Master Python Virtual Environments: Install and Use virtualenv & virtualenvwrapper

virtualenv is a tool for creating isolated Python environments. It builds a folder containing all necessary executables and packages required by a Python project.

After installing Python, pip, and setuptools, you can create a virtualenv, allowing multiple Python versions to coexist on the same machine without interference. Environments can be activated, deactivated, or removed as needed.

1. Install virtualenv

# Install Python (macOS example)
brew install python
# Install pip and setuptools (if not already present)
sudo easy_install pip
# Install virtualenv via pip
pip install virtualenv

2. Using virtualenv

# Create a new environment named pythonEnv
virtualenv pythonEnv
# Activate the environment
cd pythonEnv
source bin/activate
# Deactivate when done
deactivate

3. Manage environments with virtualenvwrapper

Install virtualenvwrapper: pip install virtualenvwrapper Configure environment variables (add to ~/.bash_profile): source /usr/local/bin/virtualenvwrapper.sh Reload the profile: source ~/.bash_profile Create a new environment:

mkvirtualenv pythonEnv          # creates ~/Envs/pythonEnv
mkvirtualenv python3Env -p python3.5  # creates a Python 3.5 environment

Switch between environments: workon pythonEnv Deactivate the current environment: deactivate Remove an environment:

rmvirtualenv pythonEnv

4. Other useful commands and troubleshooting

lsvirtualenv

– list all virtual environments. cdvirtualenv – navigate to the directory of the currently active environment. cdsitepackages – go directly to the site-packages folder. lssitepackages – list contents of site-packages.

If installing pip with easy_install raises ImportError: No module named extern, the macOS‑bundled Python 2.7 lacks the extern module. Download it from https://pypi.python.org/pypi/extern/0.1.0 , extract, and install:

tar zxf extern-0.1.0.tar.gz && python setup.py install