x', 'y', 'y', 'x'],
                   'b': [2, 0, 0, 0],
                   'c': [8, 2, 5, 6]
                  }
                 )
df

Initial dataframe

    a   b   c
0   x   2   8
1   y   0   2
2   y   0   5
3   x   0   6

Looking for:

    a   b   c  d
0   x   2   8  [6, 8]
1   y   0   2  [2, 5]
2   y   0   5  [2, 5]
3   x   0   6  [6, 8]

BENY · Accepted Answer · 2020-07-14 03:15:01Z

2

Try with transform

df['d']=df.groupby('a').c.transform(lambda x : [x.values.tolist()]*len(x))
0    [8, 6]
1    [2, 5]
2    [2, 5]
3    [8, 6]
Name: c, dtype: object

Or

df['d']=df.groupby('a').c.agg(list).reindex(df.a).values

answered Jul 14, 2020 at 3:15

BENY

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2 Comments

Adil Blanco Over a year ago

Thank you for your answer. Out of curiosity, why the multiplication by len(x) in lambda function ?

BENY Over a year ago

@AdilBlanco transform have the problem , when there is only list , it will explode it by default, so we need the use the list multiple the len of sub-group ~

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