Pandas new column from groupby averages

by

You need transform:

df['avg_result'] = df.groupby(['a', 'b'])['result'].transform('mean')

This generates a correctly indexed column of the groupby values for you:

   a   b  result  avg_result
0  1  10     100         100
1  1  20     200         250
2  1  20     300         250
3  2  10     400         400
4  2  20     500         550
5  2  20     600         550

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