Hi,

I was trying to work with python today and it required me to use a
virtual environment. I noticed that python-mode already had some basic
virtualenv facilites, and it all worked. The only thing that annoyed me
was that I couldn't configure the setup to be properly persistent, since
python-shell-virtualenv-root couldn't be set as a directory local
variable.

The commentary section mentions

> ;; (setq python-shell-virtualenv-root "/path/to/env/")

but I couldn't find any other details on how the mode expects me to set
the variable.

Therefore I patched the few lines, and it works great.