Hello all,

I'm trying to override the unlink method in the class account_invoice. To be able to erase invoice when I make tests. 

I want the shortest possible code. What I do wrong?

from openerp import models, fields, api

class account_invoice(models.Model):
    _inherit = ['account.invoice']

    @api.multi
    def unlink(self):
        _inherit = ['unlink']
        for invoice in self:
            if invoice.state not in ('draft', 'cancel'):
                raise Warning(_('You cannot delete an invoice which is not draft or cancelled. You should refund it instead.'))
        return super(account_invoice, self).unlink()

thanks all