python报错： Resource u'tokenizers/punkt/english.pickle' not found.

错误详解
Resource u’tokenizers/punkt/english.pickle’ not found. Please
use the NLTK Downloader to obtain the resource: >>>
nltk.download()

解决方法：

terminal输入一下代码：
python
import nltk
nltk.set_proxy("**.com:8080","username","password")
nltk.download()
然后弹出nltk downloader对话框，然后选择下载，结果如下图示。