Hello, I use Odoo14, I want to transform this:

if order.state not in ('draft', 'cancel'):
    raise UserError(_('You can not delete a sent quotation or a confirmed sales order. You must first cancel it.'))

and add 'custom_state' to the 'not in' condition. 

I still want to call the super() so I will not loose chance of third party modules extension for sale order unlink method.

I tried to monkey patch by doing the following, which cause Python maximum recursion error when condition is verified.

import odoo
from odoo import models, fields, api, _

class SaleOrder(models.Model):
    _inherit = 'sale.order'

def unlink_patched(self):
    for order in self:
        if order.state not in ('draft', 'cancel', 'custom_state'):
            raise UserError(_('You can not delete a sent quotation or a confirmed sales order. You must first cancel it.'))
    return super(SaleOrder, self).unlink()

odoo.addons.sale.models.sale.SaleOrder.unlink = SaleOrder.unlink_patched

For now I solved on xml side, by showing 'cancel' button on 'custom_state' so I can put state on 'cancel' then unlink record manually, but I'd like to be able to unlink without put the order in cancel state. Possible somehow ?