python - How to get virtualenv to use dist-packages on Ubuntu?

Question

I know that virtualenv, if not passed the --no-site-packages argument when creating a new virtual environment, will link the packages in /usr/local/lib/python2.7/site-packages (for Python 2.7) with a newly-created virtual environment. On Ubuntu 12.04 LTS, I have three locations where Python 2.7 packages can be installed (using the default, Ubuntu-supplied Python 2.7 installation):

1. /usr/lib/python2.7/dist-packages: this has my global installation of ipython, scipy, numpy, matplotlib – packages that I would find difficult and time-consuming to install individually (and all their dependences) if they were not available via the scipy stack.
2. /usr/local/lib/python2.7/site-packages: this is empty, and I think it will stay that way on Ubuntu unless I install a package from source.
3. /usr/local/lib/python2.7/dist-packages: this has very important local packages for astronomy, notably those related to PyRAF, STScI, etc., and they are extremely difficult and time-consuming to install individually.

Note that a global directory such as /usr/lib/python2.7/site-packages does not exist on my system. Note also that my global installation of ipython, scipy, etc. lets me use those packages on-the-fly without having to source/activate a virtual environment every time.

Naturally, I now want to use virtualenv to create one virtual environment in my user home directory which I will source/activate for my future projects. However, I would like this virtual environment, while being created, to link/copy all of my packages in locations (1) and (3) in the list above. The main reason for this is that I don't want to go through the pip install process (if it is even possible) to re-install ipython, scipy, the astro-packages, etc. for this (and maybe other) virtual environments.

Here are my questions:

1. Is there a way for me to specify to virtualenv that I would like it to link/copy packages in these two dist-packages directories for virtual environments that are created in the future?
2. When I eventually update my global installation of scipy, ipython, etc. in the two dist-packages directories, will this also update/change the packages that my virtual environment uses (and which it originally got during virtualenv creation)?
3. If I ever install a package from source on Ubuntu, will it go in /usr/local/lib/python2.7/dist-packages, or /usr/local/lib/python2.7/site-packages?

Thanks in advance for your help!

Answer 1

This might be a legitimate use of PYTHONPATH - an environmental variable that virtualenv doesn't touch, which uses the same syntax as the environmental variable PATH, in bash PYTHONPATH=/usr/lib/python2.7/dist-packages:/usr/local/lib/python2.7/dist-packages in a .bashrc or similar. If you followed this path,

1. You don't have to tell your virtual environment about this at all, it won't try to change it.

2. No relinking will be required, and

3. That will still go wherever it would have gone (pip install always uses /usr/local/lib/python2.7/dist-packages/ for my Ubuntu) if you install them outside of your virtual environment. If you install them from within your virtual environment (while it's activated) then of course it'll be put in the virtualenvironment.