With some installations of python, you have to use sudo with pip in order for it to install anything system wide. (ex: OS X's preinstalled python).
brew intall python
/usr/local/bin/pip install --upgrade setuptools
/usr/local/bin/pip install --upgrade pip
/usr/local/bin/pip install --upgrade virtualenv
/usr/local/bin/virtualenv --python=/usr/local/bin/python py.env
. ~/py.env/bin/activate
pip install butterfly pip install virtualenvwrapper
source /usr/local/bin/virtualenvwrapper.sh
mkvirtualenv -p /usr/local/bin/python venv
pip install butterflyWhy exactly? I find it convenient to have some packages installed system wide so that they can be used by quickly loading up a python shell without having to activate a virtualenv first or if they need to be used outside any specific project eg. requests, nose, jedi, pyflakes, sphinx etc.
Personally I enjoy having a few bits installed under ~/opt/py-venv and simply add ~/opt/py-ven/bin to my path. It's usually no need to activate a venv to use it -- just call that venv/bin/{python|pip|hg|ipython|<whatever>}.
In other words, whenever I "pip install something" that something is installed in my "default" virtualenv. And if/when things get out of hand/I need to upgrade to a new python -- I can just recreate the virtualenv and install whatever is needed.
(One might argue that in case of one uid per mac, doing a system wide install isn't a problem -- but I'd counter with adding a new user and testing if whatever break-age is introduced with various local package installs is easier/quicker than having to reinstall os x...).