On Debian, how do I add a Python package in dist-packages to a virtualenv?

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http://unix.stackexchange.com – Given some python package in /usr/share/pyshared/ or /usr/lib/pythonX.Y/dist-packages/ (take python-numpy or python-scipy for example), how do I add that package to a virtualenv? When adding the package to the virtualenv, does it require that it uses the "normal" python installation method (setpy.py etc.) to install, or have it's own? Basically, I want to be able to easily add python packages, which have C extensions that are complex (numpy, scipy, pyQt/pyside, pygtk etc.), to a virtualenv. (HowTos)