在python中安装非自带python模块，有三种方式:
1.easy_install
2.pip
3.下载压缩包(.zip, .tar, .tar.gz)后解压， 进入解压缩的目录后执行python setup.py install命令
本文主要针对pip安装时可能会碰到的一种情况，及解决办法:
假如我要安装pylint模块，该模块非python自带模块，用import肯定不能导入，需要额外安装
>>> import pylint
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: No module named pylint
【现象】
执行pip install <modulename>命令，报错如下:
D:\>pip install pylint --allow-external pylint
Downloading/unpacking pylint
Requirement already satisfied (use --upgrade to upgrade): six in c:\python27\lib\site-packages\six-1
.8.0-py2.7.egg (from pylint)
Downloading/unpacking astroid>=1.3.6 (from pylint)
Real name of requirement astroid is astroid
Could not find any downloads that satisfy the requirement astroid>=1.3.6 (from pylint)
Some insecure and unverifiable files were ignored (use --allow-unverified astroid to allow).
Cleaning up...
No distributions at all found for astroid>=1.3.6 (from pylint)
Storing debug log for failure in C:\Users\aaa\pip\pip.log
【分析】
在Perl中安装新模块，一般可以用PPM图形化工具，也可以用CPAN来安装，比如说: cpan>install Test::Class， 非常方便，不会碰到这种情况，这种情况主要是因为pip版本问题: pip最新的版本(1.5以上的版本), 出于安全的考
虑，pip不允许安装非PyPI的URL，因为该安装文件实际上来自pylint.org,因而导致上面的错误！
NOTE:
1. 可以在官方changelog里面查看更改的信息
2. 可以用pip --version来查看pip的版本信息
C:\>pip --version
pip 1.5.6 from C:\Python27\lib\site-packages (python 2.7)
【办法】
针对上面的情况，既然这个问题是因为pip版本的原因，可以改用pip低一点的版本
方法一: 用pip 1.4版本，再执行pip install pylint命令来安装
方法二: 执行命令时，加上--allow-all-external, --allow-unverified及依赖包版本(astroid==1.3.6)
pip install pylint --allow-all-external pylint astroid==1.3.6 --allow-unverified pylint
NOTE:
1. --allow-all-external # 允许所有外部地址的标签，只有打上该标签pip方可下载外部地址模块
2. --allow-unverified # pip没有办法校验外部模块的有效性，所以必须同时打上该标签
3. astroid==1.3.6 # 依赖包必须要添加上，并赋予其版本号，pip方能从列表下载
方法三: 在当前目录下，新增requirements.txt,内容如下:
# requirements.txt
--allow-all-external pylint
--allow-unverified pylint
pylint
--allow-all-external astroid==1.3.6
再执行: pip install -r requirements.txt
【结论】
1. pip这个设计不够友好，使用也很不方便，远不如Perl中的PPM，期待Python中也有这么个工具。
2. 如果碰到这种错，导致不能安装模块的话: 直接下载压缩包安装好了。 >>>下载包地址<<<
3. 执行pip -h命令查看更新pip相关的帮助信息
Usage:
pip <command> [options]
Commands:
install Install packages.
uninstall Uninstall packages.
freeze Output installed packages in requirements format.
list List installed packages.
show Show information about installed packages.
search Search PyPI for packages.
wheel Build wheels from your requirements.
zip DEPRECATED. Zip individual packages.
unzip DEPRECATED. Unzip individual packages.
bundle DEPRECATED. Create pybundles.
help Show help for commands.
General Options:
-h, --help Show help.
-v, --verbose Give more output. Option is additive, and can be used up to 3 times.
-V, --version Show version and exit.
-q, --quiet Give less output.
--log-file <path> Path to a verbose non-appending log, that only logs failures. This log is active by default at pip.log.
--log <path> Path to a verbose appending log. This log is inactive by default.
--proxy <proxy> Specify a proxy in the form [user:passwd@]proxy.server:port.
--timeout <sec> Set the socket timeout (default 15 seconds).
--exists-action <action> Default action when a path already exists: (s)witch, (i)gnore, (w)ipe, (b)ackup.
--cert <path> Path to alternate CA bundle.
