Python模块搜索概念介绍及模块安装方法介绍
【import模块】
和C中的#include不同，Python中的import语句并不是简单的把一个文件插入另外一个文件。
导入其实是运行时的运算，程序第一次导入指定文件时，会执行以下步骤，
1. 找到模块文件
2. 编译成位码
3. 执行模块中的代码来创建所定义的模块
并生成.pyc字节码文件，这三个步骤只在程序执行时，模块第一次导入时会进行。之后导入相同的模块时，会跳过这三个步骤，而只提取内存中已加载的模块对象，速度要快的多。
NOTE:
1. Python把已加载的模块放在内置的sys.modules字典中，在import操作开始时会确认引用的模块是否已加载。如果想看哪些模块已加载，可以导入sys，打印sys.modules.keys()
>>> sys.modules.keys()
['heapq', 'functools', 'pyreadline.console.ansi',...,'pyreadline.modes']
2. 可以import的四种文件类型
1.源文件(.py)
2.字节码文件(.pyc)
3.C扩展文件(Windows: .dll或.pyd文件 | Linux: .so文件)
4.相同变量名的目录(用于包导入)
【搜索路径】
比如说:import math,不需要加路径和后缀名，系统会自动搜索路径，按照搜索先后顺序，列出模块搜索路径.
1. 程序的主目录(程序当前运行的目录)
2. PYTHONPATH目录(在环境变量里面配置与加到Path中一样)
3. 标准库目录(比如说: C:\Python27\Lib)
4. .pth文件所在内容(一般放在Lib\site-packages目录下，如下:easy-install.pth)
import sys; sys.__plen = len(sys.path)
./setuptools-0.6c11-py2.7.egg
./py-1.4.26-py2.7.egg
./sphinx-1.3b1-py2.7.egg
./colorama-0.3.2-py2.7.egg
./babel-1.3-py2.7.egg
./snowballstemmer-1.2.0-py2.7.egg
./docutils-0.12-py2.7.egg
./pygments-2.0.1-py2.7.egg
./jinja2-2.7.3-py2.7.egg
./six-1.8.0-py2.7.egg
./pytz-2014.9-py2.7.egg
./markupsafe-0.23-py2.7.egg
./pyzmq-14.4.1-py2.7-win-amd64.egg
./tornado-4.0.2-py2.7-win-amd64.egg
./backports.ssl_match_hostname-3.4.0.2-py2.7.egg
./certifi-14.05.14-py2.7.egg
./egg-0.2.0-py2.7.egg
./chicken-0.1.0-py2.7.egg
import sys; new=sys.path[sys.__plen:]; del sys.path[sys.__plen:]; p=getattr(sys,'__egginsert',0); sys.path[p:p]=new; sys.__egginsert = p+len(new)
而所有这些路径可以通过sys.path来看到
>>> import sys
>>> sys.path
['', 'C:\\Python27\\lib\\site-packages\\setuptools-0.6c11-py2.7.egg', ...,'C:\\Python27\\lib\\site-packages\\wx-3.0-msw']
临时的工作路径加载，可以用
>>> sys.path.append('d:\\test')
>>> sys.path
['', 'C:\\Python27\\lib\\site-packages\\setuptools-0.6c11-py2.7.egg', 'C:\\Python27\\lib\\, 'd:\\test']
>>> sys.path.remove('d:\\test')
【模块安装】
但上面的方法只是临时性的，一旦退出程序或当前交互环境，就失效。要想永久生效就必须让其加入到上述目录中
Perl的强大在于CPAN，可喜的是，Python也开始有类似的技术及功能，比如说pip,可以用包括pip来安装模块。
1. 自写模块，直接拷贝到上述任意目录
2. 第三方模块，执行python setup.py install安装
3. 用easy_install安装
下载ez_setup.py文件
执行: python ez_setup.py
安装所要安装的模块: easy_install py
4. 用pip安装
先用easy_install pip命令来安装pip
再用pip install Markdown来安装具体模块，假设模块名为: Markdown
5. 用第三方模块管理库(比如说Canopy就可以管理科学计算模块,可惜很多需要收费)
用此工具，就可以安装ipython环境了
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