Re: BUG #18882: Unexpected write skew at SERIALIZABLE with psycopg2.connect() - Mailing list pgsql-bugs

On Tue, Apr 8, 2025 at 9:18 PM PG Bug reporting form
<noreply@postgresql.org> wrote:
>     cur1.execute("SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
> SERIALIZABLE;")
>     cur1.execute("START TRANSACTION;")

I think it sends an implicit BEGIN, so you're already in a
transaction, and then START TRANSACTION (= BEGIN) is ignored, and SET
SESSION will affect the *next* transaction.  I bet your log says
"WARNING:  there is already a transaction in progress".  I think there
is a psycopg2 way to request SERIALIZABLE, something like
conn1.set_isolation_level(extensions.ISOLATION_LEVEL_SERIALIZABLE), or
you could probably use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
instead of the SESSION version.