interfaces_and_objects_1.py

def create_invoice(db, customer_id, line_items):
    customer = customer_by_id(db, customer_id)
    invoice_id = new_invoice_id(db)
    insert_record(db, 'invoice', id=invoice_id, customer_id=customer_id)
    for item in line_items:
        insert_record(db, 'line_item', invoice_id=invoice_id,
                      amount=item.amount, product=item.product_id)
    return invoice_id

def customer_by_id(db, customer_id)
    cursor = db.cursor()
    cursor.execute("select * from customer where id=%s", (customer_id,))
    return next(cursor)