问按组滚动求和

EN

df = pd.DataFrame({'date' : [pd.to_datetime('2018-01-01'), 
                             pd.to_datetime('2018-01-01'), 
                             pd.to_datetime('2018-01-01'), 
                             pd.to_datetime('2018-01-01')],
                   'group' : ['a','a','b','b'],
                   'value' : [1,2,3,4],
                   'value_useless' : [2,2,2,2]})

df
Out[78]: 
        date group  value  value_useless
0 2018-01-01     a      1              2
1 2018-01-01     a      2              2
2 2018-01-01     b      3              2
3 2018-01-01     b      4              2

df['rolling_sum'] = df.groupby('group').value.rolling(2).sum()
TypeError: incompatible index of inserted column with frame index

df['rolling_sum'] = df.groupby('group').apply(lambda x: x.value.rolling(2).sum())
TypeError: incompatible index of inserted column with frame index