Creating completely separate and isolated Python environments for each project is essential. Pip and VirtualEnv make it easy!

1. VirtualEnv and Pip
Isolated Python environments
Ryan Blunden
Brisbane Python Group, May 2011

2. Summary
• Global site-packages management
• Per project package management
• VirtualEnv creates isolated Py envs
• VirtualEnvWrapper makes things easy
• Pip to install your packages
• Next... Automate your deploys

3. About Me
• Have been the Front End Tech Lead at Flight Centre
• Interested in all things web and am a fan of the new
RESTful back-end => thick client movement
• Python at work for back office and Systems Admin tasks.
Don’t get why SA’s love bash scripts so much
• Can’t build Django apps at work so I build them for
myself and external clients
• I’m hardly a Python expert, but I love it!

4. Global Package Management
• Starts out fine
• Overtime, things get messy
• Not great for development in teams (sync issues)
• Challenges in keeping dev, stage and prod environments
in sync
• Sometimes you don’t even have control of the prod
global site packages
• Easy for things to break overtime when versions and
dependencies not clearly set

5. Per Project Packages
• Enter virtualenv, a tool for creating isolated Python
environments
• Better than workingenv as it doesn’t rely on
$PYTHONPATH trickery
• Better than virtual-python as it works on Windows
(doesn’t rely on symlinks)
• Any of these are better than nothing, I just think
virtualenv does it best.
• Version 1.6.1 supports PyPy 1.5. PyPy!

6. Installing virtualenv
• sudo easy_install virtualenv, then add this to your .bashrc
• Create a new Python environment
• virtualenv myproject --no-site-packages
Don’t inherit from global site-packages directory
• virtualenv myproject --no-site-packages --distribute
Uses distribute instead of setup_tools
• I think it’s best to ensure your new env does not inherit
packages from your global site-packages directory
• This is ok but to make things easier to manage, let’s use
virtualenvwrapper (Thanks Doug Hellmann)

7. virtualenvwrapper
• Organises your virtualenvs into one spot
# Put this in your .bash_profile or .bashrc
export WORKON_HOME=$HOME/.virtualenvs
source /usr/local/bin/virtualenvwrapper_bashrc
• Separates your virtualenvs from your project source
• Nice environment event hooks for automation of
repetitive tasks (cd project on post_activate)
• Basically, it makes your use of virtualenv more efficient
• Now we need the perfect package management tool for
installing packages in our env...

8. VirtualEnvWrapper scripts
• mkvirtualenv (create a new virtualenv)
• rmvirtualenv (remove an existing virtualenv)
• workon (change the current virtualenv)
• add2virtualenv (add external packages in a .pth file to current
virtualenv)
• cdsitepackages (cd into the site-packages directory of current
virtualenv)
• cdvirtualenv (cd into the root of the current virtualenv)
• deactivate (deactivate virtualenv, which calls several hooks)

9. Virtual EnvWrapper Hooks
• postmkvirtualenv
• prermvirtualenv
• postrmvirtualenv
• postactivate
• predeactivate
• postdeactivate

10. Introducing pip
• Pip - Pip Installs Packages, nothing more, nothing less
• It’s good, James Bennet explains - http://goo.gl/F9m2N
• Great at re-producing builds of specific versions of
packages, just remember to use versioning for each
package.
• Every project needs at least one requirements file

11. Installing packages
• By default, searches PyPi just like easy_install
• Can also install from:
- tarballs containing a setup.py script
- Editable checkouts from VCS repos
- Editable checkouts from VCS repos with revision info
• Can install packages from Pip bundles

12. Installing packages
• Installation - sudo easy_install pip
• Workflow
1. Make or activate virtual env
2. Install (and search for) packages
• pip search
pip install package-name (latest)
pip install package-name==1.4.5 (specific)
pip install package-name>=1.4.5 (at least...)
• Install packages from requirements file
pip install -r requirements.txt
• Freeze current packages to produce requirements file

13. Terminal action!
• Finally, let’s write some code
• For those watching at home, go checkout
Mathemtism’s excellent screen cast at
http://goo.gl/wWkti

14. What about my editor?
• Good Python editors and IDE’s should support
virtual env
• PyCharm does by you telling it which python
interpreter should be used for each project, it then
nows which site packages directory to use.

15. Summary
• Isolated Python environments are a must
• VirtualEnv,VirtualEnvWrapper are THE way
to manage them

16. Docs
• VirtualEnv
http://www.virtualenv.org/en/latest/
• VirtualEnvWrapper
http://www.doughellmann.com/docs/virtualenvwrapper/
• Pip
http://www.pip-installer.org/en/latest/

17. References
• Pip and VirtualEnv (Awesome!)
http://mathematism.com/2009/07/30/presentation-pip-
and-virtualenv/
• Tools of the Modern Python Hacker:Virtualenv, Fabric
and Pip
http://www.clemesha.org/blog/modern-python-hacker-
tools-virtualenv-fabric-pip
• Notes on using pip and virtualenv with Django
http://www.saltycrane.com/blog/2009/05/notes-using-pip-
and-virtualenv-django/

18. Thanks!
• Ryan Blunden
ryan.blunden@gmail.com
@ryan_blunden