熊猫：将值在一组内下移一行

Shift对groupby子句的输出起作用：

>>> df = pandas.Dataframe(numpy.random.randint(1,3, (10,5)), columns=['a','b','c','d','e'])>>> df   a  b  c  d  e0  2  1  2  1  11  2  1  1  1  12  1  2  2  1  23  1  2  1  1  24  2  2  1  1  25  2  2  2  2  16  2  2  1  1  17  2  2  2  1  18  2  2  2  2  19  2  2  2  2  1for k, v in df.groupby('a'):    print k    print 'normal'    print v    print 'shifted'    print v.shift(1)1normal   a  b  c  d  e2  1  2  2  1  23  1  2  1  1  2shifted    a   b   c   d   e2 NaN NaN NaN NaN NaN3   1   2   2   1   22normal   a  b  c  d  e0  2  1  2  1  11  2  1  1  1  14  2  2  1  1  25  2  2  2  2  16  2  2  1  1  17  2  2  2  1  18  2  2  2  2  19  2  2  2  2  1shifted    a   b   c   d   e0 NaN NaN NaN NaN NaN1   2   1   2   1   14   2   1   1   1   15   2   2   1   1   26   2   2   2   2   17   2   2   1   1   18   2   2   2   1   19   2   2   2   2   1