使用字符串搜索Pandas系列会生成一个KeyE

Question

我试图使用df[df['col'].str.contains("string")]（在这两个SO问题中描述：1&；2）来根据部分字符串匹配选择行。这是我的代码：

import requests
import json
import pandas as pd
import datetime

url = "http://api.turfgame.com/v4/zones/all" # get request returns .json 
r = requests.get(url)
df = pd.read_json(r.content) # create a df containing all zone info

print df[df['region'].str.contains("Uppsala")].head()

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-23-55bbf5679808> in <module>()
----> 1 print df[df['region'].str.contains("Uppsala")].head()

C:\Users\User\AppData\Local\Enthought\Canopy32\User\lib\site-packages\pandas\core\frame.pyc in __getitem__(self, key)
   1670         if isinstance(key, (Series, np.ndarray, list)):
   1671             # either boolean or fancy integer index
-> 1672             return self._getitem_array(key)
   1673         elif isinstance(key, DataFrame):
   1674             return self._getitem_frame(key)

C:\Users\User\AppData\Local\Enthought\Canopy32\User\lib\site-packages\pandas\core\frame.pyc in _getitem_array(self, key)
   1714             return self.take(indexer, axis=0, convert=False)
   1715         else:
-> 1716             indexer = self.ix._convert_to_indexer(key, axis=1)
   1717             return self.take(indexer, axis=1, convert=True)
   1718 

C:\Users\User\AppData\Local\Enthought\Canopy32\User\lib\site-packages\pandas\core\indexing.pyc in _convert_to_indexer(self, obj, axis, is_setter)
   1083                     if isinstance(obj, tuple) and is_setter:
   1084                         return {'key': obj}
-> 1085                     raise KeyError('%s not in index' % objarr[mask])
   1086 
   1087                 return indexer

KeyError: '[ nan  nan  nan ...,  nan  nan  nan] not in index'

我不明白我得到的是哪个KeyError，因为df.columns返回：

Index([u'dateCreated', u'id', u'latitude', u'longitude', u'name', u'pointsPerHour', u'region', u'takeoverPoints', u'totalTakeovers'], dtype='object')

所以Key在列列表中，并在internet浏览器中打开页面，我可以找到739个“Uppsala”实例。

我正在搜索的列是一个嵌套的.json表，看起来像这样{"id":200,"name":"Scotland","country":"gb"}。我有什么特别的东西在{}字符之间搜索吗？有人能解释一下我在哪里犯了错吗？