运行nltk示例 Resource u'tokenizers punkt english.pickle' not found解决

nltk安装完毕后，编写如下示例程序并运行，报Resource u‘tokenizers/punkt/english.pickle‘ not found错误

import nltk

sentence = """At eight o‘clock on Thursday morning Arthur didn‘t feel very good."""
tokens = nltk.word_tokenize(sentence)
print(tokens)

解决方法：
编写如下程序并运行，有代理情况下配置代理，运行成功后弹出NLTK Downloader，下载所有包后再次运行示例程序结果正常

import nltk
nltk.set_proxy("**.com:8080","username","password")
nltk.download()