If installing on ubuntu first step is to install the python-setuptools to get 'easy_install'
(poseidon appears to have it by default out of the box) (ref:http://www.devpicayune.com/entry/200608221749)

 sudo apt-get install python-setuptools

To install install SQLAlchemy on linux for use in Python:

 sudo easy_install sqlalchemy

To install install GeoAlchemy on linux for use in Python:

 sudo easy_install geoalchemy

For Python to establish conenction with the PostgreSQL database you need this installed:

 sudo apt-get install python-psycopg2