熊猫set_index不设置索引

Question

假设我创建了一个带有两列b（一个DateTime）和c一个（整数）的pandas DataFrame 。现在，我想根据第一列（b）中的值创建一个DatetimeIndex ：

import pandas as pd

import datetime as dt

a=[1371215423523845, 1371215500149460, 1371215500273673, 1371215500296504, 1371215515568529, 1371215531603530, 1371215576463339, 1371215579939113, 1371215731215054, 1371215756231343, 1371215756417484, 1371215756519690, 1371215756551645, 1371215756578979, 1371215770164647, 1371215820891387, 1371215821305584, 1371215824925723, 1371215878061146, 1371215878173401, 1371215890324572, 1371215898024253, 1371215926634930, 1371215933513122, 1371216018210826, 1371216080844727, 1371216080930036, 1371216098471787, 1371216111858392, 1371216326271516, 1371216326357836, 1371216445401635, 1371216445401635, 1371216481057049, 1371216496791894, 1371216514691786, 1371216540337354, 1371216592180666, 1371216592339578, 1371216605823474, 1371216610332627, 1371216623042903, 1371216624749566, 1371216630631179, 1371216654267672, 1371216714011662, 1371216783761738, 1371216783858402, 1371216783858402, 1371216783899118, 1371216976339169, 1371216976589850, 1371217028278777, 1371217028560770, 1371217170996479, 1371217176184425, 1371217176318245, 1371217190349372, 1371217190394753, 1371217272797618, 1371217340235667, 1371217340358197, 1371217340433146, 1371217340463797, 1371217340490876, 1371217363797722, 1371217363797722, 1371217363890678, 1371217363922929, 1371217523548405, 

b=[dt.datetime.fromtimestamp(t/1000000.) for t in a]

c = {'b':b, 'c':a[:]}

df = pd.DataFrame(c)

df.set_index(pd.DatetimeIndex(df['b']))

print df

一切似乎都正常，除了我打印DataFrame时，它说它具有Int64Index。

<class 'pandas.core.frame.DataFrame'>

Int64Index: 100 entries, 0 to 99

Data columns (total 2 columns):

b    100  non-null values

c    100  non-null values

dtypes: datetime64[ns](1), int64(1)

我是在做错事还是不正确理解Indeces的概念？