How to perform a left join in SQLALchemy?

Here is how to use isouter:

select_from(db.join(Table1, Table2, isouter=True).join(Table3, isouter=True)) 

Option 1 - LEFT JOIN and select ALL columns from both tables

# Query the db
results = db.session.query(Table_1, Table_2).join(
      Table_2, Table_2.column_name == Table_1.column_name, 
      isouter=True).all()
# Iterate results and do stuff
for result in results:
    try:
        # Use [0] for accesing table_1 columns (left table) and use [1] for accesing table_2 columns (right table)
        print(result[0].column_name_x)
        print(result[0].column_name_y)
        print(result[1].column_name_x)
        print(result[1].column_name_y)
     except Exception as e:
        print(str(e))

Option 2 - LEFT JOIN and select SOME columns from both tables

# Query the db
results = db.session.query(Table_1.column_name_x, Table_1.column_name_y Table_2.column_name_z).join(Table_2, Table_2.column_name == Table_1.column_name, isouter=True).all()
# Iterate results and do stuff
for result in results:
    try:
        # Use dot notation for accesing column from any table
        print(result.column_name_x)
        print(result.column_name_y)
        print(result.column_name_z)
    except Exception as e:
        print(str(e))

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)