PythonVirtualEnv

A virtual environment is an isolated working copy of Python, which allows you to work on specific projects without affecting others. It creates an environment that has its own installation directories and that does not share libraries with other virtualenv environments (and optionally doesn’t access the globally-installed libraries either).

Basic Usage

The following steps outline how to create a virtual environment using the system Python.

Creating the virtual environment on Python 3

Python 3 comes with the virtualenv module built in.

$ python3 -m venv env

Creating the virtual environment on Python 2 (deprecated)

$ git clone https://github.com/pypa/virtualenv.git
$ python virtualenv/virtualenv.py env
New python executable in env/bin/python
Installing setuptools.............done.

Activating the VirtualEnv

You will need to use the 'source' command to load it into your shell environment.

$ source env/bin/activate

We also suggest the first thing you do is to ensure you have an updated version of pip installed in your environment. Make sure you have sourced your environment, and then run the following:

$ pip install --upgrade pip

In bash and tcsh, the environment can be deactivated by typing deactivate

Installing Python Modules

Once you have created your virtual environment and sourced it, you can install additional modules using the 'pip' command.

$ pip install nose
Collecting nose
  Downloading nose-1.3.7-py3-none-any.whl (154 kB)
     |████████████████████████████████| 154 kB 12.5 MB/s
Installing collected packages: nose
Successfully installed nose-1.3.7

Listing installed Python Modules

(env)$ pip freeze
nose==1.3.7

Uninstalling Python Modules

'pip' can also be used to remove a module from the environment.

$ pip uninstall nose
Found existing installation: nose 1.3.7
Uninstalling nose-1.3.7:
  Would remove:
    /nfshomes/liam/env/bin/nosetests
    /nfshomes/liam/env/bin/nosetests-3.4
    /nfshomes/liam/env/lib/python3.7/site-packages/nose-1.3.7.dist-info/*
    /nfshomes/liam/env/lib/python3.7/site-packages/nose/*
    /nfshomes/liam/env/man/man1/nosetests.1
Proceed (y/n)? y
  Successfully uninstalled nose-1.3.7

Switching between virtual environments

To switch between different environments, simply deactivate your current virtual environment, and source another.

(env)$ which python
~/env/bin/python

(env)$ deactivate 

$ source my-other-env/bin/activate

(my-other-env)$ which python
~/my-other-env/bin/python

Using a different Python version

To create a virtual environment that uses a version of python that is different than the system default, create the virtualenv with your target version of Python. This could be a Python build we provide via Modules, or one you've built yourself. Virtualenv will pick up the first python version it finds in your $PATH, or you can direct it to a specific location with the "--python" flag.

$ module load Python3

$ which python3
/opt/local/stow/Python3-3.8.1/bin/python3

Important: virtualenv will include any modules listed in your PYTHONPATH when initializing the virtual environment. To ensure a vanilla environment, it might be a good idea to verify your PYTHONPATH is empty.

$ echo $PYTHONPATH

$ python3 -m venv env36

$ source env36/bin/activate
(env36) $ which python
~/env36/bin/python