Merge two lists in pandas groupby and apply

Question

I have a dataframe such as:

    make     model     year range
0   Audi     A3        [1991, 1992, 1993]
1   Audi     A3        [1997, 1998]

I need:

    make     model     year range
0   Audi     A3        [1991, 1992, 1993, 1997, 1998]

I have tried

df = df['year range'].groupby([df.make, df.model]).apply(list).reset_index()

However I end up with the year range being a list of lists instead of a single list.

It seems simple enough but I can't figure it out!

Answer 1

Concatenating lists is done by addition, so you can simply apply sum to the relevant column:

In [24]: df
Out[24]:
   make model                year
0  Audi    A3  [1991, 1992, 1993]
1  Audi    A3        [1997, 1998]

In [25]: df.groupby([df.make, df.model]).year.apply(sum)
Out[25]:
make  model
Audi  A3       [1991, 1992, 1993, 1997, 1998]
Name: year, dtype: object