import psycopg2
cur = psycopg2.connect(host='localhost',
                    user='myuser',
                    password='mypassword',
                    database='mydb').cursor()

try:
    cur.execute("UPDATE users SET prenames=NULL WHERE id=1")
except psycopg2.Error, e:
    print e.diag.severity
    print e.diag.column_name
    print e.message