Yes. you will need to install then exclusively for this virtualenv.

See: http://virtualenv.openplans.org/#the-no-site-packages-option

If you build with virtualenv --no-site-packages ENV it will not inherit any packages from global site package directory.

Yes. you will need to install then exclusively for this virtualenv.

See: http://virtualenv.openplans.org/#the-no-site-packages-option

If you build with virtualenv --no-site-packages ENV it will not inherit any packages from global site package directory.

If you see the files inside the site-packages directory, the packages are made via symlinks.

drwxr-xr-x 3 ashish staff 102 Nov 24 20:52 .. lrwxr-xr-x 1 ashish staff 85 Nov 24 20:52 UserDict.py -> /opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/UserDict.py ..... 

So you could definitely add packages manually by creating these symlinks for specific package.

Adam Vandenberg correctly answers that.