SQLAlchemy (Postgres) and transaction

I want a record from a table (queue) to be selected, locked (no other process can edit this record) and updated later. I assumed that if I put the whole request and update in a transaction, no other process can edit / request the same record. But I am not able to achieve this.

def move(one, two):
  from settings import DATABASE_USER, DATABASE_PASSWORD, DATABASE_HOST, DATABASE_PORT, DATABASE_NAME
  from sqlalchemy.orm import sessionmaker, scoped_session
  from sqlalchemy import create_engine
  engine = create_engine('postgres://%s:%s@%s:%s/%s' % (DATABASE_USER, DATABASE_PASSWORD, DATABASE_HOST, DATABASE_PORT, DATABASE_NAME), echo = False)
  conn = engine.connect()
  tran = conn.begin()
  Session = scoped_session(sessionmaker())
  session = Session(bind=conn)
  url = session.query(URLQueue).filter(URLQueue.status == one).first()
  print "Got record: " + str(url.urlqueue_id)
  time.sleep(5)
  url.status = two
  session.merge(url)
  session.close()
  tran.commit()

move('START', 'WIP')

      

        
        
        
      

    

If I run 2 processes, they both update the same entry. I'm not sure if I created connections / sessions / transactions correctly. Any pointers?