python - 为什么 DataFrame.nlargest 没有排序结果？

Question

这是 Pandas 文档 v0.22.0 中的示例:

In [330]: df = pd.DataFrame({'a': [-2, -1, 1, 10, 8, 11, -1],
                             'b': list('abdceff'),
                             'c': [1.0, 2.0, 4.0, 3.2, np.nan, 3.0, 4.0]})

In [332]: df.nlargest(5, ['a', 'c'])
Out[332]: 
     a  b    c
 6  -1  f  4.0
 5  11  f  3.0
 3  10  c  3.2
 4   8  e  NaN
 2   1  d  4.0

为什么结果不是先按'a'再按'c'排序？

同时单列应用nlargest()总是对结果进行排序:

df.nlargest(5,['a'])
Out[110]: 
     a  b    c
5  11.0  f  3.0
3  10.0  c  3.2
4   8.0  e  NaN
2   1.0  d  4.0
1  -1.0  b  2.0

Answer 1

这不是错误，但在意料之中，检查 here :

nlargest doesn't sort the values (which is part of why it's faster).

You can sort afterwards.

In [32]: df.sort_values(['a', 'c'], ascending=False).head(5)
Out[32]:
    a  b    c
5  11  f  3.0
3  10  c  3.2
4   8  e  NaN
2   1  d  4.0
6  -1  f  4.0

In [33]: df.nlargest(5, ['a', 'c']).sort_values(['a', 'c'], ascending=False)
Out[33]:
    a  b    c
5  11  f  3.0
3  10  c  3.2
4   8  e  NaN
2   1  d  4.0
6  -1  f  4.0