loc和ix之间的意外差异

我注意到在Pandas中对DataFrame进行子集化时loc和之间存在一些奇怪的区别ix.

import pandas as pd

# Create a dataframe
df = pd.DataFrame({'id':[10,9,5,6,8], 'x1':[10.0,12.3,13.4,11.9,7.6], 'x2':['a','a','b','c','c']})
df.set_index('id', inplace=True)

df
      x1 x2
id         
10  10.0  a
9   12.3  a
5   13.4  b
6   11.9  c
8    7.6  c


df.loc[[10, 9, 7]] # 7 does not exist in the index so a NaN row is returned
df.loc[[7]] # KeyError: 'None of [[7]] are in the [index]'
df.ix[[7]] # 7 does not exist in the index so a NaN row is returned

为什么在使用NaN返回行时df.loc[[7]]抛出错误df.ix[[7]]？这是一个错误吗？如果不是,为什么loc而ix这样设计？

(注意我在Python 3.5.1上使用Pandas 0.17.1)