virtualenv 的作用相当于 Sandbox，它通过隔离包目录和系统环境参数来实现多个相对独立的虚拟环境。如此可避免过多的第三方库因版本依赖造成问题。同时每个独立的虚拟环境只需通过打包即可分发，也大大方便了系统部署。
$ sudo easy_install virtualenv
现在我们可以创建虚拟环境了。
$ virtualenv test1
New python executable in test1/bin/python
Installing setuptools............done.
我们可以看到虚拟目录下已经被安装了基本所需的运行环境。
$ ls test1/bin
activate activate_this.py easy_install easy_install-2.6 pip python
$ ls test1/include/
python2.6
$ ls test1/lib
python2.6
$ ls test1/lib/python2.6/
_abcoll.py copy_reg.pyc linecache.py os.pyc sre_compile.py stat.py
_abcoll.pyc distutils linecache.pyc posixpath.py sre_compile.pyc stat.pyc
abc.py encodings locale.py posixpath.pyc sre_constants.py types.py
abc.pyc fnmatch.py locale.pyc re.py sre_constants.pyc types.pyc
codecs.py fnmatch.pyc ntpath.py re.pyc sre_parse.py UserDict.py
codecs.pyc genericpath.py ntpath.pyc site-packages sre_parse.pyc UserDict.pyc
config genericpath.pyc orig-prefix.txt site.py sre.py warnings.py
copy_reg.py lib-dynload os.py site.pyc sre.pyc warnings.pyc
进入 test1 目录，激活虚拟环境。
$ cd test1
test1$ source bin/activate
(test)test1$ which python
/home/yuhen/projects/test1/bin/python
(test)test1$ which easy_install
/home/yuhen/projects/test1/bin/easy_install
(test1)test1$ python
Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['',
'/home/yuhen/projects/test1/lib/python2.6/site-packages/setuptools-0.6c11-py2.6.egg',
'/home/yuhen/projects/test1/lib/python2.6/site-packages/pip-0.7.2-py2.6.egg',
'/home/yuhen/projects/test1/lib/python2.6',
'/home/yuhen/projects/test1/lib/python2.6/plat-linux2',
'/home/yuhen/projects/test1/lib/python2.6/lib-tk',
'/home/yuhen/projects/test1/lib/python2.6/lib-old',
'/home/yuhen/projects/test1/lib/python2.6/lib-dynload',
'/usr/lib/python2.6',
'/usr/lib64/python2.6',
'/usr/lib/python2.6/plat-linux2',
'/usr/lib/python2.6/lib-tk',
'/usr/lib64/python2.6/lib-tk',
'/home/yuhen/projects/test1/lib/python2.6/site-packages',
'/usr/local/lib/python2.6/dist-packages/virtualenv-1.4.9-py2.6.egg',
'/usr/local/lib/python2.6/dist-packages/simplejson-2.1.1-py2.6-linux-x86_64.egg',
'/usr/local/lib/python2.6/site-packages',
'/usr/local/lib/python2.6/dist-packages',
'/usr/lib/python2.6/dist-packages',
'/usr/lib/pymodules/python2.6',
'/usr/lib/pymodules/python2.6/gtk-2.0',
'/usr/lib/python2.6/dist-packages/wx-2.8-gtk2-unicode']
>>>
可以看到使用 "souce bin/active" 激活以后，命令提示行多了一个 "(test1)" 前缀，同时 python 和 easy_install 默认会使用虚拟环境 bin 目录下的程序。sys.path 显示当前虚拟环境的库目录被添加到搜索路径列表中。
(test1)$ easy_install MySQL-python
Searching for MySQL-python
Reading http://pypi.python.org/simple/MySQL-python/
Reading http://sourceforge.net/projects/mysql-python
Best match: MySQL-python 1.2.3c1
Downloading http://sourceforge.net/.../1.2.3c1/MySQL-python-1.2.3c1.tar.gz/download
Processing download
Running MySQL-python-1.2.3c1/setup.py -q bdist_egg --dist-dir /tmp/easy_install-Em0wfb/MySQL-python-1.2.3c1/egg-dist-tmp-vzoJ2t
In file included from _mysql.c:36:
/usr/include/mysql/my_config.h:1050:1: warning: "HAVE_WCSCOLL" redefined
In file included from /usr/include/python2.6/Python.h:8,
from pymemcompat.h:10,
from _mysql.c:29:
/usr/include/python2.6/pyconfig.h:808:1: warning: this is the location of the previous definition
zip_safe flag not set; analyzing archive contents...
Adding MySQL-python 1.2.3c1 to easy-install.pth file
Installed /home/yuhen/projects/python/test1/lib/python2.6/site-packages/MySQL_python-1.2.3c1-py2.6-linux-x86_64.egg
Processing dependencies for MySQL-python
Finished processing dependencies for MySQL-python
(test1)$ ls -l lib/python2.6/site-packages/
total 444
-rw-r--r-- 1 yuhen yuhen 283 2010-06-02 09:46 easy-install.pth
-rw-r--r-- 1 yuhen yuhen 106325 2010-06-02 09:46 MySQL_python-1.2.3c1-py2.6-linux-x86_64.egg
drwxr-xr-x 4 yuhen yuhen 4096 2010-06-02 09:45 pip-0.7.2-py2.6.egg
-rw-r--r-- 1 yuhen yuhen 333447 2010-06-01 23:58 setuptools-0.6c11-py2.6.egg
-rw-r--r-- 1 yuhen yuhen 30 2010-06-02 09:45 setuptools.pth
(test1)$ cat lib/python2.6/site-packages/setuptools.pth
./setuptools-0.6c11-py2.6.egg
(test1)$ python
>>> import MySQLdb
>>> MySQLdb.version_info
(1, 2, 3, 'gamma', 1)
>>>
MySQL-python 被安装到了虚拟环境中，且 easy-install.pth 中正确添加了 egg 搜索路径。
最后我们可以用 "deactivate" 命令退出虚拟环境。
(test1)test1$ deactivate
在创建虚拟环境时，我们可以添加 "--no-site-packages" 参数指示虚拟环境不要访问 global site-packages。
$ virtualenv --no-site-packages test2
New python executable in test2/bin/python
Installing setuptools............done.
$ cd test2
test2$ source bin/activate
(test2)test2$ python
>>> import sys
>>> sys.path
['',
'/home/yuhen/projects/python/test2/lib/python2.6/site-packages/setuptools-0.6c11-py2.6.egg',
'/home/yuhen/projects/python/test2/lib/python2.6/site-packages/pip-0.7.2-py2.6.egg',
'/home/yuhen/projects/python/test2/lib/python2.6',
'/home/yuhen/projects/python/test2/lib/python2.6/plat-linux2',
'/home/yuhen/projects/python/test2/lib/python2.6/lib-tk',
'/home/yuhen/projects/python/test2/lib/python2.6/lib-old',
'/home/yuhen/projects/python/test2/lib/python2.6/lib-dynload',
'/usr/lib/python2.6',
'/usr/lib64/python2.6',
'/usr/lib/python2.6/plat-linux2',
'/usr/lib/python2.6/lib-tk',
'/usr/lib64/python2.6/lib-tk',
'/home/yuhen/projects/python/test2/lib/python2.6/site-packages']
>>>
搜索路径中除了 Python 基本库路径外，已经没有了 global site-packages。这样即便我们在安装了大量开发包的的系统里，也可以隔离出一个干净的测试环境。
要判断一个虚拟环境是否 --no-site-packages，除了检查 sys.path 外，还可以通过 "lib/python2.6/no-global-site-packages.txt" 文件来判断。