Detailed explanation of the installation and use of python virtual environment virualenv

Preface
After installing python, pip, setuptools and other tools, you can create a virualenv virtual environment. This tool, similar to a virtual machine, can run multiple programs on the same computer. Different versions of python programs do not affect each other. You can exit or delete them when not in use. It is a good development tool.
1. Install virtualenv

#install pip on mac
brew install python
curl https://bootstrap.pypa.io/ez_setup.py -o - | sudo python
sudo easy_install pip
#install virtualenv by pip
pip install virtualenv

2. Use of virtualenv

#创建一个叫做pythonEnv的新环境
virtualenv pythonEnv
#激活再使用
cd pythonEnv
source bin/activate
#退出环境
deactivate

3. Use virtualenvwrapper manages virtual environments
Install virtualenvwrapper

pip install virtualenvwrapper

Configure environment variables:

vim ~/.bash_profile
# Virtualenv/VirtualenvWrapper
source /usr/local/bin/virtualenvwrapper.sh
# exit vim
source ~/.bash_profile

Create environment

mkvirtualenv pythonEnv #在 ~/Envs 中创建 pythonEnv文件夹
mkvirtualenv python3Env -p python3.5 #创建python3.5的环境

Switch environment:

workon pythonEnv

Exit environment:

deactivate

Delete environment:

rmvirtualenv pythonEnv

Others
1. Other commands

lsvirtualenv #列举所有的环境。
cdvirtualenv #导航到当前激活的虚拟环境的目录中，比如说这样你就能够浏览它的 site-packages 。
cdsitepackages #和上面的类似，但是是直接进入到 site-packages 目录中。
lssitepackages #显示 site-packages 目录中的内容。

2. When using the easy_install command to install pip, an ImportError: No module named extern error occurs
Cause: The extern module of python2.7.12 that comes with mac is not installed
Solution:

#download from https://pypi.python.org/pypi/extern/0.1.0 tar zxf extern-0.1.0.tar.gz && python setup.py install

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