df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar', 'foo', 'bar', 'foo', 'foo'],
                   'B' : ['one', 'one', 'two', 'three', 'two', 'two', 'one', 'three'],
                   'C' : np.random.randn(8),
                   'D' : np.random.randn(8)})
df 值：
     A    B         C         D
0  foo  one -1.202872 -0.055224
1  bar  one -1.814470  2.395985
2  foo  two  1.018601  1.552825
3  bar  three -0.595447  0.166599 

4  foo  two  1.395433  0.047609
5  bar  two -0.392670 -0.136473
6  foo  one  0.007207 -0.561757
7  foo  three  1.928123 -1.623033


df.groupby('A').sum()
得：
            C        D
A
bar -2.802588  2.42611
foo  3.146492 -0.63958

df.groupby(['A','B']).sum()
                 C          D
A   B
bar one    -1.814470  2.395985
    three  -0.595447  0.166599
    two    -0.392670  -0.136473
foo one    -1.195665  -0.616981
    three  1.928123   -1.623033
    two    2.414034   1.600434