'write_function' transfer the options to 'write' method from the popup form
Steps to reproduce:
1. Create custom model to add a product quantity on hand(readonly) field on stock.move
2. Show this field on Warehouse>All operations>Create a Transfer> Create: Internal Moves
3. Add a Internal Move, then open it again, the quantity on hand field's value show 0. But change the product, the value is correct.