[Twisted-Python] PyPgSQL not locking in adbapi

[Carolina Ardohain]

I'm using Twisted 2.0, and pgSQL 2.4.

I have two function in plpgsql that access the same table, I have to
look if one id is in the table persons, if not then add to the table.
I try to lock a table with "LOCK TABLE persons IN ACCESS EXCLUSIVE
MODE;", so no both functions try to add the same id.

I get 'duplicate key violates unique constraint' when run the two
queries at the same time, if I delay one of the functions 1 sec then
everything is fine.