Python psycopg transaction isolation level

Hi,

I have a simple python script:

connection = psycopg.connect("host="+server+" dbname="+database+" 
user="+login+" password="+password)
cursor_modify = connection.cursor()
cursor_modify.execute("update messages set flag=1 where flag=0")
connection.commit()

On running it sometimes I got:

psycopg.ProgrammingError: ERROR:  could not serialize access due to 
concurrent update

How can I setup isolation level for waiting end of executing other 
transaction and execute update without any error?

--
Thanks,
Eugene Prokopiev