interfaces_and_objects_2.py

    _db = None

    def set_db(db):
       global _db
       _db = db

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

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