Mimicing source virtualenv/bin/activate in Emacs.

For a while now I've had a problem where at work we rely heavily on virtualenv and I occasionally want to do things like run pyflakes or run trial from the Twisted installed in that virtualenv. This following activate-virtualenv function appears to be all that is necessary to mimic source virtualenv/bin/activate for Emacs. Enjoy.

(defun add-to-PATH (dir)
  "Add the specified path element to the Emacs PATH"
  (interactive "DEnter directory to be added to PATH: ")
  (if (file-directory-p dir)
      (setenv "PATH"
              (concat (expand-file-name dir)
                      path-separator
                      (getenv "PATH")))))

(defun activate-virtualenv (dir)
  (setenv "VIRTUAL_ENV" dir)
  (add-to-PATH (concat dir "/bin"))
  (add-to-list 'exec-path (concat dir "/bin")))

Updated: I forgot to add my add-to-PATH implementation.