virtualenv: Specifing which packages to use system-wide vs local [duplicate]

Question

The simplest way to do this is to create a virtualenv which includes the system site packages and then install the versions that you need:

$ virtualenv --system-site-packages foo
$ source foo/bin/activate
$ pip install Django==1.4.3

You can also clean up the virtualenv afterwards by checking the output of pip freeze and removing the packages that you do not want. (removing system-site-packages with pip uninstall does no longer work for newer versions of virtualenv)

Another way would be to create a clean virtualenv and link the parts that you need:

$ virtualenv --no-site-packages foo
$ source foo/bin/activate
$ ln -s /usr/lib/python2.7/dist-packages/PIL* $VIRTUAL_ENV/lib/python*/site-packages

The commands might be slightly different on a non-unixish environment. The paths also depend on the system you are using. In order to find out the path to the library start up the python shell (without an activated virtualenv), import the module and check module_name.__path__. e.g.

Python 2.7.3 (default, Sep 26 2012, 21:51:14) 
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import PIL
>>> PIL.__path__
['/usr/lib/python2.7/dist-packages/PIL']

Also if you have created your virtualenv with --system-site-packages, it is possible to install newer version than what is in the system with pip install --upgrade --ignore-installed numpy.