Pandas: Creating aggregated column in DataFrame

In [20]: df = pd.DataFrame({'A':[1,1,2,2],'B':[1,2,1,2],'values':np.arange(10,30,5)})

In [21]: df
Out[21]:
   A  B  values
0  1  1      10
1  1  2      15
2  2  1      20
3  2  2      25

In [22]: df['sum_values_A'] = df.groupby('A')['values'].transform(np.sum)

In [23]: df
Out[23]:
   A  B  values  sum_values_A
0  1  1      10            25
1  1  2      15            25
2  2  1      20            45
3  2  2      25            45