qgis - How to connect a PyQGIS plugin with Postgres?

I have some questions for you:

I created a python-plugin in QGIS,

then I created a Qt gui fom my plugin

and at last I created a Postgres DB

now I need to make a connection between Qt-gui and Postgres-DB

I wrote this

 from PyQt4.QtSql import *
 db = QSqlDatabase.addDatabase("QPSQL");
 db.setHostName("localhost");

 db.setDatabaseName("siti");
 db.setUserName("postgres");
 db.setPassword("bina");
 bool ok = db.open();

but it doesn't works, I can't understand what I do wrong...

I wish someone could help me...

thank you for all! lisa