Python Virtual Environments on Pegasus

Users can create their own Python virtual environments to maintain different module versions for different projects. Virtualenv is available on Pegasus for Python 2.7.3. By default, virtualenv does not include packages that are installed globally. To give a virtual environment access to the global site packages, use the --system-site-packages flag.

Creating Virtual Environments

These example directories do not need to be named exactly as shown.

Create a project folder, cd to the new folder (optional), and create a ``virtualenv``:

[username@pegasus ~]$ mkdir ~/python2
[username@pegasus ~]$ cd ~/python2
[username@pegasus python2]$ virtualenv ~/python2/test1
PYTHONHOME is set.  You *must* activate the virtualenv before using it
New python executable in test1/bin/python
Installing setuptools, pip...done.

Create a ``virtualenv`` with access to global packages:

[username@pegasus python2]$ virtualenv --system-site-packages test2

Activating Virtual Environments

Activate the virtual environment with the source command and relative or absolute path/to/env`/bin/activate`. The environment name will precede the prompt.

[username@pegasus ~]$ source ~/python2/test1/bin/activate
(test1)[username@pegasus ~]$ which python

Installing Python modules in Virtual Environments

Once the virtual environment is active, install Python modules normally with PIP, easy_install, or Any package installed normally will be placed into that virtual environment folder and isolated from the global Python installation. Note that using --user or --prefix=... flags during module installation will place modules in those specified directories, NOT your currently active Python virtual environment.

(test1)[username@pegasus ~]$ pip install munkres

Deactivating Virtual Environments

(test1)[username@pegasus ~]$ deactivate
[username@pegasus ~]$

Comparing two Python Virtual Environments

PIP can be used to save a list of all packages and versions in the current environment (use freeze). Compare using sdiff to see which packages are different.

List the current environment, deactivate, then list the global Python environment:

(test1)[username@pegasus ~]$ pip freeze > test1.txt
(test1)[username@pegasus ~]$ deactivate
[username@pegasus ~]$ pip freeze > p2.txt

Compare the two outputs using ``sdiff``:

[username@pegasus ~]$ sdiff p2.txt test1.txt
matplotlib==1.2.1                                             <
misopy==0.5.0                                                 | munkres==1.0.7
[username@pegasus ~]$

As seen above, the test1 environment has munkres installed (and no other global Python packages).

Recreating Python Virtual Environments

To recreate a Python virtual environment, use the r flag and the the saved list:

(test2)[username@pegasus ~]$ pip install -r test1.txt
Installing collected packages: munkres
  Running install for munkres
Successfully installed munkres
Cleaning up...
(test2)[username@pegasus ~]$

Python virtual environment wrapper

Users can install virtualenvwrapper in their own home directories to facilitate working with Python virtual environments. Once installed and configured, virtualenvwrapper can be used to create new virtual environments and to switch between your virtual environments (switching will deactivate the current environment). Virtualenvwrapper reads existing environments located in the WORKON_HOME directory.

Install a local copy of virtualenv with --user:

Recall that --user installs Python 2.7 modules in ~/.local/lib/python2.7/site-packages To specify a different location, use --prefix=$HOME/local/python2mods (or another path).

[username@pegasus ~]$ pip install --user virtualenvwrapper
[username@pegasus ~]$ easy_install --user --always-copy virtualenvwrapper

Set virtual environment home directory and source:

WORKON_HOME should be the parent directory of your existing Python virtual environments (or another directory of your choosing). New Python virtual environments created with virtualenv will be stored according to this path. Set source to in the same location specified during installation.

[username@pegasus ~]$ export WORKON_HOME=$HOME/python2
[username@pegasus ~]$ source ~/.local/bin/

Create a virtual environment using virtualenvwrapper:

This will also activate the newly-created virtual environment.

[username@pegasus ~]$ mkvirtualenv test3
PYTHONHOME is set.  You *must* activate the virtualenv before using it
New python executable in test3/bin/python
Installing setuptools, pip...done.
(test3)[username@pegasus ~]$

Activate or switch to a virtual environment:

(test3)[username@pegasus ~]$ workon test1
(test1)[username@pegasus ~]$

Deactivate the virtual environment:

(test1)[username@pegasus ~]$ deactivate
[username@pegasus ~]$