How can I use cumsum within a group in Pandas?
You can call transform and pass the cumsum function to add that column to your df:
In [156]:df['cumsum'] = df.groupby('id')['val'].transform(pd.Series.cumsum)dfOut[156]: id stuff val cumsum0 A 12 1 11 B 23232 2 22 A 13 -3 -23 C 1234 1 14 D 3235 5 55 B 3236 6 86 C 732323 -2 -1With respect to your error, you can't call cumsum on a Series groupby object, secondly you're passing the name of the column as a list which is meaningless.
So this works:
In [159]:df.groupby('id')['val'].cumsum()Out[159]:0 11 22 -23 14 55 86 -1dtype: int64