How to perform a left join in SQLALchemy?

The isouter=True flag will produce a LEFT OUTER JOIN which is the same as a LEFT JOIN.

With your code:

(sa.select([idc.c.Code])
        .select_from(
            t1.join(t2, and_(t1.c.attr == 1, t2.c.attr2 = 1))
            .join(t3, t3.c.Code == t1.c.Code, isouter=True)))

Declarative example:

session = scoped_session(sessionmaker())
session.query(Model).join(AnotherModel, AnotherModel.model_id == Model.id, isouter=True)