How to catch OperationalError that happened anywhere in Pyramid+SQLAlchemy?

Question

I have a typical Pyramid+SQLAlchemy+Postgres app. In stress testing or during moments of exceptional load and with low max_connections setting in PG it might happen that OperationalException is raised:

OperationalError: (psycopg2.OperationalError) FATAL:  sorry, too many clients already

Now, obviously I do not want to do this everywhere:

try:
    DBSession.query(Item)...
except OperationalError as e:
    log.error(...)

Is there some way of catching this exception "globally" to be properly handled?

My app uses ZopeTransactionExtension in typical Pyramid manner:

DBSession = scoped_session(sessionmaker(extension=ZopeTransactionExtension()))

Answer 1

I managed to develop a tween that can do this (example):

def catch_pg_exc_tween_factory(handler, registry):
    def catch_pg_exc_tween_clos(request):
        response = None
        try:
            response = handler(request)
        except Exception as e:
            log.error('\n\n\n +++ problem: %s', e)
        return response
    return catch_pg_exc_tween_clos

The strange thing is that nothing but explicit tween ordering in development.ini works (no amount of over= or under= tuning of config.add_tween call seems to have worked):

pyramid.tweens = pyramid_debugtoolbar.toolbar_tween_factory
                 pyramid.tweens.excview_tween_factory
                 pyramid_tm.tm_tween_factory
                 mypkg.util.tweens.catch_pg_exc_tween_factory