Python Module of the Week - virtualenv

继之前部署 python 环境后发现自己误入了传说中的 dependency hell,总是会遇到些奇怪的问题,要纠结用哪个 python 啊,PYTHONPATHPYTHONHOME 的设置啊等等问题。这个时候就要果断用上神器 virtualenv (包括一个易用的扩展 virtualenvwrapper)啦。先看一下为什么它要解决的问题(来自官方的 documentation):

The basic problem being addressed is one of dependencies and versions, and indirectly permissions. Imagine you have an application that needs version 1 of LibFoo, but another application requires version 2. How can you use both these applications? If you install everything into /usr/lib/python2.7/site-packages (or whatever your platform’s standard location is), it’s easy to end up in a situation where you unintentionally upgrade an application that shouldn’t be upgraded.

Or more generally, what if you want to install an application and leave it be? If an application works, any change in its libraries or the versions of those libraries can break the application.

Also, what if you can’t install packages into the global site-packages directory? For instance, on a shared host.

virtualenv 提供的解决办法就是建立虚拟的环境,有自己独立的 dependency,这样不同的应用可以跑在不同的环境中互不干扰。这篇文就记录一下基于它们的常用 work flow。


# create the virtual environment in current directory
$ virtualenv MYENV
# or inherit system packages
$ virtualenv --system-site-packages MYENV

# get into that environment: basically it only changes your PATH
$ source MYENV/bin/activate
# get out
$ deactivate 

# specify which python to use
$ export VIRTUALENV_PYTHON=$HOME/.local/bin/python
$ virtualenv MYENV
# or
$ virtualenv --python=/opt/python-3.3/bin/python MYENV


virtualenvwrapper 只是将 virtualenv 包装了一下,使得整个过程更加简单。

# add those to .bashrc or .zshrc, 
# path of `virtualenvwrapper.sh` may vary
export WORKON_HOME=$HOME/.virtualenvs
source /usr/local/bin/virtualenvwrapper.sh
# then it's good to go!

# basic work flow
$ mkvirtualenv MYENV
$ workon MYENV
$ deactivate

# if need customization, add those in .*shrc before sourcing the
# virtualenvwrapper.sh file
export VIRTUALENVWRAPPER_PYTHON=/usr/local/bin/python
export VIRTUALENVWRAPPER_VIRTUALENV=/usr/local/bin/virtualenv # unnecessary

$ mktmpenv  # temp env, deleted after deactivating
$ rmvirtualenv MYENV # remove
$ lsvirtualenv  # list, `workon` alone is similar
$ allvirtualenv pip install -U pip # universal action
$ lssitepackages # packages in current env
$ mkproject MYPROJ # an application dir along with virtual env

除此之外,还可以在 $WORKON_HOME 中对应的 env 里编辑一些 hooks 做自己的事情。比如在 postactivate 里添加 cd /path/to/proj, 这样启动一个环境后会直接进入工作目录(其实和 mkproject 类似了)。各种 hooks 可以戳这里看官网说明

另外还有些高端的玩法比如改变 cd 的语义这样进入某些目录会自动激活对应的 virtual env,有兴趣的话去官网看看 Tips and Tricks 吧。