运行程序时报出了如下警告，
:0: UserWarning: You do not have a working installation of the service_identity
module: 'No module named service_identity'. Please install it from <https://pyp
i.python.org/pypi/service_identity> and make sure all of its dependencies are sa
tisfied. Without the service_identity module and a recent enough pyOpenSSL to s
upport it, Twisted can perform only rudimentary TLS client hostname verification
. Many valid certificate/hostname mappings may be rejected.
从字面意思就是说缺少service_identity模块。
好吧 安装一下试试：
pip install service_identity
提示：
Requirement already satisfied
....:)
Stack Overflow上有类似问题
- https://stackoverflow.com/questions/24089484/python-no-module-named-service-identity
- https://stackoverflow.com/questions/23959336/scrapy-install-version-error
但是都是安装service_identity模块。尴尬。
仔细研究了一下之前的提示信息： 提到
make sure all of its dependencies are sa tisfied.
去service_identity的文档里找找看。 存在几个依赖包.
逐一安装试试，发现果然缺少了一个依赖pyasn1-modules.
教训：
-- 小心依赖。
