This works really well for a RPi connected to an Arduino.
On your RPi, you need to run these as root (ie run sudo -i first):
apt-get update
apt-get install python-pip
pip install pyserial
TODO: check that this works on fresh RPi.
You can program an Arduino from your laptop then plug it into the PI when you are ready to run. The Arduino always runs the last program that was flashed onto it.