session.begin()
try:
    1/0
except:
    session.rollback()
    log.err()
else:
    session.commit()
session.close()
тогда уж лучше так:

session.begin()
try:
    ...
except Exception:
    session.rollback()
    log.err()
    
except BaseException:
    session.rollback()
    session.close()
    log.err()
    raise
else:
    session.commit()
session.close()
или покороче:
session.begin()
try:
    ...
except BaseException, exobj:
    session.rollback()
    log.err()

    if not isinstance(exobj, Exception):
        session.close()
        raise
else:
    session.commit()
session.close()