为什么我在python pandas中遇到属性错误？

Question

当我运行时，

nfl_frame.columns

然后，我有这个输出：

Index(['Rank ', 'Team ', 'Won ', 'Lost ', 'Tied* ', 'Pct. ', 'First Season ',
       'Total Games ', 'Conference'],
      dtype='object')

但是，当我写道：

nfl_frame.Rank

它显示以下错误：

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-180-49fcdc59c710> in <module>()
----> 1 nfl_frame.Rank

C:\Anaconda3\lib\site-packages\pandas\core\generic.py in __getattr__(self, name)
   2148                 return self[name]
   2149             raise AttributeError("'%s' object has no attribute '%s'" %
-> 2150                                  (type(self).__name__, name))
   2151 
   2152     def __setattr__(self, name, value):

AttributeError: 'DataFrame' object has no attribute 'Rank'

*我使用的是Python 3.4和Ipython笔记本。